LL Demos & Labs: Rock Profile: Gneiss, Lab: Mashed Sediment Review
Gneiss, which looks like a sandwich of black, gray and white layers, is the very end-product for rocks that have been heated to very high temperatures and smashed under tons and tons of pressure. It has two possible origins. First, it may have started out as mudstone, a sedimentary rock and changed as it went through the metamorphosis to slate then slate to phyllite, phyllite to schist, and finally, schist to gneiss. The second option, granites or other igneous rocks, could have changed to gneiss when they were exposed to high temperatures and pressures.