# 5.2: Plate Tectonics

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• 5.2.1: The Earth’s Crust; Not Very Well Designed
The principal cause of crustal weakness is geothermal heat. Isotopes of radioactive elements within the Earth decay to other isotopes, producing heat that is trapped beneath the surface. Because of this trapped heat, the crust is warmer with increasing depth, as anyone knows who has ever descended into a deep mine. Geothermal heat warms the City of Klamath Falls, Oregon, heats the hot springs of the Pacific Northwest, and causes the eruption of great volcanoes like Mt. St. Helens.
• 5.2.2: Continents and Ocean Basins
Unlike the other inner planets, the surface of the Earth is at two predominant levels, one averaging 2,750 feet (840 m) above sea level, making up the continents, where we all live, and the other averaging 12,100 feet (3,700 m) below sea level, making up the ocean basin. If you were able to look at the Earth with the water removed, the continents, together with their submerged continental shelves, would appear as gigantic plateaus, with steep slopes down to the ocean basins below.
• 5.2.3: The Dance of the Plates; We Know the Beat but Not the Tune
The dominant cause of the tectonic activity that takes place at the Earth’s surface is the extremely slow flow of rock in the mantle that is solid, yet ductile. This leads us now to a discussion of plate tectonics. We would have no earthquake problem if the lithosphere, 60 miles thick, completely encircled the Earth without any breaks. Unfortunately, the 60-mile thickness of the lithosphere is not enough to withstand the stresses coming from the roiling currents of the asthenosphere below.
• 5.2.4: A Brief Thirty-Million-Year History of Western North America
Using sophisticated computer models, it is fairly straightforward to work out the plate-tectonic history of the Earth for hundreds of millions of years. This is illustrated in five stages in Figure 2-7, based on a copyrighted video created by Prof. Tanya Atwater of the University of California Santa Barbara through its Educational Multimedia Visualization Center. For an explanation in greater depth, see Atwater (1989). What did the Pacific Northwest look like thirty million years ago?

This page titled 5.2: Plate Tectonics is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Robert S. Yeats (Open Oregon State) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.