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Author Topic:   How do geologist know what they are looking at really is what they say it is?
Dr Adequate
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Message 31 of 88 (790340)
08-29-2016 3:31 PM
Reply to: Message 29 by jar
08-29-2016 12:54 PM


Re: great stuff so far, now three sheets to the wind.
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.


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edge
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Joined: 01-09-2002
Member Rating: 2.7


Message 32 of 88 (790343)
08-29-2016 3:51 PM
Reply to: Message 24 by jar
08-29-2016 11:56 AM


Re: Great so far but I'm slow so humor me by expanding some.
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.


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edge
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Joined: 01-09-2002
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Message 33 of 88 (790348)
08-29-2016 4:01 PM
Reply to: Message 30 by herebedragons
08-29-2016 1:06 PM


Re: Great so far but I'm slow so humor me by expanding some.
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.
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jar
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Message 34 of 88 (790351)
08-29-2016 4:16 PM


so short summary so far.
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:


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Dr Adequate
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Member Rating: 4.2


Message 35 of 88 (790373)
08-29-2016 6:32 PM
Reply to: Message 34 by jar
08-29-2016 4:16 PM


Re: so short summary so far.
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.


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jar
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Member Rating: 1.7


Message 36 of 88 (790374)
08-29-2016 6:46 PM
Reply to: Message 35 by Dr Adequate
08-29-2016 6:32 PM


Re: so short summary so far.
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?


My Sister's Website: Rose Hill Studios     My Website: My Website

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Dr Adequate
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Posts: 15984
Joined: 07-20-2006
Member Rating: 4.2


Message 37 of 88 (790375)
08-29-2016 7:23 PM
Reply to: Message 36 by jar
08-29-2016 6:46 PM


Re: so short summary so far.
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.


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Pressie
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Joined: 06-18-2010
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Message 38 of 88 (790391)
08-30-2016 6:27 AM
Reply to: Message 14 by Percy
08-29-2016 7:56 AM


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.


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Pressie
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Posts: 1851
From: Pretoria, SA
Joined: 06-18-2010
Member Rating: 2.2


Message 39 of 88 (790393)
08-30-2016 7:07 AM
Reply to: Message 36 by jar
08-29-2016 6:46 PM


Re: so short summary so far.
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.


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Percy
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From: New Hampshire
Joined: 12-23-2000
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Message 40 of 88 (790398)
08-30-2016 7:49 AM
Reply to: Message 33 by edge
08-29-2016 4:01 PM


Re: Great so far but I'm slow so humor me by expanding some.
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


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jar
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Posts: 29747
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.7


Message 41 of 88 (790448)
08-30-2016 11:50 AM


What are examples of biological rocks and how are they identified?
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?


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jar
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Posts: 29747
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.7


Message 42 of 88 (790525)
08-31-2016 9:56 AM
Reply to: Message 37 by Dr Adequate
08-29-2016 7:23 PM


Return to lithification for a moment.
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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edge
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Posts: 4002
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 2.7


Message 43 of 88 (790557)
08-31-2016 6:30 PM
Reply to: Message 39 by Pressie
08-30-2016 7:07 AM


Re: so short summary so far.
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?
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edge
Member
Posts: 4002
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 2.7


Message 44 of 88 (790558)
08-31-2016 6:35 PM
Reply to: Message 40 by Percy
08-30-2016 7:49 AM


Re: Great so far but I'm slow so humor me by expanding some.
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.
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edge
Member
Posts: 4002
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 2.7


Message 45 of 88 (790559)
08-31-2016 6:43 PM
Reply to: Message 41 by jar
08-30-2016 11:50 AM


Re: What are examples of biological rocks and how are they identified?
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 ...


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