USGS - science for a changing world

USGS Geology in the Parks

Basalt.
Basalt from lava flows at Sunset Crater National Mounument.

North Cascades Geology

Metamorphic Rocks

Progressive metamorphism of a basalt

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.

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.

Greenstone.
Greenstone.
Amphibolite.
Amphibolite.
Return to Metamorphic Rocks
| North Cascades National Park geology home | | North Cascades National Park | | Geologic & Field Trip Maps | | Geology field trip | | Site contents |

Material in this site has been adapted from a 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.

Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://geomaps.wr.usgs.gov/parks/noca/nocageol2cm.html
Page Contact Information: Webmaster
Page Last Modified: 13-Dec-2016@16:42