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plate tectonics

Oregon’s geologic history cannot be told without understanding the process of plate tectonics. Plate tectonics describes the movement of continent-sized slabs of the Earth’s crust and lithosphere as they are moved around the surface of the globe by slow, gigantic currents of hot rock deep in the asthenosphere. Where plates pull apart, lava wells up to form new ocean floor along submarine mountain ranges called spreading ridges. Where an ocean plate collides with a continent, as it currently does off Oregon, the ocean floor subducts, or slides beneath the edge of the continent, and is drawn deep into the Earth, where it causes rock to melt. This process feeds a chain of volcanoes called a volcanic arc. Inland from the volcanic arc a rift zone may open, tearing apart the continent along dozens of faults. Where ocean plates collide with each other, subduction builds a volcanic island arc. The island arc, attached to its underlying plate, then moves along the surface. Hot spots, small areas where intense heat wells up from deep in the Earth, feed huge volcanoes at the surface. Hot spots are fixed in place, so as the crust and lithosphere drift across the spot, a trail of volcanoes is left on the plate, like the Hawaiian island chain. The processes of plate tectonics, particularly subduction, have shaped Oregon throughout its entire geologic history and continue to shape the state today.

This diagram shows plate tectonics processes that have been active at one time or another in Oregon's geologic history. Today’s plate tectonics look very much like the right half of the diagram, with the active Cascadia Subduction Zone off the Oregon coast, an active volcanic arc in the Cascade Range, and a rift zone forming in eastern Oregon. Tens of million years from now, as Oregon continues to change, the picture will look very different.

Subduction zone diagram modified from an image by José F. Vigil and Robert I. Tilling, courtesy of U.S. Geological Survey.

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