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

I would like to suggest are slow baby step at a time look at geology.How do geologists really know what a sample is and how it came to be?Too often we seem to see very general terms used that then get qualified as a different term but without an explanation of what the differences are and why they are made. I'd like to see if by my asking question those good folk who actually know what they are talking about can 'splain it to me.Hopefully we can move slowly enough to avoid gross generalities but still keep things simply enough that even I can understand them.The discussion should move from general terms like sedimentary rock to more specific points like the basic methods sedimentary rocks get produced; to how specific designations are identified and what original materials produce specific final types of rock.First I like to explore just sedimentary rocks before attempting to identify igneous or metamorphic rocks.To begin, are all sedimentary rocks produced the same way?Please, let's go slow and until I understand something put off new questions.Geology & the Great Flood maybe?

Thread copied here from the How do geologist know what they are looking at really is what they say it is? thread in the Proposed New Topics forum.

Hi jar. I wanted to take this "class" with you---as I never understood Geology that well. Maybe I can learn something along the way. I was curious as to your question regarding sedimentary rocks.I started reading this article on Sedimentary Rocks from a Tulane University Geology Teacher, but I have never studied Geology before so I wanted some of our EvC folks to help explain it to me better.

It all starts with some assumptions. The first one is 'uniformitarianism'. This principle was formulated by James Hutton the Scottish farmer, physician, and geologist. He was the first to realize that processes going on today could produce the effects that we see in the geological record and have been going on for a very long time.While our idea of uniformitarianism has changed with time, it is an assumption that has worked well and allowed us to construct a meaningful geologic history for much of the earth's existence. Otherwise the pattern of rocks and fossils and minerals would make no sense.No one has overturned the modern view of uniformitarianism.Of course, some people have a hard time with this principle because the past cannot be directly observed. However, they can offer nothing in its place as far as interpretation of the geological record is concerned.I'm going to leave this subject for discussion rather than get into the ramifications of uniformitarianism. Let's just say that it was based on Hutton's observations during his experience in Great Britain and is applicable to the Phanerozoic sedimentary record.

...specifically are all sedimentary rocks produced the same way?

Not at all.The kind we are familiar with is clastic sedimentation, meaning it consists of small rock and mineral fragments that have been eroded and transported to a depositional center.There are also chemical sediments that precipitate out of water such as chert, or evaporites, or travertine, or various types of iron formations.Some others are accumulations of biological materials such as coral reefs coal and other types of bioherms.And that's only dealing with composition or source. There are a number of physical modes of deposition as well such as hot springs, turbidites, mudflows, deltas, clastic fans, eolian, etc., etc.And then you can get into the transition between volcanic and sedimentary deposits.It's not rocket science, but it can be complex, kind of difficult to explain in a forum such as this.I offer all of this just by way of support for my initial statement: "No".

How can I tell if a sample is clastic instead of chemical or biological?

You would look at the grains that make up the rock. Clastic rocks would consist of rock and mineral fragments from older rocks.Biogenic rocks would be mostly composed of recognizable fragments of organic remains, usually carbonate or silica.There is also a gray area where biological remains can be transported and deposited as a clastic rock. It is also very common to have some amount of clastic material mixed in with the biological carbonate.

I have several question but the first is about "rock and mineral fragments from older rocks".From that I get a sense that the first thing needed is older rocks of most any kind that get weathered and eroded and that material forms the basic components of clastic sedimentary rocks.Is that correct?

Yes. You might think, well, why can't you start with, for example, mud? But you can't get mud without starting with a rock, there's no process that just produces mud de novo.

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.Edited by Pressie, : No reason given.

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.Edited by Pressie, : No reason given.

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...Edited by Pressie, : No reason given.Edited by Pressie, : No reason given.Edited by Pressie, : No reason given.

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"?--PercyEdited by Percy, : Grammar.

Before we move on to the biological sedimentary rocks can we spend a little more time at clastics? I know I'm slow but I still have a few more questions about identifying clastics before we move on to chemical and biological sedimentary rocks.Hopefully we will get there though.