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North Cascades Geology

Plate Tectonics and Tectonic Terranes

Understanding the basic foundation rocks of the North Cascades became possible only with the birth of the concept of plate tectonics in the late 1960s and early 1970s. Plate tectonics holds that the Earth’s outer shell is made up of many large, relatively rigid plates floating on a denser layer of plastic rock, the lower part of the mantle. These plates move about at speeds no greater than a few inches per year. Where plates collide, mountains rise up; where they pull apart, oceans are formed. At the collision zone between major plates in the Pacific Ocean on the one side and the North American continental plate on the other, not only were many of the rocks now seen in the North Cascades born, but they were smashed on a mind-boggling scale as the plates slowly collided or slid by each other along faults.

The crust floats on the denser mantle.
The crust floats on the denser mantle.
Geophysicists have long known that the Earth is composed of three principal layers: a nickel-iron core, an intermediate layer of very dense rock called the mantle, and a relatively thin (2 to 50 miles) crust of lighter rocks more or less floating on the mantle. Geophysicists discovered this structure by studying sound waves generated by earthquakes as they pass through the Earth. The outermost crust can be further divided into the light rocks of the continents and the heavier, denser rocks of the ocean floors. The distinction between the crust (continental or oceanic) and the underlying mantle is primarily one of chemical composition. Geophysicists also infer another boundary, within the upper part of the mantle, that separates cooler, more rigid rock above from hotter, more plastic rock below. The upper, more rigid rocks both crust and uppermost mantle are collectively termed the lithosphere and form the plates of the Earth’s outer shell.
On to Sea-Floor Spreading
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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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