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

Thanks.

So let's return to the question asked in Message 42.

quote:

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So from what has been said lithification is basically compaction and cementation.

From that I would guess that the deeper something is buried the greater the compaction. Also the longer something is buried the greater the compaction and cementation.

For cementation though I assume what is needed would be water and minerals leached from the overburden. Is that correct?

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So it seems that lithification can produce hard or soft rocks, cemented or compressed rocks, rocks that are compressed and cemented, rocks that can be dissolved by different chemicals and that even within one grouping (mudstones as an example) we can see a vast range of physical characteristics but in every case the resulting product is a direct indicator of how that sample was formed.

Is that correct?

Next, it also seems that before any sedimentary rocks are formed there must exist the primary (or in many cases secondary or tertiary) sources of material and the earlier materials must first be weathered and eroded and transported to the location where they are found. (Is this the Party to whom I am speaking?)

Can someone explain the mechanisms for formation of a couple of the biological rock types for me?

First limestone.

I have heard that limestone can be formed is several ways, by accumulation of shell or coral and by direct precipitation or evaporation.

How can geologists tell which process was the primary one in a given sample?

What about accumulations of diatoms? What do they produce?

The send big question I have is what different types of rocks get produced by buried marshes and wetlands?

It seems that coal is often found between layers of sandstone or limestone or mudstone or shale. Why is that the case?

So it would be another indicator of environments and landscapes changing over time with one landscape being buried and replaced at the surface by a different landscape?

Edited by jar, : there's a "d" in lanscapes

So are there different types of coal and if so what does each type tell us about the source and process that produced the end product?

edge writes: ... from peat to lignite, to bituminous to anthracite and eventually to graphite.

And these different types are recognizable and each results from a different history and process?

Do the different types point to different initial origins or only to the formation process?

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?

Is part of the answer related to where in a series of processes the sample has completed?

Pressie writes: 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.

Okay, but then can you walk us through the cycle from some surface landscape to each of the resulting coal types?

Great information.

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. All begin as marshy wetlands that remain marshy wetlands long enough for deep deposits of dead plant material to accumulate?

And for the other Spy, can we say chalk results from long term deposition of dead micro-organisms with calcite shells in moderately deep water while other forms of limestone result from different original environments?

So when the various types of coal that have been mentioned (peat to lignite, to bituminous to anthracite and eventually to graphite) so far are being considered...

Pressie writes: 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.

... 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?

I'd like to take a short side trip simply to satisfy a question that has haunted me since it was first brought to my attention many many moons ago.

The Rock of Gibraltar is a massive limestone deposit but strangely it seems to be upside down with the oldest rocks at the top and the youngest near the bottom.

?????????????

What is that story? How was that determined?

But how was it determined that things were overturned?

So there really are specific indicators that will show when a portion of a geological column has been disturbed, even turned upside down.

still a few questions.

In your explanations there are just so many things still open.

Pressie writes: 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.

That begs the question "What is different, the classification system, the actual coal, the power plants, all three?"

Pressie writes: 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.

Okay, next step.

You added another qualifier, "economically exploitable", and while I realize that has significance when it comes to commercial applications, we are hopefully just dealing with the actual geology in this thread. I also understand that most of the data available will come from those sites that were economically exploitable and that is expected but there is also likely an similar body of data on sites where it was not economically exploitable.

Let's step away from the economic considerations.

The general designations of coal were listed as peat to lignite, to bituminous to anthracite and eventually to graphite.

Can you give us a short physical description of how someone identifies each of those in the field?