During our recent cruise I found this unusual rock in Ketchikan, Alaska. The thin, flat rock was about the size of the palm of my hand, and I just had to pick it up. This tortured rock was once buried miles beneath the surface of the earth and was forcefully thrust upward by collisions of continental and oceanic tectonic plates. The tremendous pressures crushed and changed the original rock, whatever it was, into this layered, flaky form known as schist. It is quite common where there has been great movement of the crust of the earth.
It is hard to believe that solid rocks can be compressed and crushed into new forms, but even granite can undergo such metamorphism. The resulting rock generally has banded layers of light and dark shades and is known as gneiss, pronunced “nice.” Most forms of gneiss are formed from granite, but other rocks can also become gneiss, so just because a rock is gniess, don’t take it for granite. We saw plenty of both schist and gneiss on our Alaska trip, and when I described the rocks to family, they thought that the word “gneiss” sounded nice, but the word “schist,” not so much.
#gneiss #schist #tectonics #ketchikan #alaska
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