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Part of Hall of Planet Earth.
AMNH/R.Mickens
Dense metamorphic rocks, such as granulite and gneiss, make up most of the lower crust. This sample was transported to the surface by erupting lava. It is made up of the minerals gray quartz, white feldspar, purple garnet, and white sillimanite, and comes from a depth of 12 to 26 kilometers.
Provided by Prof. Roberta Rudnick, Harvard University, Cambridge, Massachusetts