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Author Topic:   Growing the Geologic Column
herebedragons
Member (Idle past 71 days)
Posts: 1513
From: Michigan
Joined: 11-22-2009


(1)
Message 226 of 740 (734265)
07-27-2014 4:20 PM
Reply to: Message 199 by Faith
07-26-2014 10:17 PM


Re: Layer / Sill
I'm picturing a layer of igneous rock BETWEEN layers of sedimentary rock as what I've never seen unless it's a sill, because as usual I'm thinking of a column, or stack, of layers. Does such a thing exist or are all these other kinds of examples all there is?

I have a couple of examples of volcanic layers between layers of sediment. But before I show the examples, I just want to clarify a couple of terms.

Basalt - common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava exposed at or very near the surface.

Extrusive - the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff.

Tuff - a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption.

Volcanic Ash - consists of fragments of pulverized rock, minerals and volcanic glass, created during volcanic eruptions, less than 2 mm (0.079 inches) in diameter

* definitions are from the Wikipedia articles of the same name

So where you see "basalt", "tuff" or "volcanic ash" on these stratigraphic columns, it means that it is volcanic material that was deposited on top of a sedimentary layer and subsequentially covered with more sediment.

This one (below) is kinda hard to see but there is a basalt layer at 60 and volcanic ash layers at 55 and 79

(above) 3.5 km of lava!! about 6 km at Vancouver Is. !!!!!!!! But they are all bound by limestone top and bottom.

HBD


Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for... I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca

"Nothing is easier than to persuade people who want to be persuaded and already believe." - another Petrarca gem.

Ignorance is a most formidable opponent rivaled only by arrogance; but when the two join forces, one is all but invincible.


This message is a reply to:
 Message 199 by Faith, posted 07-26-2014 10:17 PM Faith has responded

Replies to this message:
 Message 229 by Faith, posted 07-27-2014 4:31 PM herebedragons has responded

  
herebedragons
Member (Idle past 71 days)
Posts: 1513
From: Michigan
Joined: 11-22-2009


Message 227 of 740 (734266)
07-27-2014 4:25 PM
Reply to: Message 225 by edge
07-27-2014 4:19 PM


Ash flows are quite dangerous and kill a lot of people because they can outrun a vehicle.

And can be extremley HOT! Up to 1000oC!!! and can move at a couple hundred miles an hour.

HBD


Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for... I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca

"Nothing is easier than to persuade people who want to be persuaded and already believe." - another Petrarca gem.

Ignorance is a most formidable opponent rivaled only by arrogance; but when the two join forces, one is all but invincible.


This message is a reply to:
 Message 225 by edge, posted 07-27-2014 4:19 PM edge has not yet responded

  
Faith
Member
Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 228 of 740 (734267)
07-27-2014 4:30 PM
Reply to: Message 225 by edge
07-27-2014 4:19 PM


I was going to ask JonF another queston but I guess I'll ask you. What you described is what the article he linked described, that the "whole outcrop" is volcanic but what is the "whole outcrop?" I suppose you mean not just the black layer but the whole hill in which it occurs? Meaning the light rock is not sedimentary but also volcanic?

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Faith
Member
Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 229 of 740 (734268)
07-27-2014 4:31 PM
Reply to: Message 226 by herebedragons
07-27-2014 4:20 PM


Re: Layer / Sill
But basalt IS usually intrusive into sedimentary rock, occurring as dikes and sills.

This message is a reply to:
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Faith
Member
Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 230 of 740 (734269)
07-27-2014 4:38 PM
Reply to: Message 224 by edge
07-27-2014 4:02 PM


Re: Blackrock Escarpment is all volcanic
The thing is, that Cardenas lava between the layers looks SO layer-like in the diagram, so evenly thick, not lumpy enough to be a lava that would have flowed over a surface and later been covered by another sediment. I mean, lava usually IS pretty lumpy as it hardens on the ground, isn't it? (I keep meaning to get back to Percy's post about that.)

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edge
Member (Idle past 45 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 231 of 740 (734270)
07-27-2014 4:40 PM
Reply to: Message 229 by Faith
07-27-2014 4:31 PM


Re: Layer / Sill
But basalt IS usually intrusive into sedimentary rock, occurring as dikes and sills.

You would call the Columbia River Basalts what? Intrusives?

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 Message 229 by Faith, posted 07-27-2014 4:31 PM Faith has responded

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Percy
Member
Posts: 19078
From: New Hampshire
Joined: 12-23-2000
Member Rating: 3.2


(1)
Message 232 of 740 (734271)
07-27-2014 4:43 PM
Reply to: Message 222 by Faith
07-27-2014 3:38 PM


Re: Blackrock Escarpment is all volcanic
Faith writes:

I've read this through a number of times, and I know you've posted on it many times before, but right now I can't figure out why it can't be a sill.

Here's a diagram of the eastern Grand Canyon region. At the top is a cross section showing the layers of the Grand Canyon supergroup, which includes the Cardenas Basalt (the diagram refers to it as the Cardenas Lava). At the bottom left is a map with a line showing the location of the cross section, and at the bottom right is a color key for all the layers:

The Cardenas Basalt overlies the Dox Formation (mostly sandstone) and underlies the Nankoweap Formation (lower portion is sandstone and siltstone). The Cardenas Basalt's boundary with the overlying Nankoweap is an unconformity, meaning that the Cardenas Basalt layer was eroded before the Nankoweap was deposited. The upper part of the the Cardenas Basalt contains eroded fragments of basalt and shows evidence of contact metamorphism by overlying lava flows that are no longer there, having eroded away.

The upper couple feet of the underlying Dox Formation also shows evidence of contact metamorphism by the heat of the lava layers deposited on top of it, a characteristic completely missing from the Nankoweap's contact with the Cardenas's upper boundary.

The contact metamorphism evident in the underlying layer and the absence of contact metamorphism in the overlying layer is an identifying characteristic of a basaltic layer deposited by lava flows, as opposed to a sill where both the underlying and overlying layers display the effects of contact metamorphism. The Wikipedia article on sills explains the difference between a sill and a lava layer like this:

Wikipedia writes:

Sills can be confused with solidified lava flows; however, there are several differences between them. Intruded sills will show partial melting and incorporation of the surrounding country rock. On both contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed (contact metamorphism). Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows will typically show evidence of vesicles (bubbles) where gases escaped into the atmosphere. Because sills generally form at shallow depths (up to many kilometers) below the surface, the pressure of overlying rock prevents this from happening much, if at all. Lava flows will also typically show evidence of weathering on their upper surface, whereas sills, if still covered by country rock, typically do not.

You go on to ask:

Or, since volcanic ash has been coming up, could it be that?

Just as it is very easy to tell the difference between lava and volcanic ash lying on the surface, it is also very easy to tell the difference between cooled lava (basalt) and volcanic ash that has been deeply buried and compacted. One could never be mistaken for the other. But lava and ash can certainly be interbedded, since they are produced at the same time by the same event.

--Percy


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Faith
Member
Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 233 of 740 (734272)
07-27-2014 4:44 PM
Reply to: Message 231 by edge
07-27-2014 4:40 PM


Re: Layer / Sill
I didn't mean it ALWAYS has to be dikes and sills, of course it CAN just flow and deposit wherever. I was answering HBD's apparent idea that it isn't an intrusive when you find it as a layer between sedimentary layers.

Edited by Faith, : No reason given.


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Faith
Member
Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 234 of 740 (734273)
07-27-2014 4:47 PM
Reply to: Message 214 by JonF
07-27-2014 9:30 AM



It's a unique place, though, and really it's less like igneous layers between sedimentary layers than sedimentary layers between igneous layers since the area is a huge volcanic province.

OK. So you were wrong when you claimed no such thing existed.

Let's be fair now. I said I'd never seen it, and I hadn't. To me this is unique.


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herebedragons
Member (Idle past 71 days)
Posts: 1513
From: Michigan
Joined: 11-22-2009


Message 235 of 740 (734274)
07-27-2014 4:48 PM
Reply to: Message 222 by Faith
07-27-2014 3:38 PM


Re: Blackrock Escarpment is all volcanic
I can't figure out why it can't be a sill. Would you please explain that again. Thanks.

I am not sure edge answered the question you seem to be asking here. How do we know it was an extrusive layer (formed on the surface) rather than an intrusive layer (formed under the surface between two previous layers)? I think that was your question right?

Well, I found this page that presents a review guide for a geology course that presents a problem that helps provide understanding into this idea.

(You could use it to test yourself on some of these concepts, but the answers are already filled in, so its more of a review than a test, but anyway...)

In the diagram below which shows a portion of the Earth's crust, what is the relative age of the igneous rock?

Key:

- Limestone


- Shale

- Igneous rock formation

- Contact metamorphosis

A. It is older than the limestone but younger than the shale.
B. It is younger than the limestone but older than the shale.
C. It is older than both the limestone and the shale.
D. It is younger than both the limestone and the shale.

---------------------------

Answer:

The diagram shows an igneous intrusion. This occurs when molten rock material passes through a rock layer. When the molten material solidifies, a rock layer is left behind. In the diagram, the molten igneous material passed through the limestone layer. Wherever the molten igneous material came in contact with the limestone layer, metamorphism occurred. There is no contact metamorphism between the igneous layer and the overlying shale. This indicates that the shale formed on top of the igneous layer after it had solidified.

B is the correct answer.

HBB


Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for... I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca

"Nothing is easier than to persuade people who want to be persuaded and already believe." - another Petrarca gem.

Ignorance is a most formidable opponent rivaled only by arrogance; but when the two join forces, one is all but invincible.


This message is a reply to:
 Message 222 by Faith, posted 07-27-2014 3:38 PM Faith has responded

Replies to this message:
 Message 239 by Faith, posted 07-27-2014 5:04 PM herebedragons has responded

  
edge
Member (Idle past 45 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 236 of 740 (734275)
07-27-2014 4:48 PM
Reply to: Message 230 by Faith
07-27-2014 4:38 PM


Re: Blackrock Escarpment is all volcanic
The thing is, that Cardenas lava between the layers looks SO layer-like in the diagram,...

Actually, it isn't. Usually, volcanic rocks have a more limited areal extent, so at the edges of the outcrop area, it goes to zero thickness. And remember it is a diagram.

... so evenly thick, not lumpy enough to be a lava that would have flowed over a surface and later been covered by another sediment. I mean, lava usually IS pretty lumpy as it hardens on the ground, isn't it? (I keep meaning to get back to Percy's post about that.)

When freshly deposited, yes. However, that would not be visible at this scale and ,as the article stated, the surface is weathered and eroded anyway, so it's a moot point.

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Faith
Member
Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 237 of 740 (734277)
07-27-2014 4:54 PM
Reply to: Message 213 by herebedragons
07-27-2014 8:28 AM


Re: Layer / Sill
In her defense, I think that what we observe happening today DOES look strangely different than what we see in the rock record.

YES! Thanks for that much. It looks really strangely weirdly dramatically different. There is NO way there's ever going to be another Redwall limestone or Coconino sandstone. That's the way it just plain LOOKS when you compare those formations with the paltry depositions that are offered up as the equivalent today.

But to me it highlights the fact that it takes lots of time (as well as specific circumstances) to convert these unconsolidated sediments into recognizable rock units. We just don't see it happening year by year as it would have had to happen in a global flood. If that were the case, we would have rock units under the Mississippi Delta that were only a couple hundred years old or so and new ones forming all the time.

Yeah, Time, the Magic Ingredient that turns a delta or continental shelf into the Grand Canyon.


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Percy
Member
Posts: 19078
From: New Hampshire
Joined: 12-23-2000
Member Rating: 3.2


Message 238 of 740 (734279)
07-27-2014 4:59 PM
Reply to: Message 233 by Faith
07-27-2014 4:44 PM


Re: Layer / Sill
Faith writes:

I didn't mean it ALWAYS has to be dikes and sills, of course it CAN just flow and deposit wherever. I was answering HBD's apparent idea that it isn't an intrusive when you find it as a layer.

Both sills and lava layers are layers. As the Wikipedia article I quoted in my previous message says, it can sometimes be difficult to tell the difference between a sill layer and a lava layer, but it goes on to describe their unique characteristics by which we tell them apart.

--Percy


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 Message 233 by Faith, posted 07-27-2014 4:44 PM Faith has not yet responded

  
Faith
Member
Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 239 of 740 (734280)
07-27-2014 5:04 PM
Reply to: Message 235 by herebedragons
07-27-2014 4:48 PM


Re: Blackrock Escarpment is all volcanic
I'm sorry, I don't see how that has anything to do with the question about the Cardenas basalt.

This message is a reply to:
 Message 235 by herebedragons, posted 07-27-2014 4:48 PM herebedragons has responded

Replies to this message:
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herebedragons
Member (Idle past 71 days)
Posts: 1513
From: Michigan
Joined: 11-22-2009


Message 240 of 740 (734281)
07-27-2014 5:05 PM
Reply to: Message 229 by Faith
07-27-2014 4:31 PM


Re: Layer / Sill
Basalt is defined as an extrusive volcanic rock. Look it up for yourself.

An intrusive rock that has the same chemical composition as basalt is called Gabbro.

quote:
Gabbro

A dark, medium- to coarse-grained intrusive igneous rock composed of calcium plagioclase, pyroxene, and possibly olivine, but no quartz. It is the intrusive equivalent of basalt and the deep intrusive equivalent of diabase.


Source

Its not just the chemical make up that determines what type of volcanic rock it is, it is also grain size. Intrusive lava cools slower so can form larger crystals. Extrusive rocks cool more rapidly and don't have time to form large crystals, so are more fine grained.

When a rock is referred to as Basalt, it can be assumed to be extrusive, unless there are reasons to think otherwise. Are there reasons why you think those examples are intrusions?

HBD


Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for... I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca

"Nothing is easier than to persuade people who want to be persuaded and already believe." - another Petrarca gem.

Ignorance is a most formidable opponent rivaled only by arrogance; but when the two join forces, one is all but invincible.


This message is a reply to:
 Message 229 by Faith, posted 07-27-2014 4:31 PM Faith has not yet responded

Replies to this message:
 Message 245 by Percy, posted 07-27-2014 5:22 PM herebedragons has responded

  
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