Geology of the Pacific Northwest

The geology of the Pacific Northwest refers to the study of the composition (including rock, minerals, and soils), structure, physical properties and the processes that shape the Pacific Northwest region of the United States and Canada. The geology of the region produces much of the area's scenic beauty and also causes periodic catastrophes. There are more than three main geologic provinces in the area. Some of these are the Cascade Volcanoes, the Columbia Plateau, and the North Cascades. The Cascade Volcanoes are an active volcanic region along the western side of the Pacific Northwest. The Columbia Plateau is a region of subdued geography that is inland of the Cascade Volcanoes, and the North Cascades are a mountainous region in the northwest corner of the United States, extending into Canada.

The geology of the Pacific Northwest is vast, complex and confusing. Most of the region was formed about 200 million years ago as the North American Plate started to drift westward during the rupture of Pangaea. Since that date, the western edge of North America has grown westward as a succession of island arcs and assorted ocean-floor rocks have been added along the continental margin.

Volcanoes

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 like a string of explosive pearls.cite web|url=http://wrgis.wr.usgs.gov/docs/parks/province/cascade1.html|title=Pacific - Cascades Volcanic Province|work=USGS Geology in the Parks|accessdate=2007-04-22]

Although the largest volcanoes like Mount St. Helens get the mostattention, the Cascade Volcanic Arc is really made up of a band of thousands of verysmall, short-lived volcanoes that have built a platform of lava andvolcanic debris. Rising above this volcanic platform are a few strikingly large volcanoes that dominate the landscape.

The Cascade volcanoes define the Pacific Northwest section of the Ring of Fire, an array of volcanoes that rim the Pacific Ocean. The Ring of Fire is also known for its frequent earthquakes. The volcanoes and earthquakes arise from a common source: subduction.cite web|url=http://wrgis.wr.usgs.gov/docs/parks/province/cascade2.html|title=Pacific - Cascades Volcanic Province|work=USGS Geology in the Parks|accessdate=2007-04-22]

Beneath the Cascade Volcanic Arc, a dense oceanic plate plunges beneath the North American Plate; a process known as subduction. As the oceanic slab sinks deep into the Earth's interior beneath thecontinental plate, high temperatures and pressures allow water moleculeslocked in the minerals of solid rock to escape. The water vapor rises into the pliable mantle above the subducting plate, causing some of the mantle to melt. This newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Volcanic Arc) above the subduction zone.

A close-up look at the Cascades reveals a more complicated picture than a simple subduction zone.

Not far off the coast of the North Pacific lies a spreading ridge;a divergent plate boundary made up of a series of breaks in the oceanic crust where new ocean crust is created. On one side of the spreading ridge new Pacific Plate crust is made, then moves away from the ridge. On the other side of the spreading ridge the Juan de Fuca and Gorda Plates move eastward.

There are some unusual features at the Cascade subduction zone. Where the Juan de Fuca Plate sinks beneath the North American Plate there is no deep trench, seismicity (earthquakes) are fewer than expected, and there is evidence of a decline in volcanic activity over the past few million years. The probable explanation lies in the rate of convergence between the Juan de Fuca and North American Plates. These two plates converge at 3-4 centimeters per year at present. This is only about half the rate of convergence of 7 million years ago.

The small Juan de Fuca Plate and two platelets, the Explorer Plate and Gorda Plate are the meager remnants of the much larger Farallon oceanic plate. The Explorer Plate broke away from the Juan de Fuca about 4 million years ago and shows no evidence that it is still being subducted. The Gorda platelet split away between 18 and 5 million years ago and continues to sink beneath North America.

The Cascade Volcanic Arc made its first appearance 36 million years ago, but the major peaks that rise up from today's volcanic centers were born within the last 1.6 million years. More than 3000 vents erupted during the most recent volcanic episode that began 5 million years ago. As long as subduction continues, new Cascade volcanoes will continue to rise.

Volcanism outside the Cascades

The Garibaldi Volcanic Belt in southwestern British Columbia is the northern extension of the Cascade Volcanic Arc in the United States and contains the most explosive young volcanoes in Canada. Like the rest of the arc, it has its origins in the Cascadia subduction zone. Volcanoes of the Garibaldi Volcanic Belt have been sporadically active over a time span of several millions of years. The northernmost member, Mount Meager, was responsible for a major catastrophic eruption that occurred about 2,350 years ago. This eruption may have been close in size to that of the 1980 eruption of Mount St. Helens. Ash from this eruption can be traced eastward to western Alberta. It is also the most unstable volcanic massive Canada, which has dumped clay and rock several meters deep into the Pemberton Valley at least three times during the past 7,300 years. Hot springs in the vicinity of Mount Cayley and Mount Meager suggest that magmatic heat is still present. The long history of volcanism in the region, coupled with continued subduction off the coast, suggests that volcanism has not yet ended in the Garibaldi Volcanic Belt. A few isolated volcanic centers northwest of Mount Meager such as the Franklin Glacier Volcano and the Silverthrone Caldera, which lie in the Pemberton Volcanic Belt, may also be the product of Cascadia subduction, but geologic investigations have been very limited in this remote region. About 5-7 million years ago, the northern end of the Juan de Fuca Plate broke off along the Nootka Fault to form the Explorer Plate, and there is no definitive consensus among geologists on the relation of the volcanoes north of that fault to the rest of the Cascade Arc. However, the Pemberton Volcanic Belt is usually merged with the Garibaldi Volcanic Belt, making Mount Silverthrone the northernmost, but an uncertain Cascadia subduction-related volcano.

The most active volcanic region of the northern Pacific Northwest is called the Northern Cordilleran Volcanic Province (sometimes called the Stikine Volcanic Belt). It contains more than 100 young volcanoes and several eruptions known to have occurred within the last 400 years. The last eruptions within the volcanic belt was about 150 years ago at Lava Fork in the Iskut-Unuk River Cones volcanic field. The most voluminous and most persistent eruptive center within the belt and in Canada is the Level Mountain Range. It is a largeshield volcano that covers an area of 1,800 km² southwest of Dease Lake and north of Telegraph Creek. The broad dissected summit region consists of trachytic and rhyolitic lava domes and was considered to be dotted with several minor basaltic vents of postglacial age, although considered Holocene activity to be uncertain. The Mount Edziza volcanic complex is perhaps the most spectacular volcanic edifice in British Columbia. It is the second largest persistent eruptive center within the Northern Cordilleran Volcanic Province and is flanked with numerous young satellite cones, including the young, well-preserved Eve Cone. There are some indications that the Level Mountain Range and Mount Edziza volcanic complex may be between 11 and 9 million years old.

The Anahim Volcanic Belt is a volcanic belt that stretches from just north of Vancouver Island to near Quesnel. It is thought to have formed as a result of the North American Plate moving over a stationary hotspot, similar to the hotspot feeding the Hawaiian Islands, called the Anahim hotspot. The youngest volcano within the volcanic belt is Nazko Cone. It last erupted about 7,000 years ago, producing two small lava flows that traveled 1 km to the west, along with a blanket of volcanic ash that extends several km to the north and east of the cone. The volcanic belt also contains three large shield volcanoes that were formed between 8 and 1 million years ago, called the Ilgachuz Range, Rainbow Range and the Itcha Range.

The Chilcotin Plateau Basalts in southern British Columbia is a north-south range of volcanoes, thought to have formed as a result of back-arc extension behind the Cascadia subduction zone. The majority of the eruptions in this belt happened either 6 to 10 million years ago (Miocene) or 2-3 million years ago (Pliocene). However, there have been few eruptions in the Pleistocene. [ [http://gsc.nrcan.gc.ca/volcanoes/map/index_e.php Volcanoes of Canada] -Map of Canadian volcanoes. Retrieved on 2007-06-24 ]

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. [ [http://gsc.nrcan.gc.ca/volcanoes/cat/belt_wells_e.php Catalogue of Canadian volcanoes: Wells Gray-Clearwater volcano field] Retrieved on 2007-07-25 ] It is within the Wells Gray Provincial Park, which also includes the convert|465|ft|m|sing=on-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 180 kilometers west of the Queen Charlotte Islands, 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 is Canada's worst known geophysical disaster. The eruption produced a 22.5 km 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 happened at the same time 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 Beds Provincial Park.

Recent volcanic activity

The Pacific Northwest volcanoes continue to be a geologically active area. The most geologically recent volcanic eruptions include:
* Level Mountain Range, Canada's most voluminous and most persistent eruptive center, might have erupted sometime during 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 BC to form Crater Lake.
* Mount Meager erupted about 2350 BP, sending an ash column 20 km high into the stratosphere. (see 2350 BP eruption of Mount Meager).
* 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 AD
* Possible eruptions in the Wells Gray-Clearwater volcanic field in 1500.
* Glacier Peak erupted sometime during the 17th-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.
* Lava Fork erupted about 150 BP, producing a 22.5 km 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).

The area is also seismically active. 11 volcanoes in Canada appear related to seismic activity since 1975, including: the Silverthrone Caldera, Mount Meager, Wells Gray-Clearwater volcanic field, Mount Garibaldi, Mount Cayley, Castle Rock, Lava Fork Valley, Mount Edziza volcanic complex, Hoodoo Mountain, Crow Lagoon and Nazko Cone. [ [http://www.bcminerals.ca/pdf/CanadianVolcanoes-CH2005.pdf Volcanoes of Canada] Retrieved on 2007-06-24 ]

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 [cite web|url=http://earthquake.usgs.gov/regional/pacnw/paleo/greateq/index.html|title=Great Cascadia Earthquake Penrose Conference|work=USGS Earthquake Hazards Program|accessdate=2007-04-23] , and may have temporarily blocked the Columbia River with the Bonneville Slide.Fact|date=April 2007 More recently, in 2001, the Nisqually earthquake (magnitude 6.8) struck convert|10|mi|km northeast of Olympia, Washington, causing some structural damage and panic.

Columbia Plateau

The Columbia Plateau province is enveloped by one of the world's largestaccumulations of lava. Over 500,000 km² of the Earth's surface is coveredby it. The topography here is dominated by geologically young lava flowsthat inundated the countryside with amazing speed, all within the last 17million years.cite web
url=http://wrgis.wr.usgs.gov/docs/parks/province/columplat.html
title=Columbia Plateau Province|work=USGS Geology in theParks|accessdate=2007-04-22]

Over 170,000 cubic kilometers of basaltic lava, known as the [ Columbia River basalts, covers the western part of the province. These tremendous flowserupted between 17-6 million years ago. Most of the lava flooded out in thefirst 1.5 million years: an extraordinarily short time for such anoutpouring of molten rock.

The Snake River Plain stretches across Oregon, through northern Nevada,southern Idaho, and ends at the Yellowstone Plateau in Wyoming. Lookinglike a great spoon scooped out the Earth surface, the smooth topography ofthis province forms a striking contrast with the strong mountainous fabricaround it.

The Snake River Plain lies in a distinct depression. At the western end,the base has dropped down along normal faults, forming a grabenstructure. Although there is extensive faulting at the eastern end, thestructure is not as clear.

Like the Columbia River region, volcanic eruptions dominate the story ofthe Snake River Plain in the eastern part of the Columbia PlateauProvince. The earliest Snake River Plain eruptions began about 15 millionyears ago, just as the tremendous early eruptions of Columbia River Basaltwere ending. But most of the Snake River Plain volcanic rock is less than afew million years old, Pliocene age (5-1.6 million years ago) andyounger.

In the west, the Columbia River Basalts are just that:almost exclusivelyblack basalt. Not so in the Snake River Plain, where relatively quieteruptions of soupy black basalt lava flows alternated with tremendousexplosive eruptions of rhyolite, a light-colored volcanic rock.

Cinder cones dot the landscape of the Snake RiverPlain. Some are aligned along vents, the fissures that fed flows andcone-building eruptions. Calderas, great pits formed by explosivevolcanism, and low shield volcanoes, and rhyolite hills are also part ofthe landscape here, but many are obscured by later lava flows.

Evidence suggests that some concentrated heat source is melting rockbeneath the Columbia Plateau Province. At the base of the lithosphere (thelayer of crust and upper mantle that forms Earth's moving tectonicplates). In an effort to figure out why this area, far from a plateboundary, had such an enormous outpouring of lava, scientists establishedhardening dates for many of the individual lava flows. They found that theyoungest volcanic rocks were clustered near the Yellowstone Plateau, andthat the farther west they went, the older the lavas.

Although scientists are still gathering evidence, a probable explanation isthat a hot spot, an extremely hot plume of deep mantle material, is rising to the surface beneath the Columbia Plateau Province. Geologists know thatbeneath Hawaii and Iceland, a temperature instability develops (for reasonsnot yet well understood) at the boundary between the core and mantle. Theconcentrated heat triggers a plume hundreds of kilometers in diameter thatascends directly through to the surface of the Earth.

When the hot plume arrives at the base of the lithosphere, some of thelighter rock of the lithosphere rapidly melts. It is this moltenlithosphere that becomes the basalt lavas that gush onto the surface toform the Columbia River and Snake River Plain basalts.

The track of this hot spot starts in the west and sweeps up to Yellowstone National Park. The steaming fumaroles and explosive geysers are ampleevidence of a concentration of heat beneath the surface. The hotspot isprobably quite stationary, but the North American plate is moving over it,creating a superb record of the rate and direction of plate motion.

The Ice Age floods

With the beginning of the Pleistocene time (about one million years ago), cooling temperatures provided conditions favorable for the creation of continental glaciers. Over the centuries, as snowfall exceeded melting and evaporation, a great accumulation of snow covered part of the continent, forming extensive ice fields. This vast continental ice sheet reached a thickness of about convert|4000|ft|m in some areas. Sufficient pressure on the ice caused it to flow outward as a glacier. The glacier moved south out of Canada, damming rivers and creating lakes in Washington, Idaho and Montana.cite web|url=http://www.nps.gov/laro/naturescience/geology.htm|title=Lake Roosevelt National Recreation Area - Geology|work=nps.gov|accessdate=2007-04-23]

The ice blocked the Clark Fork River, forming the huge Glacial Lake Missoula. The lake measured about 7 770 km² (3,000 square miles) and contained about 2100 cubic kilometers (500 cubic miles), half the volume of Lake Michigan. [cite book | last =Bjornstad | first =Bruce N. | title =On the trail of the Ice Age floods : a geological field guide to the mid-Columbia basin / Bruce Bjornstad. | publisher =Keokee Books | date =c2006 | location =Sandpoint, Idaho | pages =p. 4 | id = ISBN 9781879628274]

Glacial Lake Missoula eventually broke through the ice dam, 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 Coulee, where the river roared over an convert|800|ft|m|sing=on waterfall. The eroding power of the water plucked pieces of basalt from the precipice, causing the falls to retreat convert|20|mi|km 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, where less resistant basalt layers gave way before the great erosive power of this tremendous torrent and waterfalls developed. As in the Upper 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 three and one-half miles wide, with a drop of more than convert|400|ft|m. By way of comparison, Niagara Falls, one mile (1.6 km) wide with a drop of only convert|165|ft|m, 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 convert|12000|mi|km from
Tierra del Fuego to the Alaska Peninsula, and second only to theAlpine-Himalayan chain in height. Although only a small partof the Cordillera, mile for mile, the North Cascade Range is steeper andwetter than most other ranges in the conterminous United States. cite web|url=http://wrgis.wr.usgs.gov/docs/parks/noca/nocageol1.html|title=North Cascades Geology|work=Geology of North Cascades National Park|accessdate=2007-04-22]

In geology, the range has more in common with the Coast Ranges of BritishColumbia and Alaska than it does with its Cordilleran cousins in the RockyMountains or Sierra Nevada. Although the peaks of the North Cascades do notreach great elevations (high peaks are generally in the 7,000 to convert|8000|ft|m|sing=onrange), their overall relief, that is, the relatively uninterruptedvertical distance from valley bottom to mountain top, is commonly 4,000 to6,000 feet.

Rocks of the North Cascades record at least 400 million years of history:time enough to have collected a jumble of different rocks. The range is ageologic mosaic made up of volcanic island arcs, deep ocean sediments,basaltic ocean floor, parts of old continents, submarine fans, andeven pieces of the deep subcrustal mantle of the earth. The disparatepieces of the North Cascade mosaic were born far from one another butsubsequently drifted together, carried along by the tectonic plates thatmake up the Earth's outer shell. Over time, the moving plates eventuallyaccreted the various pieces of the mosaic onto the western side of NorthAmerica.

As if this mosaic of unrelated pieces were not complex enough, some of theassembled pieces were uplifted, eroded by streams, and thenlocally buried in their own eroded debris; other pieces were forced deepinto the Earth to be heated and squeezed, almost beyond recognition, andthen raised again to view.

About 35 million years ago, a volcanic arc grew across this complex mosaicof old terranes. Volcanoes erupted to cover the older rocks with lava andash. Large masses of molten rock invaded the older rocks from below. Thevolcanic arc is still active today, decorating the skyline with the conesof Mount Baker and Glacier Peak.

The deep canyons and sharp peaks of today's North Cascades scene areproducts of profound erosion. Running water has etched out the grain of therange, landslides have softened the abrupt edges, homegrown glaciers havescoured the peaks and high valleys and, during the Ice Age, the
Cordilleran Ice Sheet overrode almost all the range and rearrangedcourses of streams. Erosion has written and still writes it own history inthe mountains, but it has also revealed the complex mosaic of thebedrock.

Coast Mountains

The Coast Mountains are the western range of the North American mainland cordillera, covering the Alaska Panhandle and most of costal British Columbia. The range is approximately Unit km|1600|0 long and Unit km|200|0 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 Unit km|1800|0 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. [ [http://dsp-psd.pwgsc.gc.ca/Collection-R/GSC-CGC/M44-2001/M44-2001-F9E.pdf U-Pb dates from the Scotia-Quaal metamorphic belt, Coast Plutonic Complex, central-western British Columbia] Retrieved on 2007-12-02] 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 is not yet fully emerged above sea level, and Vancouver Island and the Queen Charlotte Islands 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. [ [http://www.bivouac.com/ArxPg.asp?ArxId=1357 Coast Mountains] in the Canadian Mountain Encyclopedia. Retrieved on 2007-12-02] Though 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.

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. [http://bivouac.com/ArxPg.asp?ArxId=1365 Insular Mountains] in the Canadian Mountain Encyclopedia. Retrieved on 2007-12-02] 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 Unit m|6000|0 of basal sediment-sill complexes, basaltic to picritic pillow lavas, pillow breccia, and thick, massive basalt flows.

ee also

*Basic geologic features of each state
*Beaverhead crater, Idaho
*Cascade Range
*Cascade Volcanoes
*Cascadia subduction zone
*Columbia River Basalt Group
*Fort Rock
*Garibaldi Volcanic Belt
*Geology of the Lassen volcanic area
*Hole-in-the-Ground
*List of volcanoes in Canada
*Volcanism in Canada

References

External links

* [http://spot.pcc.edu/~fgransha/G207/geomap.pdf One link on Northwest geology]
* [http://geomaps.wr.usgs.gov/pacnw/ USGS site on earthquakes]
* [http://quake.usgs.gov/prepare/factsheets/PacNW/ USGS site on earthquakes]
* [http://www.eos.ubc.ca/~krussell/epapers/bv_hrs99.pdf On the eruption of Mt. Meager]