Contact Metamorphism: Field Example

Nice hot molten magma tends to metamorphose the rocks it intrudes. Because rock is such a good insulator, often this contact metamorphism extends no more than a few meters or tens of meters away from the magma body. Sometimes the contact zone is measured in centimeters or millimeters!

This is the first in a series. This is an unmetamorphosed dolomite located just a few meters from a granite pluton. Near the Race Track, Death Valley, CA.
Other parts of the dolomite formation happen to be better layered. Up close, this is a very fine-grained, generally featureless sedimentary rock.
Over the distance of about a meter, the featureless dolomite abruptly turns into a banded marble!
About a meter or two from the first marble, we pass the contact between the marble and the granite. Here the granite is very fined grained. It cooled quickly because it was tight against the cool country rock it had intruded.
This is a crummy photo, but you can still make out several large light-colored rectangular feldspar crystals and the generally speckly texture that characterizes a granite. This is less than a meter from the fine-grained granite above.
Fluids released by the slowly cooling magma or by the metamorphism of the country rock (host rock) can deposit interesting minerals and sometimes valuable metal ores. In this case a prospector found some interesting greenish minerals near the granite/marble contact and dug this short mine shaft in hopes of striking it rich. He did not get rich off of this hole. Image the life of a prospector digging into solid rock in hopes of a payoff...

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