Exploring the Grand Canyon, from the bottom up.

Okay, I retract the time question.

Thanks. I think if we work through this like those who first explored the geology that time will become a conclusion instead of an assumption.

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Aslan is not a Tame Lion

Are we done with the intrusions?

Ratel has a question in Message 59This message has been edited by jar, 03-15-2006 10:06 PM

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Aslan is not a Tame Lion

Okay. I wasn't sure if that was off-topic now.I'm going to cheat a bit and post a link to a previous post I made that briefly touches on Ratel's question, which I thought was a very good question. If it's too brief, I can elaborate. Post #8 of thread entitled 'Geologic Column.'

and see if that answers Ratels, questions.I'd rather get all the questions answered.

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Aslan is not a Tame Lion

Thanks, Jar, I appreciate it. So we see Peperites that indicate an intrusion into the sediment, and the characteristics of the peperite tells us if the sediment was wet or dry at the time. And the hyaloclastites only form in wet sediments (or other wet material). I think I got that.

Oh, and of course, we can conclude from the above that the sediment being intruded into wasn't still wet?

From all I can gather it was still sandstone, not wet at all, but not yet transformed to schist.

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Aslan is not a Tame Lion

Per the diagram of message 8, the granite is shown to be intruding only the Visnu Shist (metasediment). I'm going to ask a (sort of) variation on that question. I am going to presume that the sediments were at least already lithified (not "muddy").What was the order of the events?1) Sediments were metamorphosed and then later intruded by the granite?or2) Sediments were intruded by the granite and then later metamorphosed?or3) Sediments were intruded by the granite and metamorphosed at the same time?Now, at least superficially, the answer would seem to be #1, at least as opposed to possibility #2. If the metamorphic event were last, then the granites would have also been metamorphosed.The kicker is (as I recall), because of their mineralogy, granitic rocks are highly resistant to metamorphic effects. Pressures and temperatures that would substantially change a mudstone or a basalt might not change a granite at all.Perhaps the detailed study of the schist/granite contact would shed light on this question. Offhand, my guess is that #3 is the case. Actually, #3 may be a variation of #1, in that the granitic intrusion may be a later phase of the same event that caused the metamorphism.Comments?Moose

As far as the Vishnu Schist goes, the granite was formed at about the same time as the schist was forming. In other words, they were forming contemporaneously - at least at deeper levels in the crust.The granite may represent either a completely separate igneous body, or it is a partial melt of the Vishnu Schist itself. When rocks are subjected to high temperatures and pressures, as the Vishnu was, they can partially melt to form a magma (aka partial melt). It is called a partial melt because only a portion of the minerals present in the parent rock, in this case the Vishnu Schist, will melt. This is because temperatures during a metamorphic event generally do not get as hot as temperatures lower in the crust where ALL the minerals melt.Ferromagnesian minerals, such as pyroxene and olivine, have higher melting temperatures, and it takes very high temperatures to get those to melt. So in a [shallow?] metamorphic setting, only the lower temperature felsic minerals will melt, forming a felsic magma - granite.This granitic magma, because it was more buoyant than the surrounding rock, worked its way up into the schist, specifically exploiting and traveling up areas of weakness, such as along faults, joints, and fold axes. This suggests the schist was already somewhat brittle, though not completely cooled, when the granite intruded some distal portions of the schist.Sorry, I don't think I'm doing a very good job at describing the temporal relationship between the schist and granite.I haven't read all the pertinent data, so I don't know whether any sediments exist below the Supergroup that may have been intruded by the granite. That is certainly possible. But what we do know is that the granite intruded the Vishnu long before the Supergroup was deposited atop it. The granite did NOT intrude any of the Supergroup rocks - as far as I know.

Going back to the diagram in Message 8 it looks to me as if the intrusive dykes shown have been cut off at the top of the picture by erosion. That is, the schist and the intrusive dykes were "levelled off" before the deposition of the Bass limestone. Can the geologists among us confirm that this reading is correct ?m

correct

great, got it.

This relates to event timing.Please correct this if there is a mistake, but I'm trying to outline this part for us laymen. If I can describe it in a way I can understand, then maybe other non-geologists like me will understand as well.The Vishnu Schist consists of transformed sandstone.To get sandstone you first need an earlier source of rock that was eroded down into small particles of sand.So we have three events so far, creating some kind of mountain or large rock source, long weathering and erosion to make sand, then the sand is transported to some basin where it is compacted into sandstone.Now Question 1.Does the accumulated sand need to be under pressure of an overlying layer to change from sand to sandstone, or is it simply that the lowest layers of sand are compressed by the weight of overlying sand?Would it simply be a layer that is sand at the top gradually turning to sandstone as the weight and pressure increase with depth.Question 2.Is it likely that the magma intrusion extended through the layer of sand--->sandstone and that what happened is the the softer protosandstone layers that also contained magma were what was eroded away before the Bass Limestone and other layers were laid down?Question 3.When did the sandstone become schist and the magma become granite? Wouldn't both have to be buried under lots of material to create the temperatures and pressures need for sandstone ---> schist and magma ---> granite?

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Aslan is not a Tame Lion