The oceanic sediment limestone and shale and ocean

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The oceanic sediment (limestone and shale) and ocean floor (basalt) metamorphosed during continued subduction to form marble, greenstone and slate. Region 7: Columbia River Basalts About 17 million years ago a series of rifts appeared in southeastern Washington. From these rifts an immense amount of basalt was erupted. The source of this basalt may have been the Yellowstone hotspot plume. Some of the flows extended all the way from Central Washington (Yakima) to the Washington coast (Astoria) (Fig. 6). In places these basalts accumulated and are more than 3,000 meters thick, and cover an area of more than 170,000 km 2 . Most of the flows (96%) were erupted between 17-14.5 million years ago, but the youngest flows are only 6 million years old. The Columbia River Basalts are one of the largest preserved outpourings of lava in the history of the earth.
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Figure 7. Cascade Volcanic Arc and subduction of the Juan de Fuca Plate beneath the North America Plate. Figure 8. Map of the Cordilleran Ice Sheet and the Missoula floods. Location of ice dam Lab #8: Geology of Washington 112 Region 8: Cascades Volcanic and Plutonic Arc About 40 million years ago, the Cascadia Subduction Zone developed off the coast between Northern California and British Columbia. Great magma chambers formed as the sea floor of the Juan de Fuca plate was subducted to depths of 90-100 km beneath the western margin of North America. As the magma moved upward towards the surface, most of it crystallized deep underground as plutons of granitic rock. Some magma made it all the way to the surface, producing rhyolitic and andesitic lava and pyroclastic flows. The Cascade Arc has been active for ~40 million years. Older mountains have since eroded, and new mountains are being formed. The current Cascade range began uplift ~14 million years ago. Mt. Baker, Glacier Peak, Mt. Rainier, Mt. Adams, and Mt. St. Helens are the youngest volcanic peaks in Washington State (formation started ~700 million years ago). More than a dozen major volcanic peaks in British Columbia, Oregon and California also exist. Hundreds of deeply eroded, older volcanoes are still present. Region 9: Cordilleran Ice Sheet and Glacial Lake Missoula During the last 2 million years, continental glaciers have covered parts of Washington State at least 6 times. The most extensive ice sheet developed during the most recent event called Fraser Glaciation between ~20,000-11,000 years ago. The Cordilleran Ice Sheet map shows the maximum of this advance ~15,000 years ago (Fig. 8). During this time the glacier covered the Puget Lowlands and most of the northern part of Eastern Washington. The WWU campus was buried well over a mile deep in ice. These glaciers carried large quantities of sediment within the ice. The glaciation also eroded and carved much of the landscape in the Puget Sound Area.
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