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North Cascades Geology

Rocks Form Where Plates Collide

Sketch of plate tectonics scheme and formation of the major rock types: igneous, sedimentary, and metamorphic.
Sketch of plate tectonics scheme and formation of the major rock types: igneous, sedimentary, and metamorphic.

The majority of the rocks in mountain ranges were created where plates collided. Ocean-deposited sediment, turned into sedimentary rock, commonly is scraped off, deformed, and transferred to the overriding plate. Some ocean-floor sediments are carried down in the subduction zone, where increasing heat and pressure turn them into metamorphic rocks, such as those forming the foundation of the North Cascades. The subducted oceanic plate and overlying sedimentary rocks are rich in water, which is driven off as the rocks move deeper and get hotter. The rising vapors lower the melting point of the already-hot upper mantle and lower crust. As a result, these melt, in the process expanding and becoming less dense than the unmelted rocks around them. The melted rocks begin to rise buoyantly, working their way toward the surface and exploiting any zones of weakness, such as faults. Where the molten rock reaches the surface, volcanoes form. Much igneous rock is created in this subduction process. All around the Earth, chains of volcanoes form arcs that lie above subducting plates, such as the volcanic islands of Japan or the Aleutians.

Flattened rock with foliation versus stretched rock with lineation.
Mount St. Helens, located in Washington’s southern Cascades.

Off the coast of Washington, the Juan de Fuca plate is slowly subducting under the continent. In Washington, the volcanoes of the Cascade Range are located above where the plate has plunged deeply enough to promote melting. In the region that this guide covers, Mount Baker volcano immediately comes to mind. More notorious is Mount St. Helens, which is located in Washington’s southern Cascades, where in 1980 magma generated deep in the subduction zone reached the surface to erupt violently and subsequently push up a dome in its crater.

On to Moving Plates and Tectonic Terranes
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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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