Remember, at this moment what is today the Vishnu Schist was still but sand and sandstone and the intrusions were still just magma.
Actually, the way a geologist would read the section would be that the sandstone was already a schist before the limetone was deposited.
This actually plays to your hand, jar. It adds just another step to the whole story. The Vishnu sediments had to be metamorphosed and folded prior to the Bass depostion. This is heralded by the presence of a major disconformity. Look at the section closely.
And, no, I would say that the magma has long cooled before Bass time.
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.
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?
... 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...
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.
"Clay is a generic term for an aggregate of hydrous silicate particles less than 2 Î¼m (micrometres) in diameter. Clay consists of a variety of phyllosilicate minerals rich in silicon and aluminium oxides and hydroxides which include variable amounts of structural water. Clays are generally formed by the chemical weathering of silicate-bearing rocks by carbonic acid, but some are formed by hydrothermal activity. Clays are distinguished from other small particles present in soils such as silt by their small size, flake or layered shape, affinity for water and high plasticity index." (bold added)
Basically, most silicate minerals are susceptible to acid attack. This removes a bunch of cations and leaves behind a residue made up of clays.
In reading this website, you will notice reference to the fact that the Bass includes several rock types. This kind of refutes Faith's statements on another thread about continuous deposition of a rock type and simplification of the section for mapping and descriptive purposes.
This is something I keep trying to point out to Faith. Just because the pretty geologic images online or in books show straight lines and single-lithology formations doesn't mean it is anywhere near representative of reality. And it's not just in the Grand Canyon that this happens - it's everywhere, as you know.
Yah, probably wasting your time. Faith is impervious to facts. I would add that much of the simplification we see is due to scaling. We simply cannot show all of the detail unless we go to a one-to-one scale, which would kind of defeat the purpose of creating a map. But Faith has decided what is correct, no reason to listen to anyone who actually does the work.
First, the article referenced says that there is a big chunk of material missing betwen the Vishnu Schist and the Bass Formation. Since it is not there, just what is the evdence that at one time something was there?
Since there is deposit of eroded Vishu material on the unconformity, we can only say that there was time to weather and erode some of the Vishnu. We don't know how much, or what else might have been there. However, by judging the metamorphic grade of the Vishnu (in other words, these rocks probably formed deep in the core of a mountain range), we can assume that there was a considerable thickness that has been eroded.
Second, the article says that the Bass Formation was created by an intruding and then retreating sea. It even provides the directionality of the event.
How is that determined? What evidence led to that conclusions?
This is called a transgressive-regressive sequence. It is determined by mapping the paleoenvironments of the different formations. When a rock sequence fines upward (sand to silt to mud to limestone, for instance) it is consdered to be caused by deepening water. THe opposite would be shallowing water, or a regressive sequence. It is determined by mapping the rocks.
Third, Roxrkool mentioned a layer pinching out. What does that mean and how are such things identified?
It is the characteristic of a layer of rock thinning laterally so that it eventually disappears. This is very common in the geologic record. It can be caused by several different processes.
From what I can find out they are the result of faulty, the Horst being the relatively upthrust body while the Graben is the relatively lowered one. Together they would form a series of hills and valleys bounded by fault lines.
It would seem that these are very indicative of tectonic activity.
Is this what is refered to in the discussion of the Bass Formation?
Is what is shown this page an explanation of Horsts and Grabens?
On a small scale, yes. Keep in mind that this can occur on a regional scale, as well.
Finally, on the page referenced above there is an animation called Fold Trains in Ireland.
On the lower left of the picture I notice that many of the layers seem to be cut off at the overlay. Is this the kind of evidence that shows an Unconformity, that part of what was there is missing?
I don't see anything unusual in the lower left of the picture. What do you see? The unconformity is at the top of the folde rocks.