Oregon
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The spectacular landscape of Oregon today was shaped over millions of years by fire and flood, earthquakes and eruptions, and the movement of oceans and continents on a huge scale. Where most people see an unchanging landscape, geologists see rocks that tell the story of an ever-shifting world. It is a story that began several hundred million years ago, an eternity for humans, but only about 4% of the Earth's 4.5 billion year history. And what a story! Imagine an oceanic and a continental plate colliding, pushing hundreds of miles of ocean floor deep into the body of the Earth. Imagine some of the largest lava flows in the history of the world blanketing the state, thousands upon thousands of volcanic eruptions, and floods that carry enough water to equal the combined flow of all the rivers in the world. This is the Oregon that geologists see.

"Oregon: A Geologic History" (DOGAMI Interpretive Map 28, 2009) compiles rock units from the Oregon Geologic Data Compilation into groups reflecting the major events in the geologic history of Oregon. That history is dominated by subduction of oceanic plates beneath the western margin of North America. Late Paleozoic to Paleocene subduction accreted exotic terranes to form the foundation for the state. From the Eocene through today, arc volcanism has built a huge pile of volcanic rocks, capped by the glaciated volcanoes of the High Cascade Range. Forearc sedimentation from the Paleocene though today built a thick wedge of marine sedimentary rocks beneath the Oregon Coast Range. In the Miocene the Yellowstone hotspot triggered the Columbia River flood basalt flows which buried half of the state under thick black lava. The geologic excitement continued through the Quaternary, with ongoing eruptions from dozens of volcanoes like Mt. Mazama, colossal floods from glacial Lake Missoula and pluvial Lake Bonneville, tens of thousands of landslides, and subduction megathrust earthquakes on the Cascadia Subduction Zone every half century.
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.

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