North Cascades Geology Metamorphic Rocks Progressive metamorphism of a basalt Basalt from lava flows at Sunset Crater National Mounument. In progressive metamorphism of a basalt, the course of change is different because the original basalt reacts differently to heat and pressure. In fact, it is so stiff and resistent to the squeezing (unlike wimpy shale) that the first reincarnation as a metamorphic rock is simple recrystallization to a rock called greenstone named because it is made of many green metamorphic minerals. Further squeezing finally overcomes the basalt or greenstone resistance, forming greenschist, which has many of the same minerals as greenstone but with the flaky foliation of all schists. Greenstone. Rising temperature and continued squeezing causes new minerals to crystallize, and what was formerly basalt becomes amphibolite, a rock that looks like a dark gneiss and is rich in hornblende and feldspar, but with very little quartz. Amphibolite.

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| USGS Geology in the Parks home | NPS Park Geology Tour home | This site is a cooperative endeavor of the US Geological Survey Western Earth Surface Processes Team and the National Park Service. Please share your comments and suggestions with us! parkgeology@den.nps.gov http://www.nature.nps.gov/grd/usgsnps/noca/nocageol2c.html This page was last updated on 12/1/99

Material in this site has been adapted from a new book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle