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

So you are telling us that sometimes more than one process contributes to create a final sample? Is that correct?

Sure, mechanical and chemical processes can obviously both operate on the same lump of rock. Often one will predominate: if, for example, a lump of granite is buried in the humid acidic flood of a rainforest, then that's going to be pretty much all chemical weathering. A cliff face will get pretty much all mechanical weathering and erosion.

And would the form of things like mica and asbestos have similar origins?

Yeah, mica is a sheet silicate, which explains why mica comes in sheets.

I don't know much about asbestos, but wikipedia says that it can be formed from amphibole minerals, where the silicates form a double chain, which would explain its fibrous consistency; or from serpentine, which is a famly of minerals that can take on different crystal habits, but presumably the ones you make asbestos out of have chains or double chains.

Okay but jargon alert. Help please. What does irregular partings mean and what does bedding mean?

Planes along which a material can more easily separate.

Irregular means not perfectly flat.

Anyway, I was thinking that since the topic is "How do geologists know..." the topics of grain shape (or roundness) and sorting should be covered, since those are major factors that help determine deposition environment.

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.

Also you mentioned chemical and mechanical weathering in the context of the production of the sediment; but don't those processes also occur during and after lithification? And don't those characteristics give us clues as to the depositional environment?

Yes, however, the breakdown of rock to producing clastic sediments is kind of the opposite to lithification.

For clastic sedimentary rocks to form there first need to be other rocks and those prior rocks need to get weathered and eroded into small pieces parts. The final material will be determined by what the original material was and particles size as well as crystalline structure. Some materials will also change form should they undergo metamorphose.

Back in Message 6 edge mentioned a second family of processes, chemical sediments that precipitate out of water such as chert, or evaporites, or travertine, or various types of iron formations. I imagine that for things to precipitate out they must first be suspended in water and so once again they must exist before they can be precipitated out.

Is that correct?

What is chert and travertine.

Would limestone and chalk fall in the clastic category?

Are the mineral salts examples of precipitates?

AbE:

edge's reply while I was writing this adds another question.

What is the difference between lithification and metamorphism.

Edited by jar, : see AbE:

What is chert and travertine.

Travertine is calcium carbonate deposited by hot springs.

Chert is any fine-grained quartz, however deposited; hot springs can deposit it, but there are other ways.

Would limestone and chalk fall in the clastic category?

A clast by definition is a bit broken off a rock, so in general no, limestone (of which chalk is a subset) is formed from calcareous parts of organisms and so is a biochemical sedimentary rock.

Of course, you can have rocks made of clasts of limestone, but they had to be produced by mechanical weathering and erosion of limestone, which had to be produced biochemically. Here's some limestone breccia (breccia because the clasts are big and unrounded).

Are the mineral salts examples of precipitates?

Yes.

What is the difference between lithification and metamorphism.

Lithification is the process of turning into stone; metamorphosis changes the properties of a stone.

DrA writes: Lithification is the process of turning into stone; metamorphosis changes the properties of a stone.

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

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

And indeed the exact process. Metamorphosis has different results depending on the degree of temperature and pressure. (For example, I mentioned foliation above: this only happens when there is pressure, heat alone won't do it.) Or in sedimentation, to take a simple example, mudstone is produced by compaction by the weight of the overlying sediment/rock. Nw there is in fact continuum from loose wet mud to the hardest mudstone, and how hard it is will depend on how much it has been compacted.

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.

Edited by Pressie, : No reason given.

Edited by Pressie, : No reason given.

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.

Edited by Pressie, : No reason given.

edge writes: Also you mentioned chemical and mechanical weathering in the context of the production of the sediment; but don't those processes also occur during and after lithification? And don't those characteristics give us clues as to the depositional environment? Yes, however, the breakdown of rock to producing clastic sediments is kind of the opposite to lithification.

I guess I didn't understand the question. About chemical and mechanical weathering occurring during lithification, since lithification occurs after burial, how could there be any weathering? And about after lithification, can't weathering only occur after exposure?

--Percy

Back in Message 1 edge mentions a third type of sedimentary rocks and that was accumulations of biological materials such as coral reefs coal and other types of bioherms.

What are bioherms and how are they identified?

What are other examples of biological material rocks?

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?

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.

Just think of talc, eh?

I guess I didn't understand the question. About chemical and mechanical weathering occurring during lithification, ...

Although I can think of exceptions, this is kind of a contradiction of terms.

... since lithification occurs after burial, how could there be any weathering?

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.

And about after lithification, can't weathering only occur after exposure?

Almost by definition, although some kinds of alteration can occur at depth.

Back in Message 1 edge mentions a third type of sedimentary rocks and that was accumulations of biological materials such as coral reefs coal and other types of bioherms. What are bioherms and how are they identified?

Basically, piles of shells or fossilized organic material.

What are other examples of biological material rocks?

Outside of limestone (including chalk)? How about coral reefs?

Or coal? (actually, according to strict definition, coal is not a rock).

Or radiolarite? (https://en.wikipedia.org/wiki/Radiolarite)

Or phosphorite? (https://en.wikipedia.org/wiki/Phosphorite)

I'm sure that I'm missing some more ...