Text Box:  Cascade Mountains Introduction | Northern Cascades | Mt. Rainier and Mt. St. Helens | More Volcanic Features | Southern Cascades                                                     The Cascade Mountains province extends about 700 miles from the northern border of the Sierra Nevada province, which is just south of Lassen Peak (Mount Lassen) in northern California, northward through Oregon and Washington into southern Canada.  It is only 30-50 miles wide in its southern extremity, and approximately double that width at its widest in northern Washington; most of the province is 40-80 miles wide.  The western border of the Cascade Mountains is consistently about 60-110 miles from the Pacific Ocean.  Although the Cascade Range is crossed by only three rivers, the name is derived from the fact that whitewater rapids or cascades occur in the Columbia River where it cuts through the mountains; an early name for the range was Mountains by the Cascades.  The portion of the province in the United States is mostly a volcanic ridge on which many stratovolcanoes have developed; 15 of these volcanoes are the major landmarks of the province.  The Canadian portion of the province also has a base of volcanic materials with volcanoes on it, the most prominent of which is Mt. Garibaldi.  As noted in the chapter on the Sierra Nevada, some classifications combine the Cascade Mountains and Sierra Nevada into a single province; this is usually based largely on convenience.  The two ranges are distinct enough to be treated as separate provinces.     Geomorphic History About 100 to 90 million years ago perhaps as many as six terranes were added to the western coast of North America in the vicinity of the United States-Canada border.  This resulted from subduction that was occurring at the western edge of the continent.  These terranes were molded together to form what is now called the North Cascades terrane, which covers much of northwest Washington north of the latitude of Seattle.  The docking of this new land caused the cessation of  subduction in the trench just off the coast.  Later a new subduction zone began farther to the west, which is the location of the present-day subduction that is occurring off the coast of Washington and Oregon (see the Pacific Border province).  Slightly less than 60 million years ago the last major terranes to be added to the northwest part of the United States docked.  Folding, large-scale thrust faulting, metamorphism and intrusion of magma accompanied the arrival of many of the new terranes.  Approximately 37 million years ago, at the beginning of the Cascadian orogeny, the oceanic plate descending the trench of the newer, more western subduction zone began to produce volcanic activity slightly inland.  For the next 20 million years the often violent eruptions created many major volcanoes, which collectively are called the Western Cascades.  The volcano building slowed about 17 million years ago, probably because the slab of oceanic crust that was descending in the subduction zone stopped supplying superhot water into the rocks above the zone, although lesser volcanic activity lingered in the Western Cascades for several million more years.  Erosion has erased most of these volcanoes; parts of some survive in the Ochoco  region of central Oregon and on long fault blocks that subsided when the underlying rocks cooled and diminished in size.  As the Western Cascades were ceasing to develop 17 million years ago, a new type of volcanic activity dominated in the Pacific northwest.  Massive amounts of lava, referred to as the Columbia River flood basalt flows, covered a vast area (discussed in detail in the Columbia Plateaus province.)  The lava covered a portion of the area of the eroded Western Cascades and huge quantities of lava poured down the Columbia River Valley, through the Cascades, and into the Pacific Ocean.  There was essentially no volcano building in the region of the Cascades for much of this basalt flooding, but commencing about 13-16 million years ago a renewal of volcano-producing subduction started building another section of Cascades.  These are the High Cascades, and they formed on a nearly straight line about 50 miles east of the now-subdued Western Cascades.  This eastward migration of the line of volcanoes has been surmised to have resulted from the lesser angle of descent of the plate being subducted now, the Juan de Fuca plate, which is slightly lighter than the descending material that yielded the Western Cascades.  Analysis of seismic data indicate the angle is now only 9°, and subduction is occurring at the rate of about 1.5 inches per year.  Because a volcanic chain most commonly is produced above the zone where descending ocean floor material reaches a depth of about 60-70 miles, the lower angle of subduction requires the descending plate to move farther inland from the trench before melting and producing volcanoes.  In this case the result is a volcanic line occurring on the surface about 50 miles farther to the east than the previous volcanoes.  The High Cascades have developed on a broad north-south-trending arch that was elevated several thousand feet in part by expansion of rocks heated by deep-seated magma and partly from folding and faulting.  Basalt that is several thousand feet thick has been extruded on this arch, especially in the middle and southern portions of the province.  Several of the largest stratovolcanoes began their growth erupting primarily basalt, which filled valleys and other low areas, eventually building low shield volcanoes.  This phase of the volcanic activity constructed a broad volcanic plateau, which has served as a base on which the newer stratovolcanoes have developed.  After the initial basaltic eruptive phase, and beginning less than 1 million years ago, the volcanoes emitted progressively lighter varieties of relatively viscous andesite and pyroclastics, and later still some rhyolite.  These materials have produced the composite or stratovolcanoes that today rise above the basaltic base of much of the province.  This latest phase involves magmas that frequently entrap significant amounts of gases, which is conducive to explosive eruptions.  Although the main aspects of this geomorphic history apply generally throughout the entire province, there are regional differences in the characteristics and distribution of the stratovolcanoes, and in the degree and timing of volcanism within this somewhat heterogeneous province.  Because of this diversity, the province is divided into three sections–Northern, Middle and Southern.     Cascade Mountains Introduction | Northern Cascades | Mt. Rainier and Mt. St. Helens | More Volcanic Features | Southern Cascades