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USGS Geology in the Parks

Terranes of the North Cascades Domains of the North Cascade: Rocks of the Metamorphic Core Domain

The Eldorado Orthogneiss, a 90 million-year-old stitching pluton in the Metamorphic Core Domain.
Looking south from the North Klawatti Glacier. The rock in the foreground and at a distance is the Eldorado Orthogneiss. Mount Buckner is on the left skyline and Mount Forbidden hidden in clouds.

Sorting out the original materials that were once stewed deep in the crust and are now exposed in the Metamorphic Core Domain is challenging but not without rewards. First, sands and muds that have been well recrystallized are hard and resistant to erosion. They stand up in some of the world’s most awesome alpine scenery. Many of the North Cascades' highest, most rugged, and challenging peaks are carved from metamorphic rocks. The geologist, seeking the ultimate origin and history of these metamorphic rocks, gets to savor some wonderful scenery. And figuring out how each rock began and what it went through is fun for those who like puzzles and unsolved mysteries.

Fortunately for puzzled geologists, the birthplaces oceanic, arc, and continental of the major rocks in this domain are generally recognizable even after thorough metamorphism to schist and gneiss at great depths and pressures see Making Sense of Metamorphic Rocks and Terranes. The four metamorphic terranes, can be described in terms of their major birthplaces. The Chelan Mountains Terrane is made of volcanic arc and deep ocean sediments. The Nason, Swakane, and Little Jack terranes are derived from volcanic arcs alone, but the Swakane terrane is very ancient, and the Little Jack terrane contains pieces of the mantle.

Rocks of the three major birth places:  continental, oceanic, and arc.
Rocks of the three major birth places: continental, oceanic, and volcanic arc.
Something extra: Making Sense of Metamorphic Rocks and Terranes in General
Visit the terranes of the Metamorphic Core Domain
or continue to the Methow Domain

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.

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