Geology of the Pacific Northwest

The Cascade Volcanoes The Cascades Province forms an arc-shaped band extending from southwestern British Columbia to Northern California, roughly parallel to the Pacific coastline. Within this region, nearly 20 major volcanic centers lie in sequence. erupts on May 18, 1980 Although the largest volcanoes like Mount St. Helens get the most attention, the Cascade Volcanic Arc includes a band of thousands of very small, short-lived volcanoes that have built a platform of lava and volcanic debris. Rising above this volcanic platform are a few strikingly large volcanoes that dominate the landscape. Beneath the Cascade Volcanic Arc, a dense oceanic plate sinks beneath the North American Plate; a process known as subduction. As the oceanic slab sinks deep into the Earth's interior beneath the continental plate, high temperatures and pressures allow water molecules locked in the minerals of solid rock to escape. The supercritical water rises into the pliable mantle above the subducting plate, causing some of the mantle to melt. This newly formed magma ascends upward through the crust along a path of least resistance, both by way of fractures and faults as well as by melting wall rocks. The addition of melted crust changes the geochemical composition. Some of the melt rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Volcanic Arc) above the subduction zone. The Wells Gray-Clearwater volcanic field in south-eastern British Columbia consists of several small basaltic volcanoes and extensive lava flows that have been active for the past 3 million years. It is within the Wells Gray Provincial Park, which also includes the -high Helmcken Falls. The origin of the volcanism is unknown, but is probably related to crustal thinning. Some of the lava flows in the field are similar to those that erupted at Volcano Mountain in the Yukon, where olivine nephelinite occurs. The last eruption in the field was about 400 years ago at Kostal Cone. Numerous seamounts lie off British Columbia's coast and are related to hotspot volcanism. The Bowie Seamount located west of Haida Gwaii is perhaps the shallowest seamount in Canada's Pacific waters. Because of its shallow depth, scientists believe it was an active volcanic island throughout the last ice age. The Bowie Seamount is also the youngest seamount in the Kodiak-Bowie Seamount chain. Volcanic disasters The last eruption of the Tseax Cone around the years 1750 or 1775 is Canada's worst known geophysical disaster. The eruption produced a long lava flow, destroying the Nisga'a villages and the death of at least 2000 Nisga'a people by volcanic gases and poisonous smoke. The Nass River valley was inundated by the lava flows and contain abundant tree molds and lava tubes. The event coincided with the arrival of the first European explorers to penetrate the uncharted coastal waters of northern British Columbia. Today, the basaltic lava deposits are a draw to tourists and are part of the Nisga'a Memorial Lava Bed Provincial Park. Recent volcanic activity , Oregon, erupted roughly 5000 years BCE The Pacific Northwest volcanoes continue to be a geologically active area. The most geologically recent volcanic eruptions include: • Level Mountain, Canada's most voluminous and most persistent eruptive center, might have erupted in the Holocene. • Nazko Cone, the youngest volcano in the Anahim Volcanic Belt, erupted 7200 BP. • Hoodoo Mountain erupted 7050 BP. • Lava Butte, Oregon erupted about 7,000 years ago. • Mount Mazama, which erupted catastrophically in 5670 BCE to form Crater Lake. • Mount Meager massif erupted about 2350 BP, sending an ash column high into the stratosphere. • Mount Edziza volcanic complex, Canada's second largest eruptive center, erupted about 1340 BP. • Medicine Lake Volcano erupted about 1000 BP. • Silverthrone Caldera might have eruptions younger than 1000 BP. • Kostal Cone in the Wells Gray-Clearwater volcanic field might have erupted and formed in 1500 based on tree-ring dating. • Glacier Peak erupted in the 17th or 18th century. • Tseax Cone erupted in the 18th century. • Mount Hood erupted in 1781–82; fumaroles on the summit still spew sulfurous gas. • Mount Shasta erupted in 1786. • The Volcano erupted about 150 BP, producing a long lava flow. • Mount Rainier erupted 1854. • Mount Baker erupted in 1880; fumaroles still occur at its summit. • Ruby Mountain might have erupted in 1898. • Lassen Peak erupted in 1914–5. • Mount St. Helens erupted in 1980, killing 57 people. (see 1980 eruption of Mount St. Helens). ==Seismic activity==

after the Nisqually earthquake The Pacific Northwest is seismically active. The Juan de Fuca Plate is capable of producing megathrust earthquakes of moment magnitude 9: the last such earthquake was the 1700 Cascadia earthquake, which produced a tsunami in Japan, and may have temporarily blocked the Columbia River with the Bonneville Slide. More recently, in 2001, the Nisqually earthquake (magnitude 6.8) struck northeast of Olympia, Washington, causing some structural damage and panic. In addition, eleven volcanoes in Canada have had seismic activity since 1975, including: the Silverthrone Caldera, Mount Meager massif, Wells Gray-Clearwater volcanic field, Mount Garibaldi, Mount Cayley, Castle Rock, The Volcano, Mount Edziza volcanic complex, Hoodoo Mountain, Crow Lagoon and Nazko Cone. ==Columbia Plateau==

The Columbia Plateau province is enveloped by one of the world's largest accumulations of basalt. Over of the Earth's surface is covered by it. The topography here is dominated by geologically young lava flows that inundated the countryside with amazing speed, all within the last 17 million years. The ice blocked the Clark Fork River, forming the huge Glacial Lake Missoula. The lake measured about and contained about , half the volume of Lake Michigan. Glacial Lake Missoula broke through the ice dam many times, allowing a tremendous volume of water to rush across northern Idaho and into eastern Washington. Such catastrophic floods raced across the southward-dipping plateau a number of times, etching the coulees which characterize this region, now known as the channeled scablands. As the floods in this vicinity raced southward, two major cascades formed along their course. The larger cataract was that of the upper Grand Coulee, where the river roared over an waterfall. The eroding power of the water plucked pieces of basalt from the precipice, causing the falls to retreat and self-destruct by cutting through to the Columbia River valley near what is now the Grand Coulee Dam. The other major cataract is now known as Dry Falls. It started near Soap Lake in Washington State, where less resistant basalt layers gave way before the great erosive power of this tremendous torrent and waterfalls developed. As in the upper Grand Coulee, the raging river yanked chunks of rock from the face of the falls and the falls eventually retreated to their present location. Dry Falls is wide, with a drop of more than . By way of comparison, Niagara Falls, wide with a drop of only , would be dwarfed by Dry Falls. ==The North Cascades==

The North Cascade Range in Washington is part of the American cordillera, a mountain chain stretching more than from Tierra del Fuego to the Alaska Peninsula, and second only to the Alpine-Himalayan chain in height. Although only a small part of the Cordillera, mile for mile, the North Cascade Range is steeper and wetter than most other ranges in the contiguous United States. In geology, the range has more in common with the Coast Ranges of British Columbia and Alaska than it does with its Cordilleran cousins in the Rocky Mountains or Sierra Nevada. Although the peaks of the North Cascades do not reach great elevations (high peaks are generally in the range, their overall relief, the relatively uninterrupted vertical distance from valley bottom to mountain top, is commonly . Rocks of the North Cascades record at least 400 million years of history. The record of this long history can be read in the many rock layers deposited over time through the forces of erosion, volcanic activity and plate subduction. These different forces have made a geologic mosaic made up of volcanic island arcs, deep ocean sediments, basaltic ocean floor, parts of old continents, submarine fans, and even pieces of the deep subcrustal mantle of the earth. The disparate pieces of the North Cascade mosaic were born far from one another but subsequently drifted together, carried along by the tectonic plates that make up the Earth's outer shell or were uplifted, eroded by streams, and then locally buried in their own eroded debris; other pieces were forced deep into the Earth to be heated and squeezed, almost beyond recognition, and then raised again to view. Over time, the moving plates eventually accreted the various pieces of the mosaic onto the western side of North America. About 35 million years ago, a volcanic arc grew across this complex mosaic of old terranes. Volcanoes erupted to cover the older rocks with lava and ash. Large masses of molten rock invaded the older rocks from below. The volcanic arc is still active today, decorating the skyline with the cones of Mount Baker and Glacier Peak. The deep canyons and sharp peaks of today's North Cascades scene are products of profound erosion. Running water has etched out the grain of the range, landslides have softened the abrupt edges, homegrown glaciers have scoured the peaks and high valleys and, during the Ice Age, the Cordilleran Ice Sheet overrode almost all the range and rearranged courses of streams. Erosion has written and still writes its own history in the mountains, but it has also revealed the complex mosaic of the bedrock. ==Coast Mountains==

The Coast Mountains are the western range of the North American mainland cordillera, covering the Alaska Panhandle and most of coastal British Columbia. The range is approximately long and wide. Most of the Coast Mountains are composed of granite, which is part of the Coast Plutonic Complex. This is the single largest contiguous granite outcropping in the world, which extends approximately in length. It is a large batholith complex. Its formation is related to subduction of the Kula and Farallon tectonic plates along the continental margin during the Jurassic-to-Eocene periods. The plutonic complex is built on unusual island arc fragments, oceanic plateaus and continental margin assemblages accreted between the Triassic and the Cretaceous periods. In addition, the Garibaldi, Meager, Cayley and Silverthrone areas are of recent volcanic origin. The Coast Mountains consist of a single uplifted mass. During the Pliocene period the Coast Mountains did not exist and a level peneplain extended to the sea. This mass was uplifted during the Miocene period. Rivers such as the Klinaklini River and Homathko River predate this uplift and due to erosion occurring faster than uplift, have continued to flow right up to the present day, directly across the axis of the range. The mountains flanking the Homathko River are the highest in the Coast Mountains, and include Mount Waddington west of the river in the Waddington Range and Mount Queen Bess east of the river, adjacent to the Homathko Icefield. The Pacific Ranges in southwestern British Columbia are the southernmost subdivision of the Coast Mountains. It has been characterized by rapid rates of uplift over the past 4 million years unlike the North Cascades and has led to relatively high rates of erosion. ==Insular Mountains==

The Insular Mountains on the coast of British Columbia have not yet fully emerged above sea level, and Vancouver Island and Haida Gwaii are just the higher elevations of the range, which was in fact fully exposed during the last ice age when the continental shelf in this area was a broad coastal plain. Although the Coast Mountains are commonly considered to be the westernmost range of the American cordillera, the Insular Mountains are the true westernmost range. Through the most recent ice age about 18,000 years ago, ice enclosed nearly all of the mountains. Glaciers that ran down to the Pacific Ocean sharpened the valley faces and eroded their bottoms. on Vancouver Island was formed by erosion carving into basalt. The Insular Mountains were formed when a large island arc, called the Insular Islands, collided against North America during the Mid-Cretaceous period. The mountains are made of turbidite and pillow lavas unlike the plutons of the Coast Plutonic Complex that make the Coast Mountains. The Insular Mountains have much seismic activity, with the Juan de Fuca Plate subducting at the Cascadia subduction zone and the Pacific Plate sliding along the Queen Charlotte Fault. Large earthquakes have led to collapsing mountains, landslides, and the development of fissures. Flood basalts on Vancouver Island form a geologic formation called the Karmutsen Formation, which is perhaps the thickest accreted section of an oceanic plateau worldwide, exposing up to of basal sediment-sill complexes, basaltic to picritic pillow lavas, pillow breccia, and thick, massive basalt flows. ==See also==