Geology of the Pacific Northwest

The geology of the Pacific Northwest includes the composition (including rock, minerals, and soils), structure, physical properties and the processes that shape the Pacific Northwest region of North America. The region is part of the Ring of Fire: the subduction of the Pacific and Farallon Plates under the North American Plate is responsible for many of the area's scenic features as well as some of its hazards, such as volcanoes, earthquakes, and landslides.

The geology of the Pacific Northwest is vast and complex. Most of the region began forming about 200 million years ago as the North American Plate started to drift westward during the rifting 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.

There are at least five geologic provinces in the area: the Cascade Volcanoes, the Columbia Plateau, the North Cascades, the Coast Mountains, and the Insular Mountains. 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 British Columbia. The Coast Mountains and Insular Mountains are a strip of mountains along the coast of British Columbia, each with its own geological history.

Volcanoes

The Cascade Volcanoes

Main article: 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.

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.

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.

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 addition of crustal melt to the original mantle melt results in volcanic and plutonic rocks that differ in mineralogy from the mantle source.

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 melted mantle rises and solidifies, creating new ocean crust. 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) is less 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 centimetres (1.2–1.6 in) 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 3,000 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

Main article: Volcanism in Canada

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, the Mount Meager massif, was responsible for a major catastrophic eruption that occurred about 2,350 years ago. Ash from this eruption can be traced eastward to western Alberta. It is also the most unstable volcanic massif in Canada, which has dumped clay and rock several meters (yards) deep into the Pemberton Valley at least three times during the past 7,300 years. Hot springs near the Mount Cayley and Mount Meager massifs 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 the Mount Meager massif such as the Franklin Glacier Complex 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 The Volcano in the Iskut-Unuk River Cones volcanic field. The most voluminous and most persistent eruptive center within the belt and in Canada is Level Mountain. It is a large shield volcano that covers an area of 1,800 km² (690 sq mi) 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 Level Mountain 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 (0.6 mi) 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 Group 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.

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 142 m (465 ft)-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 km (110 mi) 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 22.5 km (14.0 mi) long lava flow, destroying the Nisga'a villages. 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

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 20 km (12 mi) 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 22.5 km (14.0 mi) 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. (see 1980 eruption of Mount St. Helens).

Seismic activity

The Pacific Northwest experiences a lot of seismic activity. The Juan de Fuca Plate can cause very powerful earthquakes, known as megathrust earthquakes, with a moment magnitude of 9. The last big one happened in 1700, known as the 1700 Cascadia earthquake, which even created a tsunami that reached Japan. More recently, in 2001, the Nisqually earthquake struck near Olympia, Washington, causing some damage and frightening people.

Eleven volcanoes in Canada have also shown signs of activity since 1975. These include places like the Silverthrone Caldera, Mount Meager massif, and Mount Garibaldi, among others.

Columbia Plateau

Main articles: Columbia Plateau and Columbia River Basalt Group

The Columbia Plateau is covered by one of the largest areas of basalt in the world, spread over more than 500,000 km². This area was shaped by huge lava flows that poured out very quickly within the last 17 million years.

The basalt here, called the Columbia River Basalt Group, erupted between 17 and 6 million years ago. Most of it came out in just the first 1.5 million years. The Snake River Plain runs from Oregon through Nevada and Idaho to Wyoming. It looks like a giant spoon scooped out the land, very flat compared to the mountains around it.

Volcanic eruptions have played a big role in shaping the Snake River Plain. The oldest eruptions here started about 15 million years ago. The plain has both quiet flows of dark lava and powerful explosions of lighter volcanic rock. There are many small volcanoes and big pits from past eruptions, though some are hidden by newer lava.

Scientists think a hot spot deep in the Earth is melting rock under the Columbia Plateau. This hot spot is probably staying still while the North American plate moves over it. This movement creates a path of volcanic activity that leads to Yellowstone National Park, where we can see steam and geysers today.

Main article: Missoula Floods

During the Ice Age, huge glaciers covered parts of North America. One glacier blocked a river in Canada, creating a giant lake called Glacial Lake Missoula. This lake broke through the ice many times, sending massive floods across northern Idaho and eastern Washington. These floods carved deep channels into the land, creating the channeled scablands we see today.

Two big waterfalls formed during these floods. The upper Grand Coulee had a waterfall 240 m high, and Dry Falls is now 5.6 km wide with a drop of more than 120 m — much bigger than Niagara Falls. The powerful water shaped the landscape by pulling away rocks and moving the falls to where they are now.

The North Cascades

Main article: North Cascades

The North Cascade Range in Washington is part of the American cordillera, a long mountain chain that stretches from Tierra del Fuego to the Alaska Peninsula. Though small compared to other ranges, the North Cascades are very steep and wet.

These mountains tell a story that is more than 400 million years old. The rocks here were formed by volcanic eruptions, deep ocean sediments, and movements of the Earth's plates. Over time, these pieces came together to form the range we see today. About 35 million years ago, volcanoes erupted, covering older rocks with lava and ash. Today, we can still see active volcanoes like Mount Baker and Glacier Peak.

The deep canyons and sharp peaks we see now were shaped by water, landslides, and glaciers. During the Ice Age, huge sheets of ice covered most of the range, changing the land forever.

Coast Mountains

Main article: Coast Mountains

The Coast Mountains are a big mountain range along the west coast of North America. They stretch through the Alaska Panhandle and most of coastal British Columbia, covering about 1,600 kilometers (1,000 miles) in length and 200 kilometers (120 miles) in width.

Most of these mountains are made of granite, forming the largest area of granite in the world. This granite formed when ancient ocean plates moved under the continent millions of years ago. Some parts of the Coast Mountains, like the Garibaldi, Meager, Cayley, and Silverthrone areas, are still active volcanoes today.

The Coast Mountains rose up during a period called the Miocene. Rivers like the Klinaklini River and Homathko River have been flowing through the area for a very long time. The highest peaks in the Coast Mountains are near the Homathko River, including Mount Waddington and Mount Queen Bess. The Pacific Ranges in southwestern British Columbia have been rising quickly in the last few million years, causing fast erosion.

Insular Mountains

Main article: Insular Mountains

The Insular Mountains are located along the coast of British Columbia. Right now, they are mostly underwater, but during the last ice age, when sea levels were lower, they were fully exposed as land. Places like Vancouver Island and Haida Gwaii are the parts of these mountains that rise above the sea level today.

These mountains were formed when a large group of islands, called the Insular Islands, moved and joined with North America millions of years ago. They are made from special kinds of rock and lava flows, different from the nearby Coast Mountains. The area still sees movement in the earth’s crust, which can cause shaking and changes in the land.