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Author Topic:   Introduction To Geology
Dr Adequate
Posts: 16112
Joined: 07-20-2006

Message 1 of 294 (634182)
09-14-2011 5:39 AM

I should like to give an introductory course on geology.

Before I start, I should like to know that there are people who want to read it, so please say so in response to this post.

It will consist of a series of articles presented in what I think is the right order for teaching geology. I shall present them perhaps at weekly intervals, except for the first few which will merely clarify definitions and which we can get through quickly.

I propose that since my main purpose is exposition, rather than debate, I should break from usual forum etiquette in the following way: if someone points out an error in one of my articles, or a point that needs clarification, then I shall thank them for their input and then change the original post, so that each particular article is as good as your collective criticism can make it.

So, is anyone interested?

Replies to this message:
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Dr Adequate
Posts: 16112
Joined: 07-20-2006

Message 3 of 294 (634184)
09-14-2011 7:03 AM
Reply to: Message 2 by Admin
09-14-2011 6:06 AM

Could you provide your qualifications?

B.Sc, mathematics and computer science. Ph.D, mathematics. Purely amateur geologist. Transcendent arch-genius of knowingness. Why do you ask?

I am not making this offer without having thoroughly prepared to meet it. If there is sufficient interest among the forum membership, I shall do so.

This message is a reply to:
 Message 2 by Admin, posted 09-14-2011 6:06 AM Admin has responded

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Dr Adequate
Posts: 16112
Joined: 07-20-2006

Message 5 of 294 (634186)
09-19-2011 7:53 PM


Course objectives

The objective of this course is to show how it is possible to reconstruct the past history of the Earth from our present observation of the rocks.

It will differ from other textbooks in that it will place a strong emphasis on asking and answering the question: "How do we know?" Most textbooks report certain aspects of geological knowledge simply as things that are known: for example, that granite is an igneous rock, or that sandstone with certain properties is aeolian; or that the Earth's core is iron; but without addressing, or at least without systematically addressing, the question of how these things are known in such a way as to satisfy the doubts of the skeptical or the inquisitiveness of the curious.

As a result, the average geology textbook does fairly poor service to the skeptical, or to those who wish to debate and convince the skeptical. It also, in my view, does a disservice to the science of geology itself: for the story of geology is in effect the world's longest-running detective story, and it is more interesting if geology is presented as such than as a collection of facts handed down from on high.

Course outline

Finding the right order in which to structure a course in geology is perhaps the most perplexing decision facing its author. No solution is ideal, because (with the exception of the definition of basic terms, which clearly should come first) it would be best if every topic could be discussed last, so that the reader can come to it with the rest of the course as context. As this is impossible, some sort of compromise has to be made.

The contents of the course will be as follows:

(1) Rocks and minerals: in which I explain what is a mineral, what is a rock, what are sedimentary, igneous, and metamorphic rocks.

(2) Surface processes: including weathering and erosion, rivers, glaciers, nearshore processes, marine sediments, etc; with a systematic look at all the different types of sediment and their corresponding sedimentary rocks --- peat and coal; glacial till and tillite; deserts and aeolian sandstone; coccoliths and chalk; etc, etc, etc.

(3) Plate tectonics: in which we describe how it is known that plates move, what is know of the mechanisms, and what effects this has in terms of faulting, folding, orogeny, ophiolites, terranes, etc.

(4) Stratigraphy: a discussion of actualism, of Steno's principles, of way-up marks, of cross-cutting relationships, of the geological column, of index fossils, and so forth.

(5) Absolute dating: those dating methods other than the relative methods of stratigraphy. This will include a look at some of the methods of more doubtful value, such as racemization.

(6) [This is added by edit in November] I shall have to add a section, hopefully a short one, about paleoclimatology. This is something of an afterthought, but having written much of the rest of the course and repeatedly said that such-and-such a thing would be put off until I talk about paleoclimatology, I don't see how I can not talk about it. So be it.

At that point I shall have done what I set out to do, in that the reader will then have a grasp of the principles of historical geology. However, it may be that the readership will have further questions. In particular, the reader may want to see some historical geology actually done, or in other words to see some case studies. It may be possible to continue the discussion along these lines.

Note on sources

It will not be necessary to give references for notions which are the common property of geologists, such as the definition of a mineral or the fact that granite is felsic. However, I shall provide references to the more abstruse or particular facts to which I allude.


Thanks are due to Pressie for volunteering to review the material. Any remaining errors are, of course, mine. As a great man once said: "We all have our little faults".

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Dr Adequate
Posts: 16112
Joined: 07-20-2006

Message 26 of 294 (634740)
09-23-2011 4:23 PM


This is a place-keeping post. I shall insert glossary items in here every few posts, as and when it seems necessary.

In books the glossary goes at the end, but I can't arrange for any post to always be the last post on this thread, whereas I can arrange for this one to go near the start.


The glossary is currently complete up to the end of the articles on paleoclimatology.


100,000 year problem

The question of why over the last million years, climatic variation has been driven by the 100,000 year Milankovitch cycle rather than the 41,000 year Milankovitch cycle.


A type of lava flow, or the cooled and solidified rock produced by it, characterized by the rough jagged surface of the resulting rock.

Ablation zone

The end of a glacier; the point at which loss of ice by melting exceeds the supply of ice by the movement of the glacier.


Erosion of rocks caused by the sediments carried by wind or water.

Absolute dating

Dating methods which tell us how old a rock or fossil is, as opposed to relative dating.

Abyssal plain

The flat terrain found at the bottom of the ocean beyond the continental margin.

Accretionary prism

An accumulation of sediment which forms in a trench.

Accretionary wedge

A synonym for accretionary prism.

Accumulation zone

The beginning of a glacier; the zone in which snowfall exceeds the loss of snow by melting or evaporation.

Acidic rock

An outdated and inaccurate term for felsic rock.


Abbreviation for atmospheric circulation model.


The observation that the geological record can be explained in terms of the sort of geological processes that actually happen.


Having to do with the wind.

Aeolian sandstone

Sandstone formed from sand deposited by the wind, i.e. desert sand.


Organic molecules produced by certain planktonic organisms, used in the temperature proxy known as Uk'37.


Having to do with flowing water, usually rivers or streams.

Alluvial fan

A fan-shaped deposit of sediment left where a mountain stream reaches a plain.

Alpha decay

Radioactive decay involving the emission of an alpha particle.

Alpha particle

A particle consisting of two protons and two neutrons.


Any of an important class of silicate minerals in which the structure involves aluminum-based as well as silicon-based tetrahedra.

Alpine glacier

Synonym for valley glacier.


A mineraloid formed from the solidified resin of trees.

Amino acid dating

A rather unreliable method of absolute dating based on measuring the racemization of organic remains.


Lacking a crystal structure.


An extinct marine mollusc.


A group of silicate minerals in which the SiO44- units are bonded to form a double chain.

Angle of repose

The maximum angle from the horizontal that a heap of a given type of sediment can assume without collapsing.

Angular unconformity

An unconformity in which the older strata meet the younger strata at an angle, the older strata being truncated by the erosional surface.


In geology, the term "anomaly" means a measurement at some place of some quantity which is different from the average or background value for that quantity.

This should not be confused with the usage of the term "anomaly" in the philosophy of science, where it means a measurement or observation which cannot be reconciled with current theory. In geology, the term has no such implication.

Antecedent river

A river which is present before the uplift of the hills through which it flows.


A very black, hard, and shiny form of coal produced by metamorphism.


A rounded dune-like structure found in rivers of the right velocity and having a sandy bottom. Because they erode by the transport of sand grains from the lee side of one antidune to the stoss side of the next, the net effect is that while the sand moves downstream, the antidunes move upstream.


An upward fold in rocks.


An igneous rock is said to be aphanatic if the crystals in it are too small to be seen with the naked eye. In our articles we have tended to use the more straightforward term "fine-grained".

Apparent polar wander

Apparent secular variation recorded in the paleomagnetic record which is actually caused by the motion of plates relative to the poles.

Ar-Ar dating

Argon-argon dating, a form of radiometric dating.

Aseismic ridge

A long linear trail of volcanic islands and seamounts caused by a plate passing over a hotspot.


Early reef-building organisms, shaped rather like goblets and secreting skeletons of calcium carbonate; they went extinct at the end of the Cambrian period.


Having to do with mud. May be used to qualify the nature of a rock, e.g. argillaceous sandstone would be sandstone with a significant amount of mud mixed in with the sand.


Having to do with sand. May be used to qualify the nature of a rock, e.g. arenaceous mudrock would be mudrock with a significant amount of sand mixed in with the mud.


Alternative term for sandstone.


Sandstone which contains an appreciable quantity of feldspar as well as the more usual quartz. The grains are often poorly sorted and not well rounded.


The portion of the mantle just below the lithosphere.

Atmospheric circulation model

A climate model which only takes into account the ciculation of the atmosphere and not the oceanic circulation.

Atomic number

The number of protons in the nucleus of an atom.

Atomic weight

The sum of the number of protons and neutrons in the nucleus of an atom.


Erosional processes whereby the clasts transported by wind or water are broken or worn down.


A ring of metamorphic rock formed around an igneous intrusion by contact metamorphism.


That part of a beach which is above the high-water line.


The merging of two or more alluvial fans.

Banded iron formation

Sedimentary rock consisting of alternating bands of iron oxide and other sedimentary rock, typically chert.


A local accumulation of sediment, usually sand, such as forms in between the channels of a braided stream or offshore from a beach.

Barrier island

An island formed a the mouth of a river running at right angles to the direction of the distributary streams.


A mafic intrusive igneous rock, black in color and aphanatic.

Basic rock

An obsolete and inaccurate term for mafic rock.


A layer in a sedimentary rock.


A structure found in sedimentary rocks in which the rock is visibly composed of numerous layers (beds).

Bedding planes

The planes dividing the beds in a bedded rock.

Beta decay

A type of radioactive decay including beta plus and beta minus decay; the term is sometimes used to include electron capture as well.

Beta minus decay

A form of radioactive decay in which one of the neutrons in the atom is converted to a proton by emitting an electron.

Beta plus decay

A form of radioactive decay in which a proton is converted into a neutron by the emission of a positron.


Abbreviation for banded iron formation.


The study of the geographical distribution of living or extinct organisms.


Changes in the structure of sediment cause by the activity of living things.

Bituminous coal

The commonest form of coal: less peat-like than sub-bituminous coal, but not as hard, black, and pure as anthracite.


A member of a group of molluscs characterized by being enclosed in two shells (valves). Common examples are mussels, clams, and oysters.

Body waves

Seismic waves which pass through the body of the Earth rather than traveling on its surface; a collective term for S-waves and P-waves.

Bottom-set beds

Horizontal beds of sediment deposited on the sea or lake floor in front of a delta.

Bouma sequence

The characteristic pattern of sediment deposited by a turbidity current.

Braided river

A river in which the current repeatedly splits into smaller streams which merge back together and then split again, and so forth.


A rock consisting of large unrounded fragments cemented together.


A material is said to be brittle if with increasing stress it undergoes very little plastic deformation between elastic deformation and shattering.

Brown clay

Term occasionally used for pelagic clay.


Having to do with calcium carbonate.

Calcareous ooze

A calcareous sediment found over large areas of the ocean floor, consisting of the shells of small organisms.


A mineral consisting of calcium carbonate in a trigonal crystal system.

Calcium carbonate

The chemical CaCO3. Most shells are formed of this, as are the rocks limestone and marble.


A molecule with the anion CO32-; also a rock consisting of carbonates, particularly limestone and dolostone.

Carbonate compensation depth

The depth at which calcium carbonate will dissolve faster than it is deposited; hence, the depth below which calcareous ooze will not accumulate.

Carbon dating

Alternative term for radiocarbon dating.

C-C dating

Alternative term for radiocarbon dating.

Carbon dioxide

The molecule CO2. A gas at temperatures and pressures found on Earth, and forming 0.038% of the Earth's atmosphere.

Carbonic acid

The acid H2CO3. Although this is a very weak acid, it is extremely common, because it can be formed from the reaction between carbon dioxide and water. Because of this, it plays an important role in chemical weathering.


A fossil produced when a mold is filled with minerals.

Cave formation

Alternative term for a speleothem.


Abbreviation for carbonate compensation depth.

14C dating

Alternative term for radiocarbon dating.


The binding of clasts together by a finer material, typically silica, calcium carbonate, or iron oxide, to form a clastic rock.

Cemented tuff

Volcanic ash which has lithified by the process of cementation, as opposed to welded tuff.

Chain silicate

A silicate mineral in which the SiO44- units are bonded together in the form of a chain, i.e. each unit is attached to just two other units (except, of course, at each end of the chain).


Rock which, under a microscope, is clearly composed of the tests of calcium carbonate-secreting micro-organisms.

Chemical sediment

A chemical sediment is one deposited by precipitation rather than by mechanical processes such as wind or water; or by biological processes such as the growth of coral. Note however that some authors will include biological processes as a subcategory of chemical processes; our articles do not follow this usage.

Chemical weathering

Weathering caused by chemical processes (most commonly by some or all of the constituent minerals of a rock being dissolved by carbonic acid); as opposed to mechanical weathering.


A sedimentary rock composed of silica, having an amorphous or very fine-grained structure.


The handedness of an organic molecule.


A large bowl-shaped depression formed at the accumulation point of a valley glacier, with the bowl lacking about a quarter of its rim to let the glacier flow out.


A piece of rock detached by erosion or weathering from a larger rock.


Composed of clasts.


The term clay can either, depending on the context, refer to a class of sheet aluminosilicate minerals, or to clasts with a diameter of less than 1/256 mm. As clay in the second sense is usually also clay in the first sense, this causes less confusion than you might think.


Sedimentary rock composed of clay.


The tendency of minerals or of certain rocks to break along particular preferred planes. As the angles between these planes varies from mineral to mineral, cleavage can be used as a diagnostic tool to distinguish between minerals.


Broad trends in the weather; i.e. the tendency of a location to be hot and humid, or dry and cold.


The point at which snow has been so far compacted into ice that the air trapped in it is completely sealed off from the atmosphere.

Closure time

The time between snowfall and closure, varying from location to location.


Coal is peat which has been lithified by compaction, heat, or both.


The chemical processes by which peat is turned into coal.


Composed of crystals of large size; the opposite of fine-grained.


A calcareous plate forming part of the shell of a coccolithophore; a common constituent of calcareous ooze.


A group of micro-organisms clad in coccoliths.


Decrease in volume of sediment, caused by the pressure induced by being buried under yet more sediment.


A tree is said to be complacent if the thickness of its growth rings is unaffected by annual variations in temperature.


Stress that produces shortening of a solid along the direction in which force is applied.


Of dates, in agreement with one another.


A conglomerate is a rock consisting of large clasts (pebble-sized or larger) cemented together; it is common usage (which we have followed in this text) to use the term to imply that the clasts are rounded, as distinct from a breccia.


Term used ambiguously to refer either to a conodont animal or a conodont structure; context usually makes it clear which.

Conodont animals

A group of extinct primitive chordates having no hard parts except for conodont structures.

Conodont structures

The hard parts of a conodont animal.

Contact metamorphism

Metamorphism caused by close proximity to a source of heat, such as an intrusion of magma; as opposed to regional metamorphism.

Continental drift

The theory that continents have shifted their positions over time; now subsumed into the theory of plate tectonics.

Continental glacier

A glacier covering a large area and flowing outwards from its accumulation zone under the pressure of its own weight, as distinct from a valley glacier.

Continental margin

The continental shelf, slope, and rise.

Continental rise

The shallow (approximately 1 degree from horizontal) terrain between the continental slope and the abyssal plain.

Continental shelf

That part of a continent which is underwater, lying between the unsubmerged portion of a continent and the continental slope.

Continental slope

A shallow slope, typically between 4 and 10 degrees from horizontal, found between the continental shelf and the continental rise.


A group of marine organisms. Hard corals secrete skeletons of calcium carbonate and so act as reef-forming organisms.


The innermost 3,400 km of the Earth, composed mainly of iron. The term also refers to a sample of rock recovered from the Earth's crust by drilling; context ensure that the two meanings are never in practice confused.

Cosmic dust

Dust fallen from outer space, i.e. micrometorites. Although they can be found in pretty much all kinds of sediment, they are proportionally most abundant in pelagic clay due to its slow rate of deposition.

Cosmic rays

Streams of high-energy particles which bombard the Earth from outer space.


Of isotopes, produced by cosmic rays.

Cosmogenic surface dating

A method of absolute dating which gives the time since a rock became exposed on the surface.

Country rock

The rock into which an igneous rock intrudes.


Transport of clasts by wind or water by means of rolling them along the ground, river bed, sea bed, etc.


Bedding in which the beds, instead of being deposited horizontally, are deposited at an angle, as a result of deposition by a current of wind or water; in the simplest case, where the current has a continuous direction, the beds will have a downward slope in the direction of the current.


An igneous rock such as a dike which cuts through the beds of country rock is said to be cross-cutting.


The correlation of dates from different sources.


The upper layer of the Earth, varying from about 5 - 50 km thick, distinct from the mantle by having a different chemical composition, being composed of less dense and more felsic rocks.


A large molecule composed of small molecules chemically bonded together in a regular repetitive arrangement.

Crystal habit

The shape or shapes in which a mineral will typically grow.

Crystal system

A crystal, as defined above, is a large molecule composed of small molecules chemically bonded together in a regular repetitive arrangement. The possible arrangements can be classified into seven crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal and cubic.

Curie temperature

Very roughly speaking, the temperature above which a material cannot be magnetized and below which it can.


A synonym for ring silicate.


A proxy for temperature based on oxygen isotope ratios.

Decay chain

A sequence of events in which one isotope decays to another via an intermediate sequence of unstable isotopes.


The erosion of fine particles from dry soil by the wind.

Deflation lake

A lake caused when deflation has caused a hollow the bottom of which lies below the water table.


The body of sediment deposited when a river flows into a lake or the sea.


A method of dating wood by studying the annual growth rings produced by the tree.


An area of exceptionally low rainfall. Note that although the stereotypical desert is hot and sandy, in geological terms a desert is defined solely by a shortage of rain or snow.

Desert pavement

A stony surface often found in deserts.

Dessication crack

Alternative term for a mud crack.


Composed of clasts; synonymous with clastic.

Diamond anvil cell

A device used in experimental petrology to subject small samples of rock to large amounts of stress.


A sedimentary stucture formed by one type of sediment flowing upwards through another as a result of pressure.


A very light and porous rock formed from diatom tests that have undergone little in the way of compaction and recrystallization.


A group of single-celled algae which produce siliceous shells; a major source of siliceous ooze.


The mechanism by which an originally homogeneous Earth separated into crust, mantle, and core.


A vertical or near-vertical sheet of igneous rock which intrudes into the country rock.


An unconformity in which the underlying strata are parallel with the overlying strata.


A smaller stream flowing out of a larger river, as opposed to a tributary, which flows in.


Occurring once daily.


The union of a terrane with the landmass to which it becomes attached.


Any sediment deposited by a glacier.


Term for the early supporters of continental drift; the opposite of "fixists".


A stone which has traveled out to sea on a "raft" of ablated glacial ice, and has been deposited when the ice melted.


A smallish hill shaped somewhat like the back of a spoon, deposited by glaciers in a manner not fully understood.


A material is said to be ductile if, under stress, it will undergo a great deal of plastic deformation before it breaks. The opposite of brittle.


A mound of sand formed by the action of wind or water.


An ultramafic rock consisting entirely of olivine.


A material is said to be elastic if it recovers from stress: that is, if when the stress is removed it returns to its original conformation. The opposite of plastic.


A particle with negative charge and negligible mass found orbiting the nucleus of an atom.

Electron capture

A form of radioactive decay in which of the radioactive atom's own electrons combines with one of its protons, converting the proton into a neutron.


Atoms are classified into elements according to their atomic numbers, which determine their chemical properties; this is a broader classification then the division into isotopes, which also takes into account their atomic weights.


Molecules which are mirror images of one another.

Entire margins

Leaf margins which are smooth rather than serrated, characteristic of warm humid climates.


The point on the Earth's surface directly above the focus of an earthquake.


A sandy desert.


Any process capable of breaking up rocks or soils and transporting the resulting clasts.

Erratic boulder

A boulder which does not fit in with the geology of its surroundings, transported from its place of origin by a glacier.


Any chemical sedimentary rock the precipitation of which was produced by the partial or complete evaporation of the water containing the dissolved minerals or which the rock is composed.

Event horizon

An extensive geological feature all of which was laid down at exactly the same time; e.g. volcanic ash from a single volcanic eruption.


In biology, heritable change in a line of descent. Outside of biology, the term may be used colloquially to refer to any sort of change or development, as in (for example) "the evolution of jazz from ragtime".

Excess argon

Argon which is not radiogenic; a potential source of error in Ar-Ar dating.

Exponential decay

A quantity is said to undergo exponential decay if its magnitude as a function of time (t) can be expressed in the form ab-ct.

Extrusive rock

Any igneous rock formed by lava pouring out on the surface (where the "surface" includes on the sea floor, under a glacier, or anywhere except under rock) as opposed to intrusive rock which remains trapped within the country rock into which it intrudes. Extrusive rock can be distinguished from intrusive rock by its larger crystal size.


A facies is a body of sediment or sedimentary rock characteristic of a particular depositional environment.


A planar fracture or discontinuity in a volume of rock.


Animals (in the broadest possible sense, including birds, fish, crustaceans, molluscs, etc).


A group of aluminosilicate minerals with a lattice structure.

Felsic rocks

Rocks which are high in silica and feldspar and low in magnesium and iron. The opposite of mafic rocks.


Composed of crystals or clasts of small size; the opposite of fine-grained.

Fining-up sequence

A form of grading upwards from coarse to fine sediments associated with rivers.


Snow which has compacted, but not so far as to become ice.


Of a rock, having the property of splitting easily in a given direction (e.g. between bedding planes).

Fission track dating

A form of absolute dating which involves counting the fission tracks in a rock.

Fission tracks

Microscopic scars left in minerals by alpha particles.


Term for the early opponents of continental drift; the opposite of "drifter".

Flame structure

A sedimentary stucture formed when a denser sediment (typically sand) is deposited on top of a less dense sediment (typically mud) which then penetrates it by seeping upwards; hence, a kind of small diapir.

Flaser deposits

Deposits in which light and heavy sediments alternate, characteristic of nearshore environments.

Flood plain

The flattened, sediment-rich area formed by the action of rivers on a landscape.


Having to do with rivers.


The point in the Earth at which an earthquake originates.


The arrangement of sheet silicates in parallel planes in some metamorphic rocks, due to pressure causing realignment of the sheets in planes at right angles to the direction of pressure.


Short form of foramineferan.


A group of micro-organisms which secrete calcareous tests; one of the most common constituents of calcareous ooze.

Foreset beds

Beds of sediment sloping down at the front of a delta into the sea or lake into which it discharges.


That part of the nearshore which is uncovered at high tide.


Mafic intrusive igneous rock; the intrusive equivalent of basalt.


Abbreviation for general circulation model.


Glycerol dibiphytanyl glycerol tetraethers, organic molecules produced by the single-celled organisms known as Crenarchaeota, used in the TEX86 temperature proxy.

General circulation model

A climate model which takes into account both the atmospheric and the oceanic circulation.

Geological column

A table showing the order of the faunal succession in the fossil record.

Geomagnetic reversal

A change in state from normal polarity to reversed polarity, or vice versa.

Geopetal structure

A structure formed when a hollow object is partially filled with sediment, allowing us to use it as a way-up structure.

Glacial outwash

Sediment carried out of a glacier by meltwater.

Glacial polish

The smooth (but striated) surface produced on a rock by the polishing action of a glacier passing over it.


Synonym for ice age.


A moving mass of ice.


Any igneous rock with an amorphous structure, produced by lava cooling too fast to allow the formation of crystals.

Global Positioning System

A method for finding one's location on the surface of the Earth; used in geology to measure the motion of plates.


A metamorphic rock of high grade with a distinctive streaky appearance produced by the separation out of chain silicates into streaks.


Abbreviation for Global Positioning System.


The degree to which a rock has undergone metamorphism, depending on the amount of heat to which it has been exposed.


Change in size of clasts between two points; most typically from large clasts at the bottom of a layer to small clasts at the top.

Grainflow lamina

A lamina formed in sand dunes when sand at the crest of the dune avalanches down the lee face of the dune.


A felsic intrusive igneous rock; the intrusive counterpart of rhyolite.


Abbreviation for Great Oxygen Event.


Sediment consisting of clasts 2mm in diameter and upwards.

Greenhouse gas

A gas such as carbon dioxide or methane which helps keep the Earth warm by trapping heat in the atmosphere.


Sandstone consisting of quartz, feldspar, and small rock fragments embedded in a clay matrix.

Ground mass

Finer material in which larger clasts (in the case of sedimentary rock) or crystals (in the case of igneous rock) are embedded. A synonym for matrix.

Growth ring

A layer of wood produced by a tree on an annual basis, used in dendrochronology.


A mineral consisting of hydrated calcium sulfate (CaSO4.H2O) with a monoclinic crystal system.


The half-life of an isotope is the length of time in which an atom of that isotope has a 50% chance of undergoing radioactive decay.


Rock salt (NaCl).

Hallam curve

A reconstruction of past variations of sea level based on sedimentary evidence of transgressions and regressions.

Herringbone crossbedding

A form of cross-bedding in which the direction of slope alternates, as a result of oscillatory flow.


A pillar of rock produced by erosion.


A large group of metamorphic rocks produced from sedimentary rocks by contact metamorphism.


A stationary point of high volcanic activity above which plates pass, creating an aseismic ridge.


Decaying organic matter in soil.

Humic coal

Coal produced by the deposition of land plants in swamps, as opposed to sapropelic coal.

Ice age

A time at which sheet glaciers are present on some regions of the Earth's surface.

Ice sheet

Synonym for continental glacier.

Igneous rock

Rock formed by the cooling of molten rock, i.e lava (in which case the rock is said to be extrusive) or magma (in which case the rock is said to be intrusive). Igneous rocks can also be classified by their mineral composition from felsic to ultramafic.

Index fossil

A fossil of a species that was sufficiently widely distributed that its fossils can be used to correlate the deposition of fossils and sediments in widely separated locations.

Index mineral

Any mineral which forms only at certain temperatures and pressures, and which can therefore be used as an index to the conditions under which certain metamorphic rocks were formed.

Index species

A species suitable for the production of index fossils.


Of an element, unable to participate in chemical reactions.


A synonym for chain silicate.


The quantity per area of solar radiation reaching a given location.

Interference ripples

Ripples caused by two currents flowing (one after the other) at or near right-angles to one another.


A time of glacial retreat during an ice age.


A complex pattern of sediments in which different sedimentary types (e.g. sand and mud) interpenetrate in interlocking wedges broadly similar to the pattern made by the fingers of two hands laced together.

Internal drainage

A drainage pattern typical of deserts, in which rivers flow into the desert and evaporate.

Intrusive rock

Rock formed by magma penetrating country rock but not reaching the surface as lava. As the magma will cool slowly, intrusive rock can be distinguished from extrusive rock by the relatively large size of the crystals of which the former is composed. Such rock is said to intrude into the country rock.

Inverse grading

Grading where the size of clasts varies from small clasts at the bottom of a layer to large clasts at the top.

Isostatic rebound

The process whereby land which has formerly been depressed by overlying weight (for example of an ice sheet) rises when the weight is removed.


Atoms are classified into isotopes according to their atomic number and their atomic weight.

K-Ar dating

Potassium-argon dating, a form of radiometric dating.

Karst topography

The distinctive landscape produced by the chemical weathering of limestone.

K-Ca dating

Potassium-argon dating, a form of radiometric dating.


A small lake formed by glacial outwash being deposited around a largish chunk of ice left behind by a retreating glacier; when the residual chunk of ice melts, this leaves a depression which will typically fill with water, producing a kettle.

La-Ba dating

Lanthanum-barium dating, a form of radiometric dating.

La-Ce dating

Lanthanum-cerium dating, a form of radiometric dating.


An intrusion between two strata, similar to a sill but thicker and lens-shaped.


A very thin bed, no more than a few millimeters thick.

Lateral moraine

Sediment which accumulates along the sides of a valley glacier, having fallen or been scraped off the walls of the valley.


A soil type characteristic of a tropical climate alternating between a monsoon season and a dry season.

Lattice silicates

Silicate minerals in which the silica tetrahedra are bonded together to form a three-dimensional lattice.


Molten rock which has reached the surface, as opposed to magma, which is sill trapped beneath it.

Leached ions

Ions dissolved in water as a result of chemical weathering; as opposed to residual minerals.


The side of a mountain, dune, antidune, or generally any hill-shaped geological feature, which is on the down-stream side of a current of wind or water. The opposite of stoss.


Abbreviation for last glacial maximum.


The softest form of coal; the next stage in the formation of coal from peat after peat itself.


Rock formed from calcium carbonate, usually in the form of calcite.

Limiting stand

Trees the growth of which we would expect to be limited by a single factor (such as temperature) because they grow in an environment with an abundant supply of other factors necessary for growth (such as rainfall).


The arrangement of chain silicates in parallel lines in certain metamorphic rocks formed under pressure: the pressure forces these silicates to orientate themselves at right-angles to the direction of pressure.


The conversion of sediment into a sedimentary rock by such processes as compaction and sedimentation.


The Earth's crust together with that portion of the mantle which, like the crust, is brittle and elastic rather than plastic and ductile.


Having to do with the coast.


Fine wind-borne sediment produced by the action of glaciers.

Longshore bars

Bars of sediment running parallel to a beach.

Longshore current

The component of a nearshore current that flows parallel to the shoreline.

Longshore drift

The motion of sediment along the shore as a result of the fact that waves that approach the shore obliquely will recede from it at right-angles to the shoreline.

Love waves

A type of seismic wave which travels along the surface of the Earth rather than through it.

Lu-Hf dating

Lutetium-hafnium dating, a form of radiometric dating.


The gloss on a mineral: pearly, greasy, metallic, dull, etc. While these are somewhat subjective qualities, they are often used in field-guides as criteria for identifying minerals.

Mafic rock

A rock rich in magnesium and iron, and poor in silica and feldspar.


Molten rock which has not reached the surface, as opposed to lava. Some authors will define magma as any molten rock, in which case it would be proper to say that "lava is magma on the surface". However, in this text we have preferred the usage which makes magma and lava two distinct non-overlapping categories of molten rock.


A zone of ultramafic rock lying below the Earth's crust and above its core.

Mantle plume

A column of hot rock rising in the mantle below a hotspot.


A metamorphic rock formed by the metamorphism of limestone.

Massive rock

Rock which does not display bedding (in the case of sedimentary rocks) or foliation (in the case of metamorphic rocks), giving the rock a uniform and homogeneous appearance. This term is not used in our articles, so as to avoid confusion with the common use of "massive" to mean "very big"; we have instead used more transparent terms such as "unbedded".

Mass wasting

Erosion caused by gravity.


Finer material in which larger clasts (in the case of sedimentary rock) or crystals (in the case of igneous rock) are embedded. A synonym for groundmass.


A broad loop in a stream.

Meandering stream

A stream which flows in a series of meanders.

Mechanical weathering

Weathering caused by mechanical processes that break up a rock, as opposed to chemical weathering.

Medial moraine

A moraine formed by the union of two lateral moraines when two valley glaciers flow together to forms a single glacier.

Metamorphic grade

The degree of metamorphism undergone by a metamorphic rock.

Metamorphic rock

A rock which has had its texture or composition changed by heat and/or pressure.


Changes in the texture or composition of a rock brought about by heat and/or pressure.


Changes associated with contact metamorphism in which the parent rock mixes and/or reacts with the intrusive igneous rock and the hot fluids associated with its eruption.


The gas CH4, a potent greenhouse gas.


A group of sheet aluminosilicate minerals.

Mid-ocean ridge

Elevated sea-floor on either side of a mid-ocean rift.

Mid-ocean rift

The rift between two plates at which sea-floor spreading occurs.

Milankocitch cycles

Periodic changes in the inclination of the Earth's axis and the shape of its orbit.


A solid with a particular chemical composition and crystal structure.


Anything which is like a mineral in some respects but doesn't quite fit the definition.

Mixing plot

A graph showing the composition of a rock on which the plotted points will fall in a straight line if the rock was produced by the mixing of different sources of magma.

Mohs scale

A comparative scale of the hardness of minerals.


A void formed when sediment is packed around organic remains, which are then destroyed, leaving a void in the sediment in the shape of the remains.


A deposit of till.

Mud crack

A small-scale geological structures produced in mud as it dries.


Rock formed from clay or silt which is not bedded: lithified mud which is bedded is known as shale.

Native metal

Metal occurring in pure form: for example, a nugget of gold.


The zone in which the sea bed is affected by waves.


A particle with no charge and approximately the same mass as a proton; together with protons, neutrons make up the nucleus of an atom.


An unconformity in which the older rocks are igneous or metamorphic.


A silicate mineral in which the silica tetrahedra are isolated from one another.

Normal polarity

The polarity of the Earth's magnetic field as it is at present. (Note that there is nothing particularly normal about this state of affairs.) The opposite of reversed polarity.

Not fully understood

How geologists say "we don't know".


A fragment of shell or stone around which an ooid forms.


The core of an atom, consisting of protons and neutrons.


A small lake found in a desert.


A process in which one plate colliding with another is thrust over it instead of beneath it; the opposite of subduction.


Felsic volcanic glass.


A silicate mineral with the chemical formula (Mg,Fe)2SiO4 and an orthorhombic crystal system.


A small roughly spherical particle consisting of calcium carbonate layers formed around a nucleus of sand or shell. Required the action of waves for formation, and is therefore formed in shallow seas.


Limestone composed of ooids cemented together.


Either a rock formed from ooids, or a single ooid, depending on context.

Oolitic limestone

Synonym for oolite.


Amorphous hydrated silica, of which the precious stone known as opal is only one particularly pretty example.

Opal compensation depth

The depth at which siliceous material will dissolve faster than it is deposited; hence, the depth below which marine chert will not form.


A section of oceanic crust which has been thrust up above sea-level.


A synonym for neosilicate.


The formation of mountains; or the faulting and folding of a large area by lateral pressure; or the formation of mountains by this process.

Oscillatory flow

The washing back and forth of water on the foreshore as a result of the action of the tide.


Light sediment carried by meltwater from the ablation zone of a glacier.

Outwash plain

A flat area of outwash sediment in front of a glacier.

Oxbow lake

A crescent-shaped lake formed when a meandering stream changes its course, leaving one of its meanders cut off from the stream.

Oxygen catastrophe

Synonym for the Great Oxygenation Event

Oxygen crisis

Synonym for the Great Oxygenation Event


A reef-forming bivalve.


Abbreviation for pascals.


A type of lava flow, or the cooled and solidified rock produced by it, characterized by a ropey and billowy surface texture.


The study of ancient climates.


Ancient currents of wind and water the direction of which can be deduced from the analysis of sedimentary rocks.

Paleomagnetic dating

A form of absolute dating based on analysis of the paleomagnetic data in the rocks.


The geological record of the past history of the Earth's magnetic field.


Fossilized soil.


The last supercontinent to exist, prior to its rifting and the formation of the Atlantic Ocean.


A supercontinent that existed before Pangea.


An unconformity without an erosional surface.

Parent isotope

A radioactive isotope which undergoes radioactive decay to produce a daughter isotope.

Parent rock

The original rock from which a metamorphic rock is formed by metamorphosis.


Unit of stress: 1 pascal = 1 newton/square meter.

Pb-Pb dating

Lead-lead dating, a form of radiometric dating.


Waterlogged and partially decomposed vegetable matter. Note that in geological usage peat does not just refer to gardeners' peat (formed from sphagnum moss) but to any vegetable matter that has undergone peatification. Peat is the sediment from which coal is formed.


The partial decomposition of waterlogged vegetable matter, turning it into peat.


The process of turning sediment into soil by chemical weathering and the activity of organisms (plants growing in it, burrowing animals such as worms, the addition of humus etc).


Having to do with the open sea.

Pelagic clay

Fine-textured sediment deposited on the abyssal plain.


An ultramafic igneous rock consisting mainly of olivine with a little pyroxene and amphibole.


A process forming mineralized fossils in which the voids in the original material are filled by minerals.


A process forming mineralized fossils in which they undergo both replacement and permineralization.


An igneous rock is said to be phaneritic if the crystals in it are large enough to be seen with the naked eye. The opposite of aphenatic.


A large crystal embedded in the more finely-grained ground mass of a porphyritic rock.

Philosophical naturalism

The rejection a priori of the existence of the supernatural; a position completely unnecessary to the practice of geology.


A synonym for sheet silicate.

Pillow basalt

Basalt with a distinctive shape consisting of a set of "pillows"; formed underwater by as a result of the more rapid cooling of lava on contact with water.

Pinstripe lamina

A very thin lamina of very fine clasts, formed in and characteristic of aeolian sand dunes.


A material is said to be plastic if it does not recover from stress: that is, having been squeezed by stress into a given form, it retains that form when the stress is removed. The opposite of elastic.


A flat-bottomed basin in a desert which periodically fills with water to form a shallow temporary lake.

Plutonic rock

Alternative term for intrusive rock.


A piece of the lithosphere bounded by faults.

Plate tectonics

The study of the motion of the Earth's plates.


The Paleoclimate Modeling Intercomparison Project, a project comparing results from climate models with the evidence from paleoclimatic proxies.

Point bar

A bar of sediment formed on the inner bank of a meander.


Two minerals having the same chemical formula but a different crystal structure are said to be polymorphs.


Of a rock, containing some large crystals embedded in a more finely-grained ground mass.


When a chemical formerly dissolved in water settles out of it as a solid sediment, this is called precipitation. (The term has a wider application in chemistry, but for geological purposes this is all you need to know.)


Preceding written human history; the fairly arbitrary line before which organic remains are considered to be fossils.

Primary rock

Term sometimes used for igneous rock.

Principle of cross-cutting relationships

The principle that when one geological feature cuts through another, the former is the younger and the latter is the older of the two features.

Principle of faunal succession

Roughly speaking, the principle that if the fauna and flora in one location are found in one stratigraphic order, the same species will not be found in a different order in another location.

Principle of least time

The principle in physics that a wave traveling through a medium will take the quickest route between two points.

Principle of original continuity

The principle that when sediment is laid down, it will extend continuously until either it meets an obstacle or tapers off with increasing distance from the source of the sediment.

Principle of original horizontality

The principle that when sediment is laid down, it is usually laid down more or less flat.

Principle of superposition

The principle that when sediment is laid down, the sediment most recently deposited will be on the top.

Proglacial lake

A lake fed by meltwater from a glacier.


The building out of a glacier into the sea by deposition of sediment.


A positively charged particle of about the same mass as the neutron; together with neutrons, protons form the nuclei of atoms.


A quantity which we can measure which bears a known relationship to a quantity that we can't measure but would like to; for example measuring past oxygen isotope ratios in shellfish as a substitute for measuring past temperatures.


A term used to describe a situation where rock is layered, but the layers do not represent successive deposition; for example, the layers found in an ophiolite.


A form of volcanic glass filled with air bubbles.

Pyroclastic flow

A current of air laden with volcanic ash, which resists dispersion into the surrounding air because of its greater density.


An important group of rock-forming chain silicates.


Body waves consisting of moving zones of compression and tension.


A mineral consisting entirely of SiO44- units in a lattice structure, so that each oxygen atom of each unit is shared with one other unit, giving quartz the chemical formula SiO2


A metamorphic rock formed by metamorphism of quartz sandstone.

Quartz sandstone

Sandstone of which the sand grains are almost entirely quartz.


Composed of equal amounts of both forms of enantiomers.


The process by which a collection of chiral molecules become racemic.

Racemization dating

An alternative term for amino acid dating.


Having a tendency to radioactive decay.

Radioactive decay

Any process by which the composition of the nucleus of an atom is changed, such as alpha decay, beta decay, and electron capture.

Radiocarbon dating

Radiometric dating of organic material by analysis of the isotopes of carbon it contains.


An atom is said to be radiogenic if it is the product of radioactive decay.


A group of single-celled organisms which produce tiny intricate shells, usually siliceous; these form a major component of siliceous ooze.

Radiometric dating

A collection of methods of absolute dating which depend on the constancy of radioactive decay rates.

Rain shadow

A dry area on the lee side of a mountain, caused by the tendency of clouds to burst on the stoss side.

Raleigh wave

A type of seismic wave which travels on the surface of the Earth rather than through it.


The degree to which coal has undergone metamorphism.

Ra-Pb dating

Radium-lead dating, a form of radiometric dating.

Rb-Sr dating

Rubidium-strontium dating, a form of radiometric dating.


Change in the texture of a rock.

Recumbent fold

A fold in a rock which has been pushed so far over as to lie on its side.


Sedimentary rocks cemented together chiefly by iron oxide, characteristic of dry climates.

Red clay

A synonym sometimes used for pelagic clay.


An underwater ridge or mound formed from the calcareous shells of organisms (typically coral in the present day, but the term is not restricted to coral reefs). Note that the geological usage is more restricted than the nautical usage, in which a sandbar or rock sufficiently near the surface of the water to cause a hazard to shipping would also be considered a reef.

Reef limestone

Limestone resulting from the intact preservation of hard parts of coral or other organisms.

Reflection seismology

A method of examining the structure of buried rocks by studying the reflections of seismic waves produced by artificial explosions.


The change of direction undergone by a wave when it passes from a material which permits travel at one speed to a material which permits travel at another speed. A consequence of the principle of least time.

Regional metamorphism

Metamorphism over a wide region, caused by deep burial or wide-acting tectonic forces; as opposed to contact metamorphism.


An event in which the shoreline moves in a seaward direction; the opposite of a transgression.

Relative dating

Dating methods which allow us to put fossils and/or rocks in order of age, but without telling us how old they are, as opposed to absolute dating, which does.

Re-Os dating

Rhenium-osmium dating, a form of radiometric dating.


A process forming mineralized fossils in which the orginal organic material is replaced by minerals.

Residence time

The average amount of time a given type of atom or molecule will spend in the ocean or in the atmosphere.

Residual minerals

Minerals which are not dissolved by chemical weathering.

Reversed polarity

A condition in which the north and south magnetic poles of the Earth were opposite in orientation to their present position. The opposite of normal polarity.


A felsic extrusive igneous rock; the extrusive counterpart of granite.


A sedimentary rock which display a repetitive vertical succession of types of sediment.

Ring silicates

Silicate minerals in which the silica tetrahedra are bonded together to form rings.


A very small dune.

Roche moutonnée

A hump of rock with one side shallow, polished, and striated and the other side steep and ragged, caused by a glacier flowing over the rock.


An aggregate of one or more minerals or mineraloids.

Rock cycle

The set of processes by which rocks are formed, altered, destroyed, and reformed.

Rock flour

Extremely fine sediment formed by the grinding action of a glacier.

Rock glacier

A glacier consisting mainly of rocks held together by ice.

Rock salt

Common salt (NaCl) when it occurs naturally as a rock; a synonym for halite.


A clast is said to be rounded if its sharp edges and corners have been worn away by erosion. Note that the term does not imply that the clast in question is spherical or near-spherical, just that its shape is smooth.

Rudaceous rocks

Term for conglomerates and breccias.


A group of reef-building molluscs that went extinct at the end of the Cretaceous period.


A salt flat found between the sea and a desert above the high-water line.


The motion of a wind-blown or water-borne particle along the ground, river bed, sea bed, etc, by a series of short hops, when the particle is too large and the current too weak for it to be transported in suspension.

Salt flat

An accumulation of minerals on dry land by the evaporation of water containing dissolved minerals. While the commonest mineral in salt flats is indeed rock salt, other minerals such a gypsum may be deposited.


Particles of sediment between 1/16mm and 2mm in diameter.


Rock formed by the cementation of sand.


Completely weathered rock.

Sapropelic coal

Coal where the original organic material comes from the deposition of algae in lakes; as opposed to humic coal.

Satelite Laser Ranging

A system in which ground-based observation stations measure the round-trip time of ultrashort pulses of light traveling to and from satelites. Used by geologists to measure plate motion and isostatic rebound.


A high-grade metamorphic rock exhibiting pronounced foliation.


The kind of foliation found in schist.


An absolute dating method based on the study of growth patterns in shells and corals.


The study of past climates by the analysis of the composition of shells.

Sea-floor spreading

The motion of two plates away from one another, producing a rift which is continuously filled by magma, producing fresh oceanic crust.


The sediments carried by a wave.


A marine mountain which is entirely underwater.

Seat earth

The paleosol underlying coalbeds.

Secular equilibrium

A condition in which the rate of production of a radioactive isotope in a rock is exactly balanced by the radioactive decay of the same isotope.

Secular variation

The wandering of the magnetic poles over time.


Particles transported and/or deposited by wind, water, glaciers, precipitation, etc; the constituents of sedimentary rocks.

Sedimentary rock

Sediment lithified by cementation and/or compaction, or as a result of simple crystal growth in the case of evaporites.

Sediment trap

A device that collects sediment as it settles.

Seismic tomography

The science of discovering the internal structure of an object (typically, the Earth) by studying the passage of body waves through it.

Seismic waves

Waves in the body or surface of the Earth generated by earthquakes.


A device for detecting earthquakes and measuring their properties.


Occurring twice daily.


A stratigraphic unit smaller than a system but larger than a stage.


A metamorphic rock produced from peridotite in the presence of heat and water.

Serrated margins

Edges of leaves which are not smooth, characteristic of a temperate climate.


A sedimentary rock formed from silt or clay which exhibits bedding.


Stress that causes an object to skew, e.g. the stress that would deform a rectangle into a parallelogram.

Sheeted dikes

Dikes which stand side by side with one another like books on a shelf rather than intruding into some other rock.

Sheet glacier

Synonym for continental glacier.

Sheet silicate

Any silicate in which the SiO44- tetrahedra bond together to form a sheet.


A geological feature which is long and thin, e.g. a river or a shoreline.


The mineral FeCO3.


Silicon dioxide, (SiO2). This occurs in many forms, such as quartz, opal, and chert

Silica tetrahedron

The ion SiO44-, consisting of four oxygen atoms arranged around a silicon atom in a tetrahedron. Such units can link together with each other by sharing oxygen atoms at their corners to form a variety of structures including sheet silicates, chain silicates and quartz.


Any of a large and important class of minerals the chemistry of which is based on the silica tetrahedron.


Composed of silica.

Siliceous ooze

Ooze on the sea floor, composed of the siliceous shells of radiolaria and diatoms.


A sheet of intrusive rock forced between strata.


Clasts between 1/16mm and 1/256mm in diameter.


Sedimentary rock composed of silt.


A rock produced from limestone and igneous rock by metasomatism.

Sky island

A mountain habitat which is home to species which are isolated by their inability to cross the drier hotter surrounding plain.


The portion of a plate being thrust into the athenosphere during subduction.


A metamorphic rock formed by metamorphism of shale, exhibiting pronounced foliation.

Slatey foliation

The sort of foliation found in slate.


Abbreviation for Satelite Laser Ranging.

Sm-Nd dating

Samarium-neodymium dating, a form of radiometric dating.

Snell's Law

A law relating the density of two mediums to the angle of refraction undergone by a wave when it passes from one medium to the other.


Sediment which has been altered by the effects of chemical weathering and and the activity of organisms (plants growing in it, burrowing animals such as worms, the addition of humus etc).

Sole mark

A mark made in sediment when it is scoured by a current.

Solid solution

A mineral in which some positions in the crystal lattice may be filled by different elements.


Silicates in which the silica tetrahedra are bonded together in pairs.


Sediment is said to be well-sorted if it consists of particles of about the same grain-size.


A feature in a cave, such as a stalactite or stalagmite, formed by the precipitation of evaporated minerals, typically calcium carbonate.

Spherical symmetry

A body (in this textbook, invariably the Earth) is said to be spherically symmetric with respect to some property if the value of that property at any given point in it depends only on the distance of that point from the center, and not on the longitude and latitude of the point.


A stratigraphic unit smaller than a series but larger than a zone.


A speleothem hanging like an icicle from the roof of a cave.


A speleothem in the form of a mound or column rising from the floor of a cave.


A rock of known age used in Ar-Ar dating.

Steno's principles

The principles of superposition, of original horizontality, and of original continuity.

Step heating

A process used in Ar-Ar dating in which a rock sample is heated in steps of progressively higher temperatures.


The deformation of a solid body as a result of stress.


A layer of sedimentary rock with distinctive mineralogical, structural, or fossil characteristics such that it can be distinguished from the strata above and below it. Not to be confused with a bed.


The characteristic deposit left behind when a mineral is scraped across a surface; a diagnostic tool for distinguishing minerals.

Stream-dominated deltas

Deltas with long distributary channels reaching seaward; deltas in which the most important factor in their formation is the river discharging via the delta.


The force per unit area exerted on a surface of a deformable body; also by extension the external pressure which creates the internal force.


Grooves left by the movement of a glacier over a rock, parallel to the direction of motion.


Synonym for striations.


Sponges which secrete a calcium carbonate skeleton and so were once important reef-forming organisms. While not actually extinct, they now live only in marginal habitats.

Subbituminous coal

Coal of a higher rank than lignite and a lower rank than bituminous coal.


The motion of one plate beneath another and into the mantle.


A landmass consisting of most or all of the continental crust joined together to form a single continent.

Supercontinent cycle

A process in which supercontinents repeatedly form and then rift again into separate continents.

Superposed river

A river which exists before the creation by erosion of the hills through which it flows.

Superimposed river

Synonym for superposed river.


A form of transport of clasts by wind or water where the particles are carried above the ground, sea bed, river bed, etc.

Suture zone

The line along which a continent becomes joined to another continent, microcontinent, or island arc.


An area of waterlogged ground in which the water is shallow enough for land plants to grow.


Body waves consisting of waves of shear: that is, of displacement at right angles to the direction of travel of the wave, resembling the waves produced by shaking the end of a rope.


The largest stratigraphic unit.


A lake that forms in the former cirque of a glacier after the glacier has melted.

Tectonic window

A place at which a rift in the Earth's crust allows us to see deeper into the crust than is normally possible.


A synonym for lattice silicate.


Stress that produces elongation of a solid along the direction in which force is applied.

Terminal moraine

A moraine deposited in the ablation zone of a glacier.


Having an origin on land.


Part of a landmass, bounded by tectonic faults, which is different in many ways from the main landmass to which it is attached.


The shell of a micro-organism such as a diatom or a foraminiferan.


A temperature proxy based on measurement of the different varieties of GDGTs in sediments.


Physical characteristics of a rock including crystal size (in igneous or metamorphic rocks), and particle size and the degree of sorting and rounding of clasts (in sedimentary rocks).

Theory of evolution

The explanation of the facts of evolution in terms of such mechanisms as mutatation, recombination, lateral gene transfer, genetic drift, and natural selection. The explanation for the law of faunal succession.

Thermohaline circulation

Deep-water circulation driven by density differences in the temperature and salinity of sea water.

Tide-dominated deltas

Deltas in which the most important factor in their dynamics is the tide; characterized by the formation of offshore bars running parallel to the direction of the tide.


Unsorted and usually unbedded sediment deposited by a glacier.

Tidal braking

Slowing of the Earth's rotation as a result of the tidal interaction between the Earth and the Moon.


The lithified equivalent of till.

Topset beds

The flat beds of sediment deposited on the top surface of a delta.

Trace fossil

A fossil such as a footprint which is not of an animal but which was produced by one.


An event in which the shoreline moves inland; the opposite of a regression.


A relation is said to be transitive if when A stands in that relation to B, and B stands in that relation to C, then A stands in that relation to C. For example, the relation "is smaller than" is a transitive relation: if A is smaller than B, and B is smaller than C, then A is smaller than C.


A depression in the sea floor formed along the line where one plate subducts beneath another.


A sea-wave caused by any high-intensity, short-duration submarine event, most usually an earthquake. Often colloquially and completely inacurately known as a "tidal wave".


Sediment deposited by a tsunami.


Lithified volcanic ash


Loaded with sediment.


Rock formed from sediment deposited by a turbidity current.

Turbidity current

A current which manages to keep from mixing with the medium through which it flows because, being turbid, it is denser than the surrounding medium.

Turbidity sediment

Sediment deposited by a turbidity current.


A proxy for temperature based on measurements of the different kinds of alkenones preserved in sediment.

Ultrabasic rock

An obsolete and inaccurate term for ultramafic rock.

Ultramafic rock

A rock which is extremely mafic; that is, particularly low in SiO44- units and high in magnesium and iron.


A surface between successive strata representing a period of erosion or of no deposition.


Synonym for seat-earth.


An alternative term for actualism, not used in this textbook because of ambiguities and inconsistencies in its meaning.


Prone to radioactive decay.

U-Pa dating

Uranium-protactinium dating, a form of radiometric dating.

Upper plane bed

A flat stratified surface formed in a river bed when the river is travelling at too great a velocity to form ripples or dunes.


The mineral UO2

U-Th dating

Uranium-thorium dating, a form of radiometric dating.

Vail curve

A reconstruction of past variations of sea level based on the study of unconformities in the geological record.

Valley glacier

A glacier which has its accumulation zone on a mountain (typically in a cirque) and which flows down through valleys under both gravity and its own pressure; as distinct from a continental glacier.


One of the two shells of a bivalve.


A lamina of coarse light sediment grading into fine dark sediment, found in lakes fed by meltwater from a glacier and representing one year's deposition.

Very Long Baseline Interferometry

A technique in astronomy involving widely separated radio telescopes observing the same object, such as a quasar. Used by geologists to measure the motion of tectonic plates by inferring the motion of the radio telescopes necessary to account for the data.


Informally speaking, the reluctance of a liquid to flow; so for example maple syrup is more viscous than water.

Vis plastica

The name of an imaginary force once thought to cause fossils to grow in rocks.


Abbreviation for Very Long Baseline Inferometry.

Volcanic ash

Fine debris formed when a volcano sprays out fine particles of lava. Note that the term "ash" is a misnomer, since volcanic "ash" is not a product of combustion.

Walther's principle

The principle that if sediment A is succeeded vertically by sediment B without an unconformity between them, then sediment A will also be succeeded horizontally by sediment B in some direction.

Wave base

The greatest depth at which the action of a wave has any effect.

Wave ripples

Ripples in sand or other sediment caused by the action of the tide.

Wave-dominated deltas

Deltas in which longshore drift forms barrier islands in front of the delta.

Way-up structure

A geological feature which enable us to discover which way up a rock was when it was originally formed.


Processes which break up rock but do not themselves transport it, as distinct from erosion.

Weathering rind

The outer, weathered volume of a rock in which the outside has undergone weathering but the weathering process has not yet penetrated all the way through the rock.

Welded tuff

Tuff which forms when a fall of volcanic ash is still hot enough to weld itself together.


The mineral YPO4, useful because it can be used in the radiometric dating of sedimentary rocks.


The mineral ZrSiO4, useful for radiometric dating because of its resistance to erosion, weathering, and metamorphosis.


The smallest stratigraphic unit. Can also be used in the usual informal sense of a region or area, as in the term "ablation zone".

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Message 27 of 294 (634741)
09-23-2011 4:25 PM

Minerals and rocks: definitions
Minerals and rocks: definitions

A crystal is defined as a solid in which the constituent atoms are arranged in an orderly repeating pattern. The diagram below, for example, is a ball-and-stick model of the molecular structure of halite. The large green balls represent atoms of chlorine; the small gray balls represent atoms of sodium; the sticks represent the bonds between atoms.

A mineral is a naturally occurring inorganic solid which is defined by a chemical formula and a particular crystal structure.

Take for example the mineral halite (more familiar to you as table salt). It has the formula NACl, because it is formed from units consisting of one atom of sodium (Na) and one atom of chlorine (Cl), and it has a cubic structure as shown in the diagram above. The formula and structure define the mineral.

It is perfectly possible to have two different minerals with exactly the same formula but different crystal structures. One commonly used example of this are the minerals diamond and graphite. Both consist entirely of atoms of carbon, and so have exactly the same chemical formula, but graphite has a hexagonal crystal system and diamond has an cubic crystal system; as a result their physical properties are very different. Two minerals having the same formula but different molecular structures are known as polymorphs.

Some minerals are what is known as solid solutions. Take for example the mineral olivine. This has the formula (Mg,Fe)2SiO4. The part of the formula which says (Mg,Fe) indicates that there are positions in the crystal structure each of which can be filled by an atom of either magnesium (Mg) or iron (Fe). What proportion of these positions is filled by iron and what proportion is magnesium will vary from sample to sample of olivine. It is convenient to treat these as varieties of the same mineral.

A mineraloid is a substance which is similar to a mineral in some respects but does not fulfill all the criteria necessary to be a mineral. There does not appear to be a complete consensus on what is a mineraloid and what is just a plain non-mineral, and the term is little-used.

One important sort of mineraloid is naturally occurring glass, such as obsidian or pumice. Glass by definition does not have a crystal structure, but is a confused mess at the molecular level. Such a structure, or rather lack of structure, is described as being amorphous.

A rock is an aggregate of one or more minerals or mineraloids. For example granite will contain the minerals quartz, potassium feldspar, sodium feldspar, biotite, and amphibole. In granite the crystals of the different minerals are actually visible to the naked eye, giving it its characteristic speckled appearance, as in the picture below.

A rock can consist of a single mineral: for example the semi-precious stone known to dealers in gemstones as "rock crystal" consists entirely of quartz.

There are thousands of known minerals, most of which are of interest chiefly to collectors, and which we can largely ignore in an introductory text such as this. Of those that we shall mention, far and away the most important class are the silicate minerals, and the next section will be devoted to their physical and chemical properties.

Edited by Dr Adequate, : No reason given.

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Message 29 of 294 (634924)
09-24-2011 9:03 PM
Reply to: Message 28 by roxrkool
09-24-2011 4:01 PM

Re: Minerals and rocks: definitions
Thank you for your comments.

"Rock crystal" is not a term I would ever use as a geologist and I doubt any other geologist (at least in the States) would know what exactly I was referring to if I used it. I would use, "quartz crystals." We tend to use terms that are exact/descriptive rather than ambiguous.

Mmm-hmm. I suspect the term "rock crystal" is used more by lapidaries than geologists. I shall change it.

A rock can also be composed of clay particles as well as organic matter: shale and coal, respectively.

My feeling was that the constituents of coal should be counted as mineraloids and that clay minerals are, well, minerals.

As far as solid solutions go, we have specific mineral names for the various phases of a solid solution series. For example, using your olivine example, forsterite is the Mg-rich end-member and fayalite the Fe-rich end-member of the olivine solid solution series. Both minerals are olivines, but geologists like to distinguish between them for a variety of reasons. Namely, compositions of minerals tell us what sorts of conditions exististed when they formed. An abundance of Mg-rich minerals in igneous intrusions usually suggests that the melt was hotter and generally more primitive (i.e., less evolved/differentiated) than one that contains an abundance of Fe-rich minerals (i.e., more evolved/differentiated).

This should go into a discussion of igneous rocks in particular. For now I just wanted to introduce the concept of a solid solution so that I could say what a mineral is without actually lying to the reader.

What you say about the heat of the melt surprised me. Knowing that iron had a higher melting point than magnesium, I would have supposed that fayalite had a higher m.p. than forsterite. How wrong I was. So much for the Fallacy of Composition.

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Message 31 of 294 (635035)
09-26-2011 12:31 AM
Reply to: Message 30 by roxrkool
09-25-2011 12:25 AM

Re: Minerals and rocks: definitions
I've not heard or read anything regarding organic material being classified as a mineraloid, but that can change or has changed(?).

Every list of mineraloids I've seen includes amber, which is organic. As to coal, for example this guy in his lecture notes classes coal as a mineraloid. On the other hand, some people mention jet but not coal, which is odd, because surely there's no qualitative difference.

I think the best thing is if I join the people who call coal a mineraloid, appart from anything else that saves one from havving to add "... or coal" to the definition of a rock.

Clays, such as illite, chlorite, etc. are minerals, but "clay" is also a particle size. So clay minerals, as well as clay-sized particles (of other minerals), comprise shale.

Yes, but as you note the other things that are clay (by particle size) are minerals... OK, maybe with a bit of organic material thrown in, but to a very large extent shales and mudstones are made of minerals, so I stand pat.

We'll get round to discussing these details when we look at sedimentary rocks in particular.

Iron, the metal, may have a higher melting point than magnesium metal, but Mg-rich forsterite is a far more stable mineral under higher temps and pressures (as present in the mantle) than the Fe-rich fayalite. Therefore, forsterite will crystallize first from higher temp ultramafic/mafic melts, rather than fayalite.

Oh yes, I know you're right --- I looked it up. I just thought it was an interesting example of counter-intuitiveness in science.

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Message 35 of 294 (635159)
09-27-2011 12:46 PM
Reply to: Message 32 by roxrkool
09-26-2011 8:35 AM

Re: Minerals and rocks: definitions
I was simply elaborating a bit for the readers.

Well, please don't. So far I have explained the distinction between a mineral and a rock. This is no place to talk about the distinction between clay as a mineral and clay as a particle size; or about the melting points of olivine endmembers. This will all come in time.

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Message 36 of 294 (635965)
10-03-2011 11:51 AM

Silicate Minerals
Silicate minerals

Silicate structures

By a silicate tetrahedron we shall mean an atom of silicon bonded with four equally spaced atoms of oxygen forming the four corners of a pyramid having a triangular base.

Each tetrahedron can share each one of its oxygen atoms with one other tetrahedron, so that two tetrahedra can join together corner-to-corner (but not edge-to-edge or face to face). Hence each tetrahedron can be linked with up to four other tetrahedra, one for each corner of the tetrahedron; or three, two, one, or none. This gives us a wide variety of structures that can be built out of tetrahedra: three-dimensional lattices; two dimensional sheets, chains, double chains, rings, et cetera; so silicate minerals can be classified according to their silicate structure as lattice silicates, sheet silicates, chain silicates, and so forth. The diagram below shows some of the possible structures.

Key: (a) an isolated tetrahedron; (b) a pair; (c) a six-member ring; (d) a chain; (e) a double chain; (f) a sheet.

Note that the chain and double chain can be extended indefinitely in one direction and the sheet in two directions.

A silicate mineral is a mineral containing silicate structures. Note that with the exception of quartz and its polymorphs, a silicate mineral will not consist entirely of such structures. Atoms of other elements must necessarily be involved so that the rings, chains, sheets or whatever form part of a three-dimensional crystal.

Most silicate structures can be described either by a descriptive English name such as "sheet silicate" or by a name which describes the same thing in Greek such as "phyllosilicate". Where a plain English term exists, I shall employ it in this text. The table below shows how the structures relate to the English and Greek names and gives examples of minerals important to this course.

Structure English name Greek Important examples
Three-dimensional Lattice silicates Tectosilicates Quartz; feldspars
Parallel sheets Sheet silicates Phyllosilicates Micas; clays; serpentine; chlorite
Single or double chains Chain silicates Inosilicates Pyroxenes; amphiboles
Three, four, or six-membered rings Ring silicates Cyclosilicates
Pairs Sorosilicates Epidotes
Isolated tetrahedra Nesosilicates or orthosilicates Olivine

Because tetrahedra link together by sharing the oxygen atoms at their corners, the structure formed by the tetrahedra is reflected in the chemical formula of a silicate. For example, quartz consists of nothing but tetrahedra linked together in a three-dimensional lattice. This means that every tetrahedron is linked to another at all four corners; which means that every oxygen atom is shared by two silicon atoms; which means that quartz has the formula SiO2. Similarly, if you look at the formula for zircon (ZrSiO4) you can see that it must be a neosilicate.


In some silicates, the structure is based not just on silicate tetrahedra but also on tetrahedra with a central atom of aluminum rather than silicon. Silicates which incorporate these aluminum-based tetrahedra as well as silicate tetrahedra are known as alumnosilicates.

Aluminum-based tetrahedra have a different charge from silicate tetrahedra. This means that you cannot have an aluminosilicate which differs from an ordinary silicate only by the substiution of atoms of aluminum for some of the atoms of silicon; there must necessarily be other differences. For example, it is chemically impossible to build a lattice just out of these two kinds of tetrahedra analogous to quartz; other atoms are required to balance the charge of the chemical formula. Hence lattice aluminosilicates such as feldspars do not have the formula (Si,Al)O2, which is impossible, but have formulas such as KAlSi3O8 and CaAl2Si2O8.

Felsic and mafic silicates

Silicate minerals which are high in silicon are called felsic minerals; the opposite of felsic is mafic; minerals which are very mafic are known, sensibly enough, as ultramafic. Note that this term only applies to minerals which are silicate minerals and so contain some silicate tetrahedra; no-one would call calcium carbonate (for example) an ultramafic mineral.

In some texts, particularly older texts and British texts, you may see the words acidic, basic, and ultrabasic used instead of felsic, mafic, and ultramafic. These terms refer to an obsolete hypothesis of mineral formation, and I shall not use them; I mention them only for the benefit of those readers who might come across them in further reading.

Some generalizations might be made about the differences between felsic and mafic minerals: as we progress from felsic to mafic the minerals are more dense (because the atoms in them which aren't oxygen or silicon are heavier elements such as magnesium or iron); they have higher melting points; and when they do melt they are less viscous (that is, they flow more easily).

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Message 37 of 294 (636684)
10-09-2011 3:02 PM

Rocks: Igneous, Sedimentary, And Metamorphic
Rocks: igneous, sedimentary and metamorphic


In this article we shall look at the most significant way in which geologists classify rocks. The reader should recall from the article on minerals and rocks that a rock is an aggregate of one or more minerals or mineraloids.

Igneous, sedimentary, and metamorphic

There are all sorts of ways that we might classify rocks. We might, for example, divide them up according to chemistry: and indeed the distinction between silicates and carbonates is a useful one. We might also classify rocks according to whether they contain felsic or mafic minerals, and as we shall see this is a good way to classify certain rocks. But the most fundamental way in which geologists classify rocks is to label them as igneous, sedimentary, or metamorphic.

Igneous rocks are rocks formed by the cooling and setting of molten rock.

Sedimentary rocks are formed by sediment (for example, sand or mud) turning into rock (such as sandstone or mudrock).

Metamorphic rocks are formed when rocks are subjected to heat, to pressure, to chemical reactions, or to any combination of these three, in such a way as to change the properties of the original rock in some way.

The rock cycle

The relation between the various kind of rocks can be summarized by a diagram of the rock cycle. Here is one picture of it which I find more or less satisfactory; I may at some point come back to this and prepare my own.

How do we know?

At this point we are touching on the main theme of this course. For to classify rocks as igneous, sedimentary, or metamorphic is implicitly to classify them not by their directly observable properties such as color or density or chemical composition --- but by their history as inferred from their present-day properties. The reader will, therefore, want to know: how do we know? This question will be answered in separate articles on igneous, sedimentary, and metamorphic rocks.

In the meantime, let us point out how intrinsically historical geology is. After we've got past the most basic of considerations such as defining a mineral and defining a rock, we are plunged into historical considerations. And this in not just because this course is about historical geology: it would be the same if it was an introduction to how to find oil. There is no geology that is not historical, and if we tried to do without historical inferences we might as well classify rocks by how pretty they are for all the good it would do us.

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Message 40 of 294 (636909)
10-12-2011 5:42 AM
Reply to: Message 39 by Pressie
10-12-2011 12:33 AM

Well, I have to disagree with you. In fact, I have already disagreed with you, since by over-reacting to Robert you are disagreeing with what I wrote in my previous post.

If we are studying historical geology, which is what we're meant to be doing here, then the history, the process, is the thing being studied. It is the underlying reality of which the rocks are merely the outward and visible sign.

But, as I pointed out, even if we're not doing historical geology, but have a more practical aim, it is still necessary to talk about it in terms that refer to process and history --- igneous, sedimentary, and metamorphic rocks, marine sedimentary rocks, aeolian sandstone, rhythmites, pedogenesis, faulting, folding, mineralization ... and if you tried to rewrite geology to get away from such terms, it would appear as a set of arbitrary rules, like chemistry without the concept of valence. Consequently it would be impossible to think about geology in such terms; you would have to think about it in historical terms and then translate the practical upshot into ahistoric terms.

For example, I can tell you easily in historical terms why oil is to be found in certain nearshore deposits. Now, it is possible to translate this into a method for looking for oil which can be expressed purely in ahistoric terms, in which we refer exclusively to the we-can't-call-it-"sedimentary"-any-more structure of rocks. But it would be impossible to say why any of this should be true --- the instructions would be as arbitrary as a magic spell: "Two nights after the dark of the moon, walk three time widdershins round the enchanted glade, muttering thrice the words 'Grimblethorpe, Mulcifer, Pellicle', and behold, the fairy gold will appear."

The same, of course, is true of Robert Byers' bullshit "history", 'cos of its wild and ludicrous inaccuracy. Someone trying to be ahistorical couldn't talk about the characteristic sedimentary deposits formed by the action of the tide because that refers to history and process, and Robert Byers can't talk about it because that refers to real history and process and not to imaginary impossibilities recounted in a disgusting and ridiculous myth.

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Message 43 of 294 (637186)
10-14-2011 1:59 AM
Reply to: Message 42 by Pressie
10-14-2011 1:37 AM

I disagree with you. The rocks are and were studied. The processes forming rocks are and were also studied. After that the processes by which the rocks under investigation were formed can be deducted as we can study the processes by which such rocks are formed today.

I'm not sure that that's disagreeing with me ...

For example: After studying the Clarens formation of the Karoo Sequence, it has been found that said formation has all the characteristics of Aeolian deposits, therefore it was deposited by the working of wind. That’s geology.

It is. But in particular the deduction that the Clarens formation is aeolian is just as much geology as looking at the structure of the rock. And considerably more interesting. Mere observational statements such as "This rock contains pinstripe laminae" without any deduction from it would be no more informative than "This rock is pretty". But the former statement is more interesting if we use it to infer that we're looking at what used to be a desert --- a notion which has predictive and explanatory power. This is what distinguishes geology from stamp-collecting.

As far as the oil deposits go, no magic. The Mississippi Delta exists. To simplify: to find oil we start looking for similar environments.

OK, but how do you do that? For example, suppose I find aeolian sandstone (which is a deduction about its history) over here, and I find deep marine sediment (which is a deduction about its history) over there, and if I find that they are the same age (obviously a historical judgement) then I can infer a coastline somewhere between the two, and then I can start looking for nearshore deposits (another historical judgement) somewhere in the line between them, and if I find, for example, interference ripples, I would infer that they were formed by the tide and know that I was getting close to the sort of structures I'm actually looking for.

Now one could express how to do this and similar things in terms of a mechanical procedure that just involves looking at rocks and making certain measurements of isotope ratios, but only by expunging any clue as to how it actually works.

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Message 44 of 294 (637187)
10-14-2011 2:09 AM

Igneous Rocks
Igneous rocks


Igneous rocks are rocks formed by the cooling and solidification of molten rock. They fall into two main categories:

* Intrusive rocks are those which are caused by the cooling of molten rock underground. Subterranean molten rock is known as magma.

* Extrusive rocks are those formed from molten rock on the surface, which is known as lava.

Igneous rocks can be identified and classified by their texture and their chemistry, as will be described in the following two sections of this article.


It is a universal law that fast crystallization makes small crystals and slow crystallization makes large crystals. This is because crystallization is a kinetic process: for a molecule to join onto a crystal it must bump into it and then align with it.

The thermal properties of rock are such that magma cooling underground will cool slowly as compared to lava cooling above ground. Hence, by looking at the texture of the rock, we can find out how it cooled: an intrusive rock will be coarse-grained; an extrusive rock will be fine-grained.

Sometimes lava is ejected from a volcano with such force that it goes shooting high up into the air, causing it to solidify so quickly that it doesn't have time for crystals to form at all, making an amorphous solid known as a glass. The glass in windows is an artificial glass produced by the rapid cooling of molten silica; examples of natural glasses are obsidian and pumice.

Occasionally magma will begin to cool below the surface and than be ejected on to the surface; in this case it will have a porphyritic texture, with a few larger crystals (phenocrysts) embedded in a finer-grained ground mass.


The simplest way to classify the chemistry of igneous rocks is by the amount of silica they contain.

An igneous rock with a high silica content is said to be felsic, and an igneous rock which is low in silica is said to be mafic. You will recall that these are the same terms used for high-silica and low-silica minerals; and in fact it is the case that felsic rocks will contain felsic minerals and mafic rocks will contain mafic minerals.

Classifying rocks by their silica content is convenient because typically the chemistry of igneous rocks lies on a continuum such that if you know the proportion of silica in an igneous rock, you can say what minerals it contains. The rules for doing so may be represented by the diagram below. Note that this applies only to igneous rocks, and not to sedimentary or metamorphic rocks.

To read the diagram, look along the bottom of the graph for the silica content of the rock: then a line drawn directly upwards from that point cuts through the minerals it will contain in their relative proportions. So, for example, if we tell you that a certain rock contains 50% silica, then you can see from the chart that it contains about 5% olivine, 75% pyroxene, and the remaining 20% will be calcium-rich plagioclase feldspar.

We have divided the rock types into fairly coarse divisions. It is possible to make finer distinctions: we could, for example, have put granodiorite between granite and diorite, as a rock type having a silica content lying between granite and diorite; or we could have placed dunnite to the right of peridotite, to denote those rocks which consist of pure olivine. The divisions we have proposed are, however, sufficient for our present purposes. It is more important that the reader realizes that whatever divisions we impose on the diagram, they are arbitrary: there is a continuum between felsic and ultramafic rocks.

Also, as we look along the continuum from felsic to ultramafic, the rocks are progressively denser; they have a higher melting point; and they have a less viscous flow when molten. This is the same progression as we see as we pass from felsic to ultramafic minerals, and is a natural consequence of the fact that felsic rocks consist of felsic minerals and mafic rocks of mafic minerals.

We should perhaps add a note on the presence of komatiite (extrusive ultramafic rock) in our diagram, as some textbooks omit it entirely from such diagrams. Komatiite is never observed forming today: as ultramafic magma rises from the hot interior of the Earth to its cool surface, it will fall below its melting point before it gets near to the surface, forming peridotite, komatiite's intrusive counterpart. Consequently komatiite is found only in rocks dated to over 2.5 billion years ago, consistent with geologists' belief that the Earth was hotter at that time.

Igneous structures

In the diagram below we show some of the structures formed by igneous rocks. The black represents igneous rock; the other colors represent sedimentary rocks.

Key: (1) Volcanic ash. (2) A volcano. (3) A volcanic conduit. (4) A fissure. (5) A lava flow. (6) A lacolith. (7) Dikes. (8) Sills. (9) Stocks. (10) A batholith.

As this is a two-dimensional cutaway diagram, it may be slightly misleading. The reader should bear in mind that a fissure is a crack in the surface; we have shown it end-on. Similarly, the lava flow which emerges from a fissure will be a sheet of lava; and a dike is not a spike of rock, but a vertical or near-vertical sheet of rock. And a sill, again, is a horizontal sheet of rock.

That last statement needs a little qualification. In the diagram, we have shown the layers of rock lying flat, except around the lacolith (item (6) on the diagram) and so we have shown the sills as horizontal structures. However, layers of rock can be folded by tectonic activity. When a sill intrudes into rocks like this, it intrudes between the layers of rock (this is the definition of a sill) and so will itself be contorted.

We shall have more to say about igneous structures when we consider stratigraphy and cross-cutting relationships, but for now this brief introduction is sufficient.

How do we know?

How do we know that igneous rocks are igneous? Like everything else in geology, this had to be proved at some point: indeed, there was once a body of thought known as "Neptunism" which asserted (amongst other things) that granite was sedimentary.

In the case of extrusive rocks, the answer is obvious: we can see basalt (for example) forming when lava flows cool: so it certainly can form as an extrusive rock. But it could not also form as an intrusive igneous rock, because under such circumstances, being thermally insulated, it could not cool quickly enough to produce a fine-grained structure, and the physics of the situation would dictate the formation of gabbro instead.

Since we can actually watch the formation of basalt, we can make further deductions about it. When basalt cools underwater (as observed by divers), it forms the distinctive shapes known as pillow basalt, which is never the case when it is observed forming on dry land. This criterion allows us to distinguish between basalt formed on land and on the sea floor; a deduction confirmed by the association of pillow basalt with marine sedimentary rocks.

But what about intrusive rocks? Take granite, for example, since it is the commonest intrusive igneous rock. If we are absolutely right about how it forms, we should never see it forming. So how do we know how it forms?

As a matter of fact, the fact that we never see it forming is one of the predictions of the theory that it is an intrusive igneous rock, and so tends to confirm the theory. We do not see granite or granite-like sediment forming by surface processes; what else can we conclude but that it is formed underground?

In the second place, as we have observed, granite has the same chemical composition as rhyolite, differing from it only in its texture. Now, as we know that larger crystals form when cooling is slower, and as the thermal properties of rock as opposed to air or water will lead to slower cooling underground, we must conclude that granite is exactly what we should expect to see if the magma that forms rhyolite when extruded onto the surface was to cool below the surface instead.

A close look at its texture through a microscope confirms the igneous nature of its formation. The picture below is a photomicrograph of granite. Note how the crystals, however bizarre their shape, fit together perfectly. We may compare this with the texture of sedimentary rocks such as sandstone, which are clearly made of non-interlocking particles cemented together.

Then we may consider the structures formed by intrusive rocks. It is difficult to see how something such as a dike, which, as explained above, is a vertical or near-vertical sheet of rock, could form by any process except the intrusion of magma into a crack in pre-existing rocks.

Finally, we may note that the rocks into which granite intrudes are typically changed in ways we would expect if they had been subjected to great heat; for example, when granite intrudes through a layer of limestone, the limestone immediately adjacent to the granite will be turned to marble. This suggests that the granite was itself once at a high temperature and has subsequently cooled, consistent with the theory that it is an intrusive igneous rock.

For these reasons, we may conclude that granite is an intrusive igneous rock; similar remarks might be made about the other rocks classified as igneous intrusive.

Note on vocabulary

Igneous rocks are sometimes called primary rocks, extrusive rocks are sometimes called volcanic rocks, and intrusive rocks are sometimes called plutonic rocks. We shall not use these terms in this text, and mention this only for the benefit of those readers who wish to pursue a course of further reading.

The rocks that we have described as fine-grained and coarse-grained are also known by the terms aphanitic and phaneritic respectively. These terms are rather commonly used by geologists, but I shall stick to the more self-explanatory terms.

Finally, just as silicate minerals are sometimes referred to (erroneously) as "acidic", "basic" and "ultrabasic" rather than felsic, mafic, and ultramafic, the same is true of igneous rocks; again, I do not intend to use these terms, as they are obsolete and misleading.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Replies to this message:
 Message 45 by Jazzns, posted 10-14-2011 10:59 AM Dr Adequate has responded
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Dr Adequate
Posts: 16112
Joined: 07-20-2006

Message 48 of 294 (637307)
10-14-2011 6:40 PM
Reply to: Message 45 by Jazzns
10-14-2011 10:59 AM

Re: Igneous Rocks
Is that a typo? Did you mean batholith?


Also, since you seem to be okay with mentioning other vocabulary, aren't batholiths also called plutons sometimes? I seem to remember that it may just be when they are small they are called plutons but it has been awhile.

Well, it's puzzling. WP says: "A pluton in geology is a body of intrusive igneous rock (called a plutonic rock) that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies." That makes sense, in that case a pluton is anything formed from plutonic rock. But then it goes on: "In practice, "pluton" usually refers to a distinctive mass of igneous rock, typically several kilometers in dimension, without a tabular shape like those of dikes and sills."

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Dr Adequate
Posts: 16112
Joined: 07-20-2006

Message 49 of 294 (637313)
10-14-2011 7:43 PM
Reply to: Message 47 by Pressie
10-14-2011 4:10 PM

We don’t even have to infer. We can see them forming today. Same characteristics.

That's what makes the inference particularly easy. It's an inference on the level of "If it looks like a duck, and it quacks like a duck, it's a duck". But it is still an inference, and what the inference is about is how the sediment of which the rock consists was laid down in the past.

No, it is certainly not historical. We can see it happening today. Look for the same characteristics..voila. We know exactly what happened for those rocks to be deposited.

And the word "deposited" is in what tense?

Geology follows the scientific method. Stamp collecting follows what looks good for making money.

What I meant was, geologists don't merely collect and classify. They have a theory. This theory has predictive and explanatory power.


Take aeolian sandstone as an example.

Theory: such-and-such sedimentary structures are aeolian.

Prediction: when we find fresh sediment having these structures, it will be on dry land; when we watch it forming in real time we shall see the wind forming the structures.

Observation: consistent with the prediction.

Explanatory inference: when we find these structures in sandstone, we should infer that it was formed in the same way.

Another prediction: hence if we find sandstone with these structures, then if we find fossils in it they should be terrestrial and not marine.

Observation: We do, hurrah!

Explanatory inference: but if we find no fossils, we should come to the same conclusion, since the theory works every time we can test its predictions.

It may be bleedin' obvious, but it is still an example of a scientific theory which we establish as true by testing its predictions and from which we can then make inferences which rest on the theory rather than on direct observation.

Now it is precisely this aspect of geology which I wish to emphasize.

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