Understanding through Discussion


Welcome! You are not logged in. [ Login ]
EvC Forum active members: 63 (9072 total)
64 online now:
AZPaul3, dwise1, Minnemooseus (Adminnemooseus) (3 members, 61 visitors)
Newest Member: FossilDiscovery
Post Volume: Total: 893,178 Year: 4,290/6,534 Month: 504/900 Week: 28/182 Day: 16/12 Hour: 0/2


Thread  Details

Email This Thread
Newer Topic | Older Topic
  
Author Topic:   How do geologist know what they are looking at really is what they say it is?
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


Message 10 of 88 (790280)
08-29-2016 1:26 AM
Reply to: Message 9 by jar
08-28-2016 10:45 PM


Re: fragments from older rocks
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.

This message is a reply to:
 Message 9 by jar, posted 08-28-2016 10:45 PM jar has replied

Replies to this message:
 Message 16 by jar, posted 08-29-2016 8:57 AM Dr Adequate has taken no action

  
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


Message 21 of 88 (790320)
08-29-2016 11:45 AM
Reply to: Message 20 by jar
08-29-2016 11:24 AM


Re: Great so far but I'm slow so humor me by expanding some.
Shale has lots of thin layers.


This message is a reply to:
 Message 20 by jar, posted 08-29-2016 11:24 AM jar has taken no action

Replies to this message:
 Message 23 by edge, posted 08-29-2016 11:48 AM Dr Adequate has taken no action

  
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


Message 25 of 88 (790326)
08-29-2016 12:02 PM
Reply to: Message 20 by jar
08-29-2016 11:24 AM


Re: Great so far but I'm slow so humor me by expanding some.
Particle size is pretty clear but what produces the different sized particles. Why does something end up as silt or mud or clay?

Well one factor is the proportion of chemical weathering on the one hand, and mechanical weathering and erosion on the other. Obviously mechanical weathering and erosion can produce chunks of pretty much any size.

Chemical weathering, on the other hand ... consider a nice typical igneous rock such as granite. It will contain quartz, mica, amphibole, and two kinds of feldspar. Broadly speaking, the mica, amphibole and feldspar will be turned by chemical weathering into clay, which doesn't hold together particularly well, and which gives you fine particles: mud, in effect. The quartz, on the other hand, is indestructible, and gives us sand.

(Because sand and clay particles are different sizes, they will be transported differently, which is why you end up with some places that are all sand and some which are all mud.)

One consequence of this is that when there's been a lot of mechanical weathering involved in the production of sand the sand is arkose: that is, the grains contain an appreciable amount of feldspar; whereas when chemical weathering predominates the feldspar is converted to clay and the resulting sand is practically pure quartz.


This message is a reply to:
 Message 20 by jar, posted 08-29-2016 11:24 AM jar has taken no action

  
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


Message 26 of 88 (790327)
08-29-2016 12:07 PM
Reply to: Message 20 by jar
08-29-2016 11:24 AM


Re: Great so far but I'm slow so humor me by expanding some.
And then there is slate???????

Oh, I missed this. Slate is a metamorphic rock, and comes apart into layers because it has undergone foliation. To explain this, consider that the silicate minerals in a rock have shapes: they can come in strings or sheets. So when a rock is heated and compressed, these minerals get pushed so that they lie perpendicular to the direction of the compression.

Have you read my book? All this stuff is in there; though I see the point of discussing these issues on this thread.


This message is a reply to:
 Message 20 by jar, posted 08-29-2016 11:24 AM jar has replied

Replies to this message:
 Message 27 by jar, posted 08-29-2016 12:20 PM Dr Adequate has replied

  
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


Message 28 of 88 (790331)
08-29-2016 12:42 PM
Reply to: Message 27 by jar
08-29-2016 12:20 PM


Re: Great so far but I'm slow so humor me by expanding some.
So far we have discussed two kinds of rock that both are found to create layers, shale and slate. The later has undergone metamorphose and it was that process through crystal alignment that produced the layering.

Are the layering found is shale produced a different way?

Yes indeed, that's just ordinary layering as found in sedimentary rock. Though it occurs to me that perhaps the fact that clay is a sheet silicates may also have something to do with the layering in shale as well: the sheet silicates would mean that clay often comes in flakes, which would tend to settle parallel to the ground and then would be forced even more into that orientation by compaction.

Clay particles, magnified:


This message is a reply to:
 Message 27 by jar, posted 08-29-2016 12:20 PM jar has replied

Replies to this message:
 Message 29 by jar, posted 08-29-2016 12:54 PM Dr Adequate has replied

  
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


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.


This message is a reply to:
 Message 29 by jar, posted 08-29-2016 12:54 PM jar has seen this message

  
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


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.


This message is a reply to:
 Message 34 by jar, posted 08-29-2016 4:16 PM jar has replied

Replies to this message:
 Message 36 by jar, posted 08-29-2016 6:46 PM Dr Adequate has replied

  
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


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.


This message is a reply to:
 Message 36 by jar, posted 08-29-2016 6:46 PM jar has replied

Replies to this message:
 Message 42 by jar, posted 08-31-2016 9:56 AM Dr Adequate has taken no action

  
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


(1)
Message 49 of 88 (790646)
09-02-2016 11:40 AM


Incidentally, can I have a quick question, I think I used to know it but I've forgotten. What do you call a conglomerate/breccia which is made of clasts from a previous conglomerate/breccia?

  
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


Message 54 of 88 (790719)
09-03-2016 8:08 PM
Reply to: Message 53 by jar
09-03-2016 6:35 PM


Re: Before moving on ...
'Cos it tended to form in swamps by the shore; it was always just a little variation in sea level away from being in a marine environment.

This message is a reply to:
 Message 53 by jar, posted 09-03-2016 6:35 PM jar has replied

Replies to this message:
 Message 55 by jar, posted 09-03-2016 8:11 PM Dr Adequate has replied

  
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


Message 56 of 88 (790722)
09-03-2016 10:03 PM
Reply to: Message 55 by jar
09-03-2016 8:11 PM


Re: landscapes come and landscapes go?
Or, y'know, seascape. But yes, you get coal measures where you can see the same cycle over and over: paleosol (fossil soil) with root traces in it and tree stumps sticking out of it into coal which is then covered over by marine sediment, and then the same sequence over and over again.

This message is a reply to:
 Message 55 by jar, posted 09-03-2016 8:11 PM jar has replied

Replies to this message:
 Message 57 by jar, posted 09-04-2016 9:18 AM Dr Adequate has taken no action
 Message 59 by edge, posted 09-04-2016 6:03 PM Dr Adequate has taken no action

  
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


Message 73 of 88 (790897)
09-07-2016 3:44 PM
Reply to: Message 72 by jar
09-07-2016 12:20 PM


Re: a short side trip to satisfy my curiosity.
It's the remains of part of a great big overturned fold. Here's a smaller one so you can see what's happening.


This message is a reply to:
 Message 72 by jar, posted 09-07-2016 12:20 PM jar has replied

Replies to this message:
 Message 74 by jar, posted 09-07-2016 4:30 PM Dr Adequate has taken no action

  
Dr Adequate
Member (Idle past 286 days)
Posts: 16112
Joined: 07-20-2006


Message 77 of 88 (790925)
09-08-2016 1:48 AM
Reply to: Message 76 by jar
09-07-2016 8:32 PM


Re: a short side trip to satisfy my curiosity.
Oh yes. I wrote this article. Also if the bit that's been overturned is big enough you could use dating methods. Or, as in the photograph, the folds may still be visible.

This message is a reply to:
 Message 76 by jar, posted 09-07-2016 8:32 PM jar has taken no action

  
Newer Topic | Older Topic
Jump to:


Copyright 2001-2018 by EvC Forum, All Rights Reserved

™ Version 4.1
Innovative software from Qwixotic © 2022