Geology of the Pacific Northwest

The geology of the Pacific Northwest includes the types of rock, minerals, and soils found in the area, as well as how they change and shape the Pacific Northwest region of North America. The area is part of the Ring of Fire. The movement of the Pacific and Farallon Plates under the North American Plate creates beautiful landscapes but can also cause volcanoes, earthquakes, and landslides.

The geology of the Pacific Northwest is large and complicated. Most of the land here began forming about 200 million years ago when the North American Plate moved westward as part of the breaking apart of Pangaea. Since then, the western edge of North America has grown outward as pieces of island arcs and ocean-floor rocks joined along the continental margin.

There are at least five main geological areas in the region: the Cascade Volcanoes, the Columbia Plateau, the North Cascades, the Coast Mountains, and the Insular Mountains. The Cascade Volcanoes are an active volcanic area along the western side of the Pacific Northwest. The Columbia Plateau is a flat area just inland from the Cascade Volcanoes. The North Cascades are mountains in the northwest part of the United States, reaching into British Columbia. The Coast Mountains and Insular Mountains are strips of mountains along the coast of British Columbia, each with its own geological story.

Volcanoes

The Cascade Volcanoes

Main article: Cascade Volcanoes

The Cascades Province is a curved line of volcanoes from southwestern British Columbia to Northern California. It runs close to the Pacific coast. There are nearly 20 major volcanoes here.

Big volcanoes like Mount St. Helens get a lot of attention. But there are also thousands of small volcanoes that built up a layer of lava and volcanic debris. A few very large volcanoes stand out and shape the land.

The Cascade volcanoes are part of the Ring of Fire, a group of volcanoes around the Pacific Ocean. The Ring of Fire also has many earthquakes. Both come from the same place: subduction.

Under the Cascade volcanoes, a dense ocean plate moves under the North American Plate. This is called subduction. As the oceanic slab goes deep into the Earth, water from the rocks rises. This water makes some of the mantle melt. The melted rock, called magma, moves up through the Earth's crust. This magma can erupt and form volcanoes. The melted crust changes the rocks, making them different from the original mantle.

The Cascades look more complicated than just a simple subduction zone.

Not far from the coast of the North Pacific is a spreading ridge. This is a divergent plate boundary where new ocean crust is made. On one side, new Pacific Plate crust forms and moves away. On the other side, the Juan de Fuca and Gorda plates move east.

There are some special features at the Cascade subduction zone. Where the Juan de Fuca Plate moves under the North American Plate, there is no deep trench. Earthquakes are fewer than expected, and volcanic activity has decreased over the past few million years. This may be because the plates are moving together slowly.

The small Juan de Fuca Plate and two small pieces, the Explorer Plate and Gorda Plate, are leftovers from a much larger Farallon oceanic plate. The Explorer Plate broke away from the Juan de Fuca Plate about 4 million years ago. The Gorda platelet split away between 18 and 5 million years ago and is still moving under North America.

The Cascade Volcanic Arc began 36 million years ago, but the big mountains we see today were formed in the last 1.6 million years. More than 3,000 volcanoes erupted in the most recent volcanic time, starting 5 million years ago. As long as subduction continues, new Cascade volcanoes will keep forming.

Volcanism outside the Cascades

Main article: Volcanism in Canada

The Garibaldi Volcanic Belt in southwestern British Columbia is the northern part of the Cascade Volcanic Arc in the United States. It has some of Canada's most explosive young volcanoes. Like the rest of the arc, it comes from the Cascadia subduction zone. Volcanoes here have been active over millions of years. The northernmost volcano, the Mount Meager massif, had a big eruption about 2,350 years ago. Ash from this eruption went far to the east. This volcano is also unstable and has let loose clay and rock several times in the past 7,300 years. Hot springs near Mount Cayley and Mount Meager show there is still heat under the ground. The long history of volcanoes here, plus ongoing subduction, means volcanism may not be over in the Garibaldi Volcanic Belt. Some volcanoes far north, like the Franklin Glacier Complex and the Silverthrone Caldera, might also come from Cascadia subduction, but we do not know much about them. The Juan de Fuca Plate broke apart about 5–7 million years ago, and scientists are not sure how volcanoes north of there relate to the Cascade Arc. The Pemberton Volcanic Belt is usually grouped with the Garibaldi Volcanic Belt, making Mount Silverthrone the northernmost volcano linked to Cascadia subduction.

The most active volcanic area in the northern Pacific Northwest is the Northern Cordilleran Volcanic Province. It has more than 100 young volcanoes and several eruptions in the last 400 years. The last eruption was about 150 years ago at The Volcano in the Iskut-Unuk River Cones volcanic field. The biggest and most lasting volcano here is Level Mountain. It is a large shield volcano covering 1,800 km² (690 sq mi) southwest of Dease Lake and north of Telegraph Creek. The top of Level Mountain has lava domes and may have had small basaltic eruptions. The Mount Edziza volcanic complex is one of the most impressive volcanoes in British Columbia. It is the second largest volcano in the Northern Cordilleran Volcanic Province and has many young satellite cones, including Eve Cone. Level Mountain and Mount Edziza may be between 11 and 9 million years old.

The Anahim Volcanic Belt runs from near Vancouver Island to near Quesnel. It formed as the North American Plate moved over a hotspot, like the one under the Hawaiian Islands, called the Anahim hotspot. The youngest volcano here is Nazko Cone. It erupted about 7,000 years ago, sending out lava and volcanic ash. The belt also has three large shield volcanoes formed between 8 and 1 million years ago: the Ilgachuz Range, Rainbow Range, and the Itcha Range.

The Chilcotin Group in southern British Columbia is a line of volcanoes formed by back-arc extension behind the Cascadia subduction zone. Most eruptions here happened either 6 to 10 million years ago or 2–3 million years ago. There have been few eruptions in more recent times.

The Wells Gray-Clearwater volcanic field in southeastern British Columbia has small basaltic volcanoes and lava flows active for the past 3 million years. It is inside Wells Gray Provincial Park, which also has Helmcken Falls. The cause of the volcanism is unknown but may be related to thinning of the Earth's crust. Some lava flows look like those from Volcano Mountain in the Yukon. The last eruption was about 400 years ago at Kostal Cone.

Many seamounts are off the coast of British Columbia and come from hotspot volcanism. The Bowie Seamount, 180 km (110 mi) west of Haida Gwaii, is perhaps the shallowest seamount in Canada's Pacific waters. Because it is so shallow, scientists think it was an island during the last ice age. The Bowie Seamount is the youngest in the Kodiak-Bowie Seamount chain.

Volcanic disasters

The last eruption of the Tseax Cone around 1750 or 1775 was Canada's worst known geophysical disaster. The eruption created a long lava flow that destroyed Nisga'a villages. The Nass River valley was covered by lava and has tree molds and lava tubes. This happened when the first European explorers arrived near northern British Columbia. Today, the lava is a tourist spot in the Nisga'a Memorial Lava Bed Provincial Park.

Recent volcanic activity

The Pacific Northwest volcanoes are still active. The most recent volcanic eruptions include:

• Level Mountain might have erupted in recent times.
• Nazko Cone erupted 7200 BP.
• Hoodoo Mountain erupted 7050 BP.
• Lava Butte, Oregon erupted about 7,000 years ago.
• Mount Mazama erupted to form Crater Lake.
• Mount Meager massif erupted about 2350 BP.
• Mount Edziza volcanic complex erupted about 1340 BP.
• Medicine Lake Volcano erupted about 1000 BP.
• Silverthrone Caldera may have erupted less than 1000 BP.
• Kostal Cone may have erupted around 1500.
• Glacier Peak erupted in the 17th or 18th century.
• Tseax Cone erupted in the 18th century.
• Mount Hood erupted in 1781–82; fumaroles still release gas.
• Mount Shasta erupted in 1786.
• The Volcano erupted about 150 BP.
• Mount Rainier erupted in 1854.
• Mount Baker erupted in 1880; fumaroles still occur.
• Ruby Mountain may 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 has a lot of seismic activity. The Juan de Fuca Plate can cause very powerful earthquakes, called megathrust earthquakes. The last big one happened in 1700, known as the 1700 Cascadia earthquake. This earthquake created a tsunami that reached Japan. More recently, in 2001, the Nisqually earthquake struck near Olympia, Washington, causing some damage.

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

Columbia Plateau

Main articles: Columbia Plateau and Columbia River Basalt Group

The Columbia Plateau has one of the largest areas of basalt in the world, covering more than 500,000 km². Huge lava flows shaped this area in 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 the first 1.5 million years. The Snake River Plain runs from Oregon through Nevada and Idaho to Wyoming. It is very flat compared to the mountains around it.

Volcanic eruptions helped shape the Snake River Plain. The oldest eruptions began about 15 million years ago. The plain has both quiet flows of dark lava and powerful explosions of lighter rock. There are many small volcanoes and pits from past eruptions.

Scientists think a hot spot deep in the Earth is melting rock under the Columbia Plateau. This hot spot may stay 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 big 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 land by moving rocks and changing where the falls 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 have 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 large mountain range along the west coast of North America. They run through the Alaska Panhandle and most of coastal British Columbia, stretching about 1,600 kilometers (1,000 miles) long and 200 kilometers (120 miles) wide.

Most of these mountains are made of granite, 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.

The Coast Mountains rose up during a time called the Miocene. Rivers like the Klinaklini River and Homathko River have flowed 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 recent millions of years, causing fast erosion.

Insular Mountains

Main article: Insular Mountains

The Insular Mountains are along the coast of British Columbia. Today, they are mostly under the water. But during the last ice age, when the sea was lower, they were above land. Places like Vancouver Island and Haida Gwaii are the parts that now stick out above the sea level.

These mountains formed when a group of islands, called the Insular Islands, moved and joined North America long ago. They are made of special rocks and lava, different from the nearby Coast Mountains. The area still has movement in the earth, which can cause shaking and changes in the land.