How do geologist know what they are looking at really is what they say it is?

jar writes: To begin, are all sedimentary rocks produced the same way?

No. Not at all. For example; ashes resulting from volcanoes may fall into a lake where they, amongst others, get deposited on the bottom by lacustrine processes. On the beaches around the lakes those same ashes would result in different phenomena. Now, do you call the ash falling on the beaches sedimentary or volcanic? The boundary between sedimentary and volcanic is not clear; there's a huge overlap. It's not either black or white; it's black and/ or white with huge grey areas in between.

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Your question was: "How do geologists know what they are looking at really is what they say"?

Economic geology. Old earth models work. In my case, the coal seams found in northern part of the Witbank Coalfield are very consistent with being deposited on deltas resulting from rivers flowing out of U-formed glacial valleys into a lake. And then the different types of vegetation forming the coal deposits and coal seams. And evidence for shore transgression and regression. Hence the different coal seams.

All those exploration and mining companies follow so-called 'uniformatism'. Every single one of them. Old earth models work. And they change their exploration and mining plans according to old earth models. Old earth models work.

If anyone has a better way of doing it, please demonstrate by putting their money where their mouths are.

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jar writes: How do geologists really know what a sample is and how it came to be?

To me a sample basically is what you get from the field. On surface rocks you get a sample from the surface and you can study the weathering and surface chemistry and all that. For a representative sample of igneous rocks I hit the bejesus out of the surface and try to get a more representative sample away from the surface. For sedimentary rocks I drill a hole to try to get to the heart of it.

But then, you must also realise that in labs, we can melt rocks and see how they crystallise under different P/T and fluid conditions and we can metamorphasise rocks under different P/T conditions and fluids. And we get samples of those, too...

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Percy writes: This would be interesting to understand better. What is it you find that tells you it was "deltas resulting from rivers flowing out of U-formed glacial valleys into a lake"

Drill through it. Produce a map of the bottom of the delta and the surrounding area.

The valleys at the bottom look very similar to glacial valleys. U-formed. And then the drumlins and striations and all that. Those deposits (the Ecca Group) started being deposited above glacial deposits in U-forming valleys, continuing deposits spilling over into deltas into a relatively shallow body of water. Then we can test for whether the lake was salty or fresh. Rocks forming from salt water have lots of different isotopes than those forming in salt water. Just like we find in deltas today.

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jar writes: Thanks. So neither term actually says how hard the stone will be or anything else specific unless the exact materials are also specified?

Serpentinite, a very common mineral, is metamorphic, but also very soft. So, metamorphism is not really a good indicator of how hard a rock is.

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Yes, the Dwyka tillites (Late Carboniferous) towards the south west of Jo'burg consist, amongst others, of clays, sands, gravels and boulders obtained from the underlying Malmani Dolomites of the Transvaal Sequence (Palaeoproterozoic in age).

jar writes: And these different types are recognizable...

Yes. Ultimate and proximate analyses do it very efficiently. Classification.

jar writes: ... and each results from a different history and process?

Yes and no.

jar writes: Do the different types point to different initial origins ...

Yes and no.

jar writes: ...or only to the formation process?

No. Lots of different processes.

I don't really understand why anyone would think that there's only one process involved in the formation of any rock.

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jar writes: If we look at the different "coal" types as presented (peat to lignite, to bituminous to anthracite and eventually to graphite), what would be the original landscape that resulted in each final product and that process paths are involved in each?

"The" original landscape? There's not one 'original' landscape. Lots of different landscapes.

jar writes: So in the Mad tradition of spy vs spy can we make a general statement that the various forms of coal products are more the result of processing than original source.

Not really. The processes are similar, but the original sources provide different products. In some circumstances leaves tend to be preserved better than branches to form coals later even undergoing similar processes.

In other circumstances branches tend to be preserved to form coal later. They produce different coal products.

Then there's also the bark and stuff like that; which also play a role.

Then also the species of plants growing, dying and getting preserved also play a role. Then depth of burial and modes of preservation and proximity to inflowing inorganic sources. etc. That's why Gondwana coals (Africa, Australia, India) differ so much in everything from northern hemisphere coals. Different plants, different time periods, different outcomes eventually.

Similar processes, though.

Gondwana anthracites have completely different properties than northern hemisphere anthracites, for example. The processes are similar, yet the original sources differ.

jar writes: ... which products or product would get produced in each situation; or. are you saying the the characteristics of bituminous coal (as one example) is different in the situations you mention. Is it a matter of specific chemical composition varying by source material?

Not really. Yes and no. It's complicated. You have to realise the official global classification system is based on northern hemisphere coals. A, B, C, etc. and how they behave when burnt in power stations. In Europe and America power plants are built around that northern hemisphere coal classification system.

Southern hemisphere coals have different properties. What is classified as a bitumionous coal in the northern hemisphere has similar properties to a sub-bituminous coal in the southern hemisphere (and India). It's not a matter of specific properties, it's basically (more or-less) based on how the coals 'behave' in power stations.

So, it's not an easy answer.

In Europe and Russia and the US, local coals classified as bituminousus are the best to provide the most efficient power in those power stations. In Africa and Australia and South America and India the coals classified as sub-bituminous are the most efficient. One of the reasons is that northern hemisphere coals were formed from different plant material from a slightly different period than those formed in what is now Southern hemisphere coal.

So, it's not an easy answer!

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Actually, all of them.

But, you have to realise that the classification systems in the northern hemisphere work for most northern hemisphere coals (and some southern hemisphere coals). In the southern hemisphere coals the official classification system more-or-less works for a lot of southern hemisphere coals (not all of them, though). Nothing ever is either black or white, lots of processes involved inbetween.

What is very obvious for all those economically exploitable coals, though, is that all those coals were formed by different natural processes over lots of time. Not one method. Lots of different methods.

The present is the key to the past.

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It's all subjective in the field. It comes from experience and a hand held magnifiers.

Peat basically is derived from plant material still recognisable as plant material by the open eye.

Lignite is usually brown coal that can be seen by the naked eye as originating from plant material.

Sub-bituminous coals are black with a dull luster.

Bituminous coals are black with a bright luster.

Anthacites are black and have a very bright luster and have a lower relative density than bituminous coals.

Graphites are black with a dull luster and crumbling like a clot of damp sand.

These are general rules and don't always apply everywhere. Most Gondwana coals have bands of bright and dull coal. As always it's not always either black or white, but somewhere in between.

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Yes, -"The best geologist is he who has seen the most rocks."- Herbert Harold Read

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