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Author Topic:   Exploring the Grand Canyon, from the bottom up.
edge
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Posts: 4450
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 4.3


Message 91 of 282 (296408)
03-17-2006 10:55 PM
Reply to: Message 90 by jar
03-17-2006 10:30 PM


Re: What sandstone???????
So, how do we turn sandstone into schist?

Rox has explained this previously. In simple terms, heat and pressure with the possible inclusion of chemical alteration. I don't know the specifics of this occurrence.

First, what is a disconformity?

Excuse me, I misspoke. Actually, the contact between the metamorphic Vishnu and the overlying Bass Limestone is a nonconformity, meaning the contact is a hiatus of deposition in which the underlying rocks have undergone a metamorphic event prior to erosion. A disconformity is a horizon that represents a non-depositional period between sedimentary layers. I always get them confused because we usually use the term 'unconformity' which includes all types of interruptions of deposition.

Cooled enough to have become granite?

That would be the implication. Since it appears that erosion has occured prior to the limestone that would mean surficial processes (streams, etc.,note the conglomerate). Very few magmas would exist at the surface for long without cooling to the point of solidification.

Can you help explain to someone as slow as I am what needs to happen to change the sandstone to schist and the magma to granite?

I thought this had been done earlier. Basically, a magma has to cool to the point that it solid. Sandstones would likewise have completed their process of metamorphism upon cooling to a their present form. This is a bit of a simplification, but the basics are there.


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 Message 90 by jar, posted 03-17-2006 10:30 PM jar has responded

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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 92 of 282 (296412)
03-17-2006 11:08 PM
Reply to: Message 91 by edge
03-17-2006 10:55 PM


Gotta go even slower for me.
Actually, the contact between the metamorphic Vishnu and the overlying Bass Limestone is a nonconformity, meaning the contact is a hiatus of deposition in which the underlying rocks have undergone a metamorphic event prior to erosion.

I'm still looking for some explanations I can understand.

...meaning the contact is a hiatus of deposition in which the underlying rocks have undergone a metamorphic event prior to erosion.

I asume that means a pause?

This brings a few more questions to mind.

How do you build enough pressure to turn sandstone into schist without piling stuff on top of it?


Aslan is not a Tame Lion
This message is a reply to:
 Message 91 by edge, posted 03-17-2006 10:55 PM edge has responded

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roxrkool
Member (Idle past 875 days)
Posts: 1493
From: Nevada
Joined: 03-23-2003


Message 93 of 282 (296413)
03-17-2006 11:16 PM
Reply to: Message 87 by jar
03-17-2006 8:42 PM


Re: What sandstone???????
To change sandstone into schist I understood that it took time, higher temperatures and pressure.

Correct.

To change the magma into granite I understood that it took time, long slow cooling and pressure.

It takes time, but not pressure. Large plutonic bodies can take millions of years to cool, but dikes and smaller igneous bodies can cool quickly when intruding cold country rock. (Country rock is the rock being intruded.)

Since we are still at that point in time were the Vishnu layer has been laid down and the magma intruding into it, there has not been any time for the transformation. In addition, so far there are no layers on top of the Vishnu layer so no pressure to cause the metamorphose.

Is that correct?


Now that I've read a little more, I have a clearer picture of what happened, but I will have to use dates to illustrate the temporal progression.

During the latest part of the Archean, deposition of the Vishnu proto-sediments begins in a marine basin located just seaward of a young North American continent, and adjacent to a volcanic island chain similar to Indonesia. The island chain is slowly moving towards the continent. I'm not sure how thick the sediments were able to accumulate in the basin, possibly several thousand feet, but the deepest sediments begin the lithification process sooner than the shallower sediments due to temperature and pressure.

Later (by 1700 Ma), Vishnu proto-sediments of unknown thickness are fully lithified (I believe) and the volcanic island chain is in the process of colliding with North America. It's at this point in time the sediments, during collision and after lithification, are metamorphosed and deformed into schist due to intense pressures associated with collision. The collision results in mountain building (think Mt. Everest). It is also during this time (~1700 Ma) that granitic bodies are forming deep in the schist and migrating up into the shallower and distal portions of the schist. The resultant mountain range is composed of the Vishnu Schist and igneous intrusions.

Sometime after 1700 Ma, uplift stops and the mountain range is eroded down to small rolling hills. At around 1200 Ma, the erosive event is for the most part complete and deposition begins with the Bass Limestone as a result of a marine incursion.

Did that make better sense?


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edge
Member
Posts: 4450
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 4.3


Message 94 of 282 (296414)
03-17-2006 11:23 PM
Reply to: Message 92 by jar
03-17-2006 11:08 PM


Re: Gotta go even slower for me.
I'm still looking for some explanations I can understand.

Try these definitions from Wikipedia:

"Disconformity
An unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

Nonconformity
An unconformity exists between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock."

These are both types of unconformities. It's all a matter of defining different types of unconformities. The old geologic tradition of making something relatively simple seem very complex.

In this case, I see a nonconformity between the underlying Vishnu and the overlying Bass. There is a conglomerate in between: a typical occurrence at an unconformity. My guess is that the conglomerate is local.

How do you build enough pressure to turn sandstone into schist without piling stuff on top of it?

Ah, that's the whole point isn't it? The best way to build pressure is with the weight of overlying sediments. Usually, however, some kind of deformation is implied by schistosity. This would be caused by folding, thrusting, or other faulting; all common processes of mountain building.

So, now you have to account for not only the metamorphism, but the cooling and erosion of the metamorphic rocks below the Bass and prior to deposition of the Bass. I believe this is your point in this thread?


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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 95 of 282 (296415)
03-17-2006 11:28 PM


Need a short break to absorb...
all this stuff you folk are throwing at me. I'll try to post a summary of what I think you folk are saying in the morning.


Aslan is not a Tame Lion
Replies to this message:
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roxrkool
Member (Idle past 875 days)
Posts: 1493
From: Nevada
Joined: 03-23-2003


Message 96 of 282 (296416)
03-17-2006 11:47 PM
Reply to: Message 95 by jar
03-17-2006 11:28 PM


Re: Need a short break to absorb...
That's perfectly understandable, Jar. :) I wish I was better at explaining this stuff in simpler terms, but it really is quite difficult.

Geology makes the most sense when we can visualize the progression of events through time and so to really understand geology, you need to think in 4-dimensions. Unfortunately, that is a skill not easily picked up by people, especially if you aren't a natural visual learner - thankfully I am or geology would have been absolute Hell.


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Replies to this message:
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Tony650
Member (Idle past 2011 days)
Posts: 450
From: Australia
Joined: 01-30-2004


Message 97 of 282 (296472)
03-18-2006 11:45 AM
Reply to: Message 96 by roxrkool
03-17-2006 11:47 PM


Re: Need a short break to absorb...
rox writes:

...you need to think in 4-dimensions.

Argh! Ok, I know this is completely off topic (please forgive the intrusion, Admins), but whenever I see such a comment by someone I've never engaged on the subject, I simply feel overwhelmed to mention my old thread.

Rox, if you've not seen it before, and if you think there is anything you can add, check out this topic and feel free to contribute something if you wish. I am always eager to learn about this from anyone who can teach me. It is a very old thread but please don't let that stop you if you think you can help.:)

My apologies again for the diversion; I just can't bear to let such opportunities go by when I see them. As you were.:P


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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 98 of 282 (296476)
03-18-2006 12:36 PM
Reply to: Message 96 by roxrkool
03-17-2006 11:47 PM


Okay, trying another summary
Alright.

Let's see if I have a few first principles down correctly and then try to sum up where we are so far.

The process of making many small things out of fewer large things is erosion.

The products of erosion collect in places lower than the origin in most cases.

So what happens is that rocks get formed and pushed up. Rain, wind, heat and cold erode those rocks and the material collects in lower, realtively level areas.

So far so good?

Next, schist is sandstone that was changed due to heat, pressure and time.

Granite is the result of magma which changed form as it slowly cooled over time.

The two formations we have considered so far are the Vishnu Scist and the Zoroaster Granite. We can tell that the Vishnu Schist is older by several indicators. One is that the formations of the granite are ones that had to happen afterwards, to be intrusions, because they would have been unsupported if there hadn't been something to support them originally.

we can see an example of a granite intrusion here

or here

and the process specific to the Vishnu Schist here.

The 4 dimensions you are talking about are height, width, length and time.

Now for the summary.

We began with the Vishnu Schist because it is the lowest exposed layer. It is schist, so originally it was sandstone. To get sandstone there first had to be some higher rock source which was eroded, accumulated in a basin, was later compressed and metamorphed into schist.

So to this point, rock was created, eroded into smaller and smaller particles until we had sand, washed or blown downhill to collect in a basin, settled, compressed and over time, under pressure and heat became schist.

At sometime during this process, magama pushed through the sandstone or schist and over long periods of time cooled to become granite.

The whole structure, Vishnu Scist and Zoroaster Granite was pushed upward or the whole area was eroded down to where everything was at about the same level. We know this happened because accumulations of the sandstone stopped and the area began to erode. We know that because there is a non-conformity between the Vishnu Schist/Zoroaster Granite and the layers above them. Another indicator that that was what happened is that the Zoroaster Granite does not intrude into the layers above.

Is that a reasonable summary of the Grand Canyon so far?

And now a few other questions related to things mentioned. You good folk mention cross bedding, conglomerates and breccia.

From my limited knowledge, conglomerates and breccia both are simply groupings of other rocks that have changed into a single rock while crossbedding refers to changes in directions of what is being laid down or bending of existing layers.

I think it would be worthwhile to explore just what those mean and how they can be identified.

Is this an example of cross bedded sandstone? Can you explain what we see here and what it tells us?

Here is a picture of a conglomerate

and and one of breccia.

To the uneducated eye these seem to be very different formations. The conglomerate shows rounded stuff incorporated that must have going through considerable weathering and erosion before being cemented back together. The breccia on the other hand shows very angular stuff incorporated.

Can you explain what it is that we should be seeing in these pictures and what that tells us about the processes involved?


Aslan is not a Tame Lion
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Minnemooseus
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Posts: 3685
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001
Member Rating: 2.9


Message 99 of 282 (296486)
03-18-2006 1:59 PM
Reply to: Message 98 by jar
03-18-2006 12:36 PM


Re: Okay, trying another summary
Seems to me that you are bringing up enough details that we could have several seperate other topics.

Basicly agree, but a few comments (warning - may contain oversimplifications):

The process of making many small things out of fewer large things is erosion.

That is weathering. It may be chemical, mechanical, or both. Erosion is the moving of the smaller particles from their location.

Next, schist is sandstone that was changed due to heat, pressure and time.

In the case of the Vishnu Shist, yes. I think it was because the protolith was a clay rich variety of sandstone. It is the clay minerals that are metamorphosed into the micas that give the schist the property of schistosity. Not all sandstones can metamorphose into schists, and not all schists are metamorphosed sandstones.

Granite is the result of magma which changed form as it slowly cooled over time.

Granite is a variety of rock that is formed when a very hot solution (magma) cools, and different chemical composition units (crystals) are formed. In general, slower cooling rates result in larger crystals. A very fast cooling rate results in glass.

We can see an example of a granite intrusion...

Your second example is a pegmatite, which is a special late stage, high volatile (water etc.) variety of igneous intrusion. Probably best not discussed further in the context of this topic.

Crossbedding - Formed from the movement of sediments to form ripples of various scales. The crossbeddings are the successive down flow direction faces of the ripples. Dunes (from either wind or water sediment movement) are essentially just large ripples.

Conglomerates - Rock formed from coarse grained sediments with the "grains" being rounded because of abrasion from movement.

Breccia - Angular broken up rock, not rounded from movement. Offhand, your photo example appears to be a fault zone breccia.

Moose

Edits: Fixed a misspelling, tweeked a couple of phrases.

This message has been edited by minnemooseus, 03-18-2006 03:06 PM


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roxrkool
Member (Idle past 875 days)
Posts: 1493
From: Nevada
Joined: 03-23-2003


Message 100 of 282 (296507)
03-18-2006 4:11 PM
Reply to: Message 99 by Minnemooseus
03-18-2006 1:59 PM


Re: Okay, trying another summary
Good point about clay in the original sediments, moose. If no clay was present in the proto-sediments and it was just sand, metamorphism would result in quartzite, not a schist.
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jar
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Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 101 of 282 (296524)
03-18-2006 8:06 PM


Well, looks like we gotta pause again before...
we can get beyond these two levels. Moose and Roxrkool done said stuff this old man needs clarified.

I'm sorry if I'm holding you folk back but if I can't understand it there's the possibility that others may not as well.

First off you folk have mentioned weathering several times and you now bring in chemical and mechanical as types of weathering.

Can you explain what the difference is and how you tell which type weathering happened?

Next you say

da moose writes:

I think it was because the protolith was a clay rich variety of sandstone. It is the clay minerals that are metamorphosed into the micas that give the schist the property of schistosity.

So what be the property of schistosity?

Second, IIRC this is the first time clay has been mentioned.

Clay is very different than other dirts or sand. For one thing, I've never been able to see a clay particle. It doesn't seem to be just small rocks like sand. It feels different, sticky and when wet slippery. It also is different than sand in that as it dries out, it retains it's form and doesn't just crumple like an old sand castle.

So what is clay and how is it produced?

Now for a question on Granite. So fast cooling magma yield glass like material and the slower it cools the larger the crystal structure.

This is important because it gives something specific that can be seen and not just wild speculation as some creationists claim.

This message has been edited by jar, 03-18-2006 11:08 PM


Aslan is not a Tame Lion
Replies to this message:
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edge
Member
Posts: 4450
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 4.3


Message 102 of 282 (296602)
03-19-2006 1:08 PM
Reply to: Message 101 by jar
03-18-2006 8:06 PM


Are you sure...
... that you want to get into all of these details?

First off you folk have mentioned weathering several times and you now bring in chemical and mechanical as types of weathering.

Can you explain what the difference is and how you tell which type weathering happened?

Chemical weathering is primarily dissolution of a rock at the surface of the earth. This is how many limestones erode in moist environments. Mechanical weathering would be the act of wearing down by movement of wind, water, ice, etc. Rounded river cobbles would be an example. Another would be mass wasting such as debris flows.

Next you say

da moose writes:
I think it was because the protolith was a clay rich variety of sandstone. It is the clay minerals that are metamorphosed into the micas that give the schist the property of schistosity.

So what be the property of schistosity?

It is a platy or foliated texture, usually brought about by deformation during metamorphism.

Second, IIRC this is the first time clay has been mentioned.

Clay is very different than other dirts or sand. For one thing, I've never been able to see a clay particle. It doesn't seem to be just small rocks like sand. It feels different, sticky and when wet slippery. It also is different than sand in that as it dries out, it retains it's form and doesn't just crumple like an old sand castle.

So what is clay and how is it produced?

Ummm, jar, are you ready for this? "Clay" can refer to two different things. One is grain size (can't remember the scale, just google 'grain size scale' and you can find it). But 'clay' is also a very complex set of minerals. They are usually soft, platy garbage cans of minerals with all kinds of odd pysical properties. And, true, normally, you cannot see a clay crystal. They are too small, and (like carbonates) they require virtually a separate mineralogical field for their study. We usually use x-ray diffractometry to study them. Clay is a very common product of chemical weathering. If you want more details on 'clay', I suggest a separate thread.

Now for a question on Granite. So fast cooling magma yield glass like material and the slower it cools the larger the crystal structure.

Yes, keeping in mind that different minerals form at different temperaturs so the cooling history of a magma can be quite complex. Let's just say that, in a general way, rapid cooling results in finer grain sizes and glass in some conditions.

This is important because it gives something specific that can be seen and not just wild speculation as some creationists claim.

Oh, I'm sure they'll come up with some ad hoc explanation that ignores every other line of evidence. REcently we discussed on another board, the rapid formation of granitic (coarse grained) textures in laboratory setting and very controlled conditions, and YEC took this to mean that giant batholiths, like the Sierra Nevada, could have cooled in just a few weeks. Kind'a flys in the faces of logic and intuition, but there you go...


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 Message 101 by jar, posted 03-18-2006 8:06 PM jar has responded

Replies to this message:
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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 103 of 282 (296604)
03-19-2006 1:28 PM
Reply to: Message 102 by edge
03-19-2006 1:08 PM


Yep, I'm sure.
Thanks. You did use yet a few more new terms, platy or foliated. Would you say that these drawings are a reasonable illustration of those and why they form?

And are these useful examples of what geologists find when actually looking at the rocks? When you say glass like I assume you mean something like the sample in the lower left box while granier would refer to things like the one on the lower right.

One more question about clays. Clays seem to be very, very fine grained. Is that from reduction in size and weathering?


Aslan is not a Tame Lion
This message is a reply to:
 Message 102 by edge, posted 03-19-2006 1:08 PM edge has responded

Replies to this message:
 Message 104 by edge, posted 03-19-2006 1:58 PM jar has responded

  
edge
Member
Posts: 4450
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 4.3


Message 104 of 282 (296612)
03-19-2006 1:58 PM
Reply to: Message 103 by jar
03-19-2006 1:28 PM


Re: Yep, I'm sure.
Thanks. You did use yet a few more new terms, platy or foliated. Would you say that these drawings are a reasonable illustration of those and why they form?

Yes, they show the development of schistosity. Some actual photos would be good. Should be readily available.

And are these useful examples of what geologists find when actually looking at the rocks? When you say glass like I assume you mean something like the sample in the lower left box while granier would refer to things like the one on the lower right.

Yes. The pictures are pretty good at showing some of the igneous textures.

One more question about clays. Clays seem to be very, very fine grained. Is that from reduction in size and weathering?

Mainly, I think, they are just not very stable in large configurations. Their grain boundaries are highly reactive. I suppose under very controlled conditions, they might be grown larger, but that probably won't occur in nature.


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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 105 of 282 (296615)
03-19-2006 2:01 PM
Reply to: Message 104 by edge
03-19-2006 1:58 PM


still on clay
so where does clay come from? How is clay produced?


Aslan is not a Tame Lion
This message is a reply to:
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Replies to this message:
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