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

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.

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".

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.

That is where data collection comes into play. We literally map rock bodies in three dimensions. We can actually see their shapes in space. We can see the paleotopography of a valley, the piles of glacial till, the lake beds and the great fans of river delta silts and sand channels.I once modeled a hill and realized that it was really a lake back in the Miocene. In fact, I'm certain that it had hot springs feeding it. Others tried to model it as a big featureless blob, but I showed that the whole environment made sense and controlled mineralization.Hey, it isn't easy and there's a lot of uncertainty that some people can't deal with, but as Pressie says, it all works.

Well, I have to say that learning from books and lectures is one thing, but there is nothing like seeing the field relationships.

That sums it up pretty well. Clastic rock definition is mostly based on grain size. That's were it gets really technical. In order of decreasing grain size:Gravel ---> conglomerate
Sand ----> sandstone
Silt ----> siltstone
Mud ----> mudstone
Clay ----> claystoneYeah, I know, it's tough.We can make it more difficult by giving them modifiers like 'calcareous' or 'organic'; or we can combine the terms to things like 'sandy carbonaceous mudstone' (which may not actually exist, but you get the idea).Hey, people spend careers studying this stuff.And we haven't gotten into carbonates or igneous rocks, etc. ... The most definitive way would be microscopically, but in the field one gets a feel for it. Basically, if you can see the grains you are dealing with siltstone or coarser.

Either chemical weathering to smaller mineral grains, or abrasion during transport. Shale is the traditional term for a mudstone. Here is the Wentworth scale for determining rock type: The different gravel sizes become conglomerates.Sands are self-explanatory.Muds include clay and silt in this diagram. In my work, I reserved claystone for very soft material that had no silt-sized grains in it. Slate is a low-grad metamorphic rock derived from mudstones. They usually have a very strong cleavage (finely space parallel partings).

Yes, shale is defined as a mudstone with a lot of irregular partings due to bedding.

Planes along which a material can more easily separate.Irregular means not perfectly flat.

Yes, but 'small steps' ...Basically, the greater the distance of transport, the finer the grain size and rounding of rock fragments. But then, some materials will break down faster, so be careful.Sediments will sort in a water medium according to grain size with finer materials being suspended longer (carried farther out to sea) or settling out last in a still column of water. Yes, however, the breakdown of rock to producing clastic sediments is kind of the opposite to lithification.

Just think of talc, eh?

Although I can think of exceptions, this is kind of a contradiction of terms. As I said, if you want to get into details, I can think of something, I'm sure. The point is that for a general discussion like this, it would be confusing. Almost by definition, although some kinds of alteration can occur at depth.

Basically, piles of shells or fossilized organic material. Outside of limestone (including chalk)? How about coral reefs?Or coal? (actually, according to strict definition, coal is not a rock).Or radiolarite? (Radiolarite - Wikipedia)Or phosphorite? (Phosphorite - Wikipedia)I'm sure that I'm missing some more ...

Essentially, yes. As in clastic rocks, yes.Edited by Adminnemooseus, : Fix 1 quote box.

The short answer is 'from fossils and rock textures'. Fossils are the main clue, but sometimes limestones are so fine-grained that you can't tell without a microscope and even then it's just a matter of naming it by grain size.Carbonate geology is a whole separate area of the science. If Petrophysics were here, we could get a better idea... Highly reduced, carbonaceous stuff, like coal ...