Something that has not been mentioned are intrusions. Intrusions are very much a part of geologic columns worldwide.
One can often determine whether the country rock (e.g., sedimentary rock) was lithified or not, was saturated with water or not, how deep the rocks were, etc., when intruded simply by examining the mineralogy and their associated textures, as well as the cross-cutting relationships.
Simply put, the priciple of cross-cutting relationships says that younger rocks cut older rocks, either via intrusion, faulting, or erosion; and by studying these relationships, we can extract depositional and intrusive histories of an area. Additionally, this principle is used to study vein systems, dike swarms, alteration history, etc.
Basically then, igneous rocks, because they intrude other rocks (i.e., country rocks), are therefore younger than the country rocks.
However, igneous intrusions can evolve over a period of time through fractionation, these magmatic changes can be reflected in associated and chemically-related dike swarm systems. The earliest magmatic composition might be more mafic rich and dikes reflect this. Younger and younger dikes might become progressively silica rich and again, dikes can reflect this. The oldest mafic dikes cut only country rock. The youngest silica-rich dikes cut everything else including dikes that are more mafic in composition.
How can one tell if unconsolidated and/or water-saturated sediments were intruded?
Peperites and hyaloclastites.
Peperites are rocks with brecciated textures that are the result of interaction between hot igneous material and wet or dry sedimentary material (which can include volcaniclastic sediments as well).
Hyaloclastites are formed as a result of magma intruding and/or lava flowing into ice, water, or water-saturated sediments and the subsequent formation and shattering of volcanic glass into smaller fragments.
Long time' is suggested and strongly supported by these relationships, because as I stated above, one can determine whether the rocks were 'soft' or 'hard' when intruded, when faulted, or when eroded. Additionally, the igneous rocks themselves (though not necessarily the dikes) exhibit evidence of slow cooling.
This message has been edited by roxrkool, 03-31-2005 01:05 PM
Slow in the sense that it can take several thousand to perhaps as much as several million years for a large intrusion to cool. I think it's entirely plausible, however, for large intrusions to cool more 'quickly' than previously thought, but even so, the time scale would still be several tens to several hundred thousand years. Mainstream geoscientists have a mountain of data to support this.
In addition to cooling rates, we also have to consider the amount of time required to emplace these large igneous bodies. Rapid emplacement (much higher pressures) affects the surrounding country much differently and would form different minerals, than gradual emplacement.
Rapid emplacement and cooling at the scale YECs require would effectively pulverize the crust and quench the magma. Quenched magma plutons would not exhibit exsolved mineral textures, would not contain large crystals, and would not result in massive hydrothermal systems like the world class deposits at Butte, MT, the Motherlode in California, or the Carline Trend, or hundreds of others scatter across the globe.
And neither would quenching result in layered intrusions (generally enriched in platinum group elements, gold, copper, nickel) such as the massive Bushveld Igneous Complex (S. Africa), Dufek Complex (Antartica), the Stillwater Complex (Montana), the Great Dyke (Zimbabwe).
Layered intrusions are the result of prolonged fractionation of a mafic/ultramafic parent magma (along with few to numerous subsequent magma injections into the same chamber) to a felsic magma. These fractionation trends and magmatic injections leave traces in the mineralogy, mineralogical textures, and geochmemistry of the complexes.
This differentiation pattern is clearly visibly in the pattern of rocks which are layered just like sedimentary rocks. The most mafic rocks occur at the bottom of the chamber, which are generally more enriched in Mg, Fe, Ca, as well as in Cu- and Ni-sulfides. With stratigraphic height, we get gabbroic rocks in the middle with felsic rocks up top.
The image below is an idealized cross-section through a layered igneous intrusion showing lithologic zones associated with mineral deposits.
This message has been edited by roxrkool, 03-31-2005 01:10 PM
For the most part, it would be safe to say igneous rocks with larger crystals likely cooled more slowly than igneous rocks with small crystals, but really, that sort of assertion needs to be backed up by other evidence as well. Many YECs like to bring up pegmatites to refute the large xls = slow cooling statement.
Pegmatite melts are exceedingly rich in incompatible elements, such as the rare earths, and they result from the residua of granitic/silicate melts. They typically contain the same mineral species as granite (e.g., K-spar, quartz), but in addition, are water-rich and gaseous. These two things directly affect crystallization rates as well as mineralogy. However, I would be hard-pressed to believe an 18 meter beryl can form in 2,000 - 3,000 years... The Largest Crystals - American Mineralogist
This message has been edited by roxrkool, 03-30-2005 05:46 PM
Yep. Under the right conditions, large crystals can form in 'short' periods of time. But like you said, any discussion of rapidly cooled granitic or gabbroic melts would require a whole lot more research and supporting evidence than "I don't see why it would take that long" or "Labs can form large crystals from silicate melts in a short time, so all melts must be able to cool rapidly and form large crystals as well."
Re: Need a new "Igneous and Metamorphic Rocks" topic
I don't know if I agree, Moose. I don't think met/ig rocks are remote to this theme. Unless of course this topic was intended to be strictly a discussion on sedimentary rocks in geologic columns. If so, that should probably be made more clear in the thread title.
The modus operandi of the YECs when discussing geology is to divorce all geologic features from their surroundings and then extrapolate a plausible means of Noachic formation for that one feature. Hell, most times it doesn't even have to be that plaussible.
Geos know that a limestone, if you follow it out far enough, will many times interfinger or grade into shale, or sandstone, or various other marine, near-shore, or shore facies rocks. And these relationships tell us what was happening at the time - be it marine transgression or or regression.
YECs will only consider the limestone - and only the limestone that does not abut against a cross-bedded sandstone with lizard tracks on it. They don't discuss what happens to that limestone laterally because that would entail explaining why that limestone becomes sandy and contains tetrapod dinosaur tracks and pterosaur fossils.
If you think most of the geologic column is composed solely of sedimentary rocks with metamorphic/igneous rocks only occurring at the base of the column (e.g., Grand Granyon), then of course it's quite easy for them to say the met/ig rocks are preflood and the sedimentary rocks are syn- and post-flood. Well, as we've found out, it's pretty difficult to refute this interpretation - especially if the YEC knows (or thinks they know) even the tiniest bit about geology. They know the buzzwords, but not much more and they're willing to ignore any mention that disputes their perfectly plausible explanation.
How do you convince a YEC (layperson) that geologists can tell if a sandstone or limestone was eroded after lithification? Geologists can use their handlens and look closely at an erosional surface and see how the individual quartz or calcite grains on that surface are flattened - like teeth after years of chewing gritty food, they are worn down. If the sand was unconsolidated at the time of erosion, the grains would simply move about, but sand or calcite is trapped by cement and can't move; it is therefore subjected to abrasion. I can explain it a hundred times, but if I don't have a photo to show people, they can continue denying the fact.
. However, what happens if you ask a YEC to explain/interpret large igneous provinces? Portions of the globe where igneous rocks cover thousands of square kilometers?
The massive Siberian Traps cover an area of 2.6 million km2 and temporally/stratigraphically coincide with the largest mass extinction in the history of the planet - the Permian extinction. These basaltic flows are stratigraphically located in rocks that, according to YECs, should have been erupted during the Noachic flood. And yet, no pillow basalts are present to suggest deposition in water. No chemical analyses of contemporaneous marine deposits indicate a massive influx of volcanic gases into the ocean. Textures of igneous-water interaction should be abundant, but they are not.
YECs can get away with assuming all sediment was deposited by flood water, but they can't when it comes to metamorphic and igneous rocks. Those are much more difficult to explain in the context of a Noahic flood. And that's why I think it's vitally important to mention metamorphic and igneous rocks in any discussion concerning geologic columns.
This message has been edited by roxrkool, 03-31-2005 03:34 PM
Re: Need a new "Igneous and Metamorphic Rocks" topic
Is it possible for magma to intrude into sediments that are not lithified?
Yes. I refer you to Message 8 for some rocks and associated textures of 1) basaltic material mixing with subaerial unconsolidated sediments; and 2) magmatic intrusion into unlithified oceanic sediments off the coast of Hawaii.
The Ocean Drilling Project is a wonderful resource for the types of alteration associated with magmatic intrusion of unconsolidated sediments.
Would lithified and unlithified sediments yield the same metamorphic features when exposed to magma?
I might have to get back to this one, but in a nutshell, yes and no. :)
The alteration and metamorphic effects of igneous intrusion into lithified vs. non-lithified material is dependent on many variables, such as: presence of water in the intruded material (marine seds vs. terrestrial seds), the source of the water itself (marine vs. magmatic vs. meteoric), the pre-existing mineral suite (100% quartz vs. limestone), chemistry of the igneous body and how much water it contains (wet vs. dry intrusives), duration of thermal activity (a dike vs. a laccolith), and so on.
For the most part, the unconsolidated equivalents of lithified rocks will generally form similar minerals because of similar chemistry, but the variables will determine the end result (different minerals and textures, alteration suites and halos, etc.).