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USGS Geology in the Parks

North Cascades Geology

Moving Plates and Tectonic Terranes

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A consequence of an Earth made up of moving plates is that large slabs of crust can be moved great distances around the globe. We have come to realize that mountain belts can contain rocks that could not have formed in the same setting as their neighbors (see Making Sense out of Metamorphic Rocks and Terranes in General). Rocks from the flanks of a volcanic arc on the continent may be cheek by jowl with rocks of the same age that formed in the middle of a deep ocean with nary a volcano in sight.

Sometimes a piece of one plate breaks off and becomes attached to another plate. Commonly such an orphaned fragment will travel along with its new plate only to shear off on yet another plate. These separated, transported pieces are called tectonic terranes or, sometimes, exotic terranes. Each of the three domains making up the foundation of the North Cascades is itself a mosaic of several distinct tectonic terranes .

On to Recognizing the Mountain Mosaic

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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