Stop 2
The second stop was to the rock outcrop behind the Tokyo Expres on the corner of Commonweath and Virginia.
Again, the first questions to ask yourself are: 1) how many different rocks are there, and 2) what type of rock are they?
While not the best image, hopefully you can get a sense of the nature of rock here.
Another view here shows some of the features a little better. (Unfortunately the top portion- kind of gray/tan- has been heavily weathered.)
Can you see the foliation? This is a clear indication of the type of rock. There is only one type of rock at this location.
Towards the bottom of the outcrop the nature of the rock changes. Notice the reddish crystal in the upper third of the image. This should be a familiar mineral and should help to confirm the type of rock.
Notice that the foliation you saw above is not present in these lower rocks.
So, you have a metamorphic rock that has foliation at the top, but none at the bottom. Also, towards the bottom, garnets become fairly common. What does this indicate about the type of metamorphism? For it to have distinct characters from the top to the bottom of one section was it likely regional or contact metamorphism? Think about why the lack of foliation and presence of garnets at the bottom and the opposite at the top.
This is exemplary of contact metamorphism. Locally, there was a heat source under the rock. It was hot enough to partially melt the bottom of the outcrop, such that you don't get the folation (crystals were melted rather than aligned) and you do get garnets (a medium-grade metamorphic mineral). At the top, since rocks are poor conductors, there was enough heat to cause foliation (alignment of the crystal) but not enough to form garnets.
Hmmm, so what is the source of the metamorphism, and how does this rock's age compare to the other two?
This rock, incidentally, is the Fork Mountain Gneiss.
