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Archive for May, 2010

A few weeks ago my structural geology class took a three day trip to the Blue Ridge Mountains and the Valley and Ridge province just west of that. I had a great time, and not just for the camaraderie and tomfoolery, but also for the chance to get out of the class again and put our collective nose to the outcrop. I have always been of the opinion that you can listen to lectures and read, but that the lesson doesn’t truly sink in until you see it in person. It’s all theory until put into practice.

On our trip we made five major stops over three days and two nights, with the final being a collection of outcrops along Route 55 in West Virginia. I will be putting together a separate blog entry for each stop over the coming days. The entries will include pictures, descriptions and maybe a few graphs or charts.

The first stop on our trip was Garth Run (Garth Run Google Earth file):

At the Garth Run outcrop there was a large outcrop on the eastern side of the road, and a creek on the western. My initial reaction to the outcrop was that it was granite, which it was/is. But, after spending some time getting a closer look it is apparent that there is so much more going on there.

The rocks seen at Garth Run are Grenvillian aged granite plutons put in place some 1.1 billion years ago, they were then metamorphosed a second time during the Alleghenian Orogeny of ~300 million years ago. This is evidenced in two ways: one there are two separate joint sets in the out crop, one striking roughly northwest (300°, 59°) and the other southwest (205°, 79°). Further evidence is seen in S and C fabrics of the foliation. Of the two foliations the NW striking one is older than the NE as determined from cross cutting relationships. The older of the two would have been formed during the Grenville Orogeny and the second during the Alleghenian. This outcrop is an example of the basement complex of the Blue Ridge Mountains, which is underneath every other formation discussed from this trip.

When I mentioned the creek earlier it was because in it was a beautiful example of mylonite progressing to ultramylonite, pictured below. This grain size reduction would have been attributed to the multiple tectonic events on these rocks.

Also, at this stop I found a nice looking hand sample of blue quartz in feldspar. I plan on devoting a future blog post to this one rock, so you will have to wait eagerly for now. But, I will include a picture to tide you over.

We then moved onto our next stop, the Swift Run Formation.

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Today as I was cooking up a batch of tapioca pudding a thought occurred to me. Given the teaching habits of one of my professors, I now have a propensity for finding geologic analogies in food. Which can sometimes be a messy habit. Today that took shape in the form of pudding. As I sat and stirred over medium-low heat as not to let the tapioca stick to the bottom of the pot, I would watch little tapiocas (?) rise in some places travel along and sink near the edges. Then it dawned on me. I was watching at a food inspired geologic analogy for the asthenosphere!

As the heat from the stove (heat from radioactive decay) warms the pudding (partially melted rock of the asthenosphere) it becomes less dense and rises. Once it reaches the surface (or the under belly of the lithosphere) it flows away from the spreading center. Being pushed by and pushing other little tapiocas in their path. Once they are far enough away they either cooled enough or collided with the scummy build-up along the edge of the pot (the continental crust).  My little tap’s would then dive back into the depths of the “asthenosphere” to be heated up again, and repeat the entire process.

I also consider that if I let enough of the scummy build-up propagate out, I could have an analogy for plate tectonics as well.

Needless to say, as I sat and watched this (and took pictures and video) I let the tapioca burn and stick to the bottom of the pot.

What do you think? Is this an accurate analogy or do I need some fine tuning?

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