Tuesday, October 18, 2011

Dr. A's course in geology: some questions

This is from his post, Igneous Rocks.
We should perhaps add a note on the presence of komatiite (extrusive ultramafic rock) in our diagram, as some textbooks omit it entirely from such diagrams. Komatiite is never observed forming today: as ultramafic magma rises from the hot interior of the Earth to its cool surface, it will fall below its melting point before it gets near to the surface, forming peridotite, komatiite's intrusive counterpart. Consequently komatiite is found only in rocks dated to over 2.5 billion years ago, consistent with geologists' belief that the Earth was hotter at that time.
What I'd like to know is where I might see some of this komatite, or how would I recognize a rock "dated to over 2.5 billion years ago." I suppose they're pretty deep rocks of course.
In the diagram, we have shown the layers of rock lying flat, except around the lacolith (item (6) on the diagram) and so we have shown the sills as horizontal structures. However, layers of rock can be folded by tectonic activity. When a sill intrudes into rocks like this, it intrudes between the layers of rock (this is the definition of a sill) and so will itself be contorted.
Are you thinking of magma intruding between the layers after the distortion? How would you know if that happened or if it had intruded before the distortion and was folded along with the sedimentary rocks?

[That bulge of sedimentary rock over the laccolith is interesting because it's similar to the much larger bulge of the strata into which the Grand Canyon is cut, shown in cross sections of the area, which is clearly (according to me) the result of the intrusive magma beneath the base of the canyon. The volcanic event that produced that magma bubble clearly displaced the rocks that make up the "Great Unconformity" and is evidenced by the granite in the form of magma fingers that also always appears on such diagrams at the base of the canyon. The point? 1) The unconformity was created while all the strata were in place, not, as is commonly believed, before. 2) The sedimentary rocks follow the curve of the magma intrusion in both cases, showing they were already in place when it erupted, which nobody is doubting in the case of the laccolith, and that they were still damp because they conform to the shape.]

He goes on to answer the question "How do we know" about the igneous nature of granite among other things. That's all good of course, as there must have been such questions about its origin or nature early on in the study of rocks, but I'd be more interested in knowing how they think they know that "the Earth was hotter 2.5 billion years ago."

Then in the following discussion Pressie raises questions that suggest he doesn't know what Dr. A. means by his wanting to emphasize the history of the rocks. But the constant refrain appears that we know how such and such a rock was formed because we see it forming before our eyes. Now THIS is going to require a lot more description than he may be planning to give. Exactly WHAT is "forming before their eyes" anyway? Exactly WHAT are they seeing? Has anyone actually seen a rock form? Has anyone seen a STRATUM of rock form?

In message 49 Dr. A says:
What I meant was, geologists don't merely collect and classify. They have a theory. This theory has predictive and explanatory power.
Take aeolian sandstone as an example.

Theory: such-and-such sedimentary structures are aeolian.

Prediction: when we find fresh sediment having these structures, it will be on dry land; when we watch it forming in real time we shall see the wind forming the structures.

Observation: consistent with the prediction.

Explanatory inference: when we find these structures in sandstone, we should infer that it was formed in the same way.

Another prediction: hence if we find sandstone with these structures, then if we find fossils in it they should be terrestrial and not marine.

Observation: We do, hurrah!

Explanatory inference: but if we find no fossils, we should come to the same conclusion, since the theory works every time we can test its predictions.

It may be bleedin' obvious, but it is still an example of a scientific theory which we establish as true by testing its predictions and from which we can then make inferences which rest on the theory rather than on direct observation.

Now it is precisely this aspect of geology which I wish to emphasize.
Theory, fine, let's see what it actually means in reality.

What I really really want explained is such things as where all the sediments came from to build the enormous depth and breadth of strata in, say, the Grand Canyon/Grand Staircase area, what sort of processes could have made it possible for an aeolian (air or wind-formed) layer to sit atop or between some water-originated layers and look for all the world like they were all created by exactly the same processes, and how an aeolian layer could ever have come to be conformed to the flat hard horizontality of a water-made layer, and how any of the layers could ever have turned into rock unless there were LOTS of layers above them compressing them soon after they were laid down, and so on and so forth.

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