![]() To the casual observer, many of the smaller vents and volcanoes are merely bumps on the landscape. Nearly all of these have been active since four million years ago. The High Cascades in Oregon are coincident with the currently active volcanic arc, a nearly continuous band of large, long-lasting volcanic centers and smaller volcanoes, cinder cones, and lava domes. Consequently, streams were able to erode unabated, deepening and broadening their courses and continuing to create relief in the Western Cascades. The Western Cascades subprovince, with only sparse small volcanoes active in the past four million years, was spared the periodic infilling of its canyons by extensive volcanic deposits. Second, at about that time, volcanism was focused along the axis of the High Cascades, filling old canyons and building up the spine of the range. First, broad uplift of the Western Cascades sometime after 8 million years ago created steep rivers that propelled erosional downcutting. In contrast, the High Cascades subprovince is little eroded. ![]() West-flowing and northwest-flowing streams have carved canyons as deep as 3,700 feet, measured from adjacent ridgetops to canyon floors. The Western Cascades, or Old Cascades in some tourist logs, encompasses volcanic rocks as old as 45 million years. In Oregon, the Cascade Range is customarily divided into two physiographic subprovinces, the Western Cascades and the High Cascades (fig. The molten rock supplies the volcanic arcs with heat and magma. Cascade Range volcanoes are part of the Ring of Fire, a popular term for the numerous volcanic arcs that encircle the Pacific Ocean. As this water rises, it lowers the melting temperature in the overlying hot mantle rocks, thereby promoting melting. The volcanoes and their eroded remnants are the visible magmatic expression of the Cascadia subduction zone, where the offshore Juan de Fuca tectonic plate is subducted beneath North America. Subduction occurs as two lithospheric plates collide, and an underthrusted oceanic plate is commonly dragged into the mantle by the pull of gravity, carrying ocean-bottom rock and sediment down to where heat and pressure expel water. On the west it reaches nearly to Interstate 5, forming the eastern margin of the Willamette Valley and, farther south, abutting the Coast Ranges.Īlong its Oregon segment, the Cascade Range is almost entirely volcanic in origin. Oregon’s Cascade Range covers roughly 17,000 square miles, or about 17 percent of the state, an area larger than each of the smallest nine of the fifty United States. In Oregon, it comprises the Cascade Range, which is 260 miles long and, at greatest breadth, 90 miles wide (fig. The Cascade mountain system extends from northern California to central British Columbia.
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