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5. Early Volcanic Arc: Oregon's tropical volcanoes

Exotic Terranes
(Lighter shading indicates original extent.)

As the subduction zonesee Plate Tectonics diagram moved westward under the Pacific Ocean about 52 million years ago, volcanic activity from this shift built a broad volcanic arcsee Plate Tectonics diagram across much of the eastern two thirds of the state. The eruptions continued until about 20 million to 6 million years ago and built a huge pile of lava flows, tuff, and interlayered river and lake sediments. Hundreds of volcanoes of all types and sizes erupted across the landscape. Huge explosive calderas erupted enormous ash clouds that deposited ash tuff over large areas. The near-tropical climate supported a wide range of now extinct plants and animals, whose fossils are spectacularly preserved in ash layers and old lake beds in the John Day Fossil Beds National Monument. Much of the ancient arc is now buried in the east by Columbia River Basaltsee Unit 8, Rift Volcanoessee Unit 9, and High Cascade Volcanoessee Unit 12.

OGDC units: Eocene through Miocene volcanic, intrusive and volcaniclastic rocks and interbedded terrestrial sedimentary rocks of the Little Butte, Early Western Cascades and Late Western Cascades, Skamania, Steens Mountains, Tower Mountain, Warner Peak, and Goble Volcanics; Colestine, John Day, Clarno, Alvord Creek, Eagle Creek, Heppsie, Mollala, Pike Creek, Rhododendron, Roxy, Sardine, Scorpion Mountain, and Wasson Formations

Age Range: 52 million to 6 million years ago
Rock Types: basalt, andesite, dacite, rhyolite, mudflow breccia, conglomerate, tuff
Did You Know? Early Volcanic Arc rocks are one of the best sources of Oregon’s state rock, the thunderegg.


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