Cascade Volcanic Arc continued

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The Cascadia subduction zone

The Cascade Volcanoes were formed by the subduction of the Juan de Fuca, Explorer and the Gorda Plate (remnants of the much larger Farallon Plate) under the North American Plate along the Cascadia subduction zone. This is a 680 mi (1,094 km) long fault, running 50 mi (80 km) off the west-coast of the Pacific Northwest from northern California to Vancouver Island, British Columbia. The plates move at a relative rate of over 0.4 inches (10 mm) per year at a somewhat oblique angle to the subduction zone.

Because of the very large fault area, the Cascadia subduction zone can produce very large earthquakes, magnitude 9.0 or greater, if rupture occurred over its whole area. When the "locked" zone stores up energy for an earthquake, the "transition" zone, although somewhat plastic, can rupture. Thermal and deformation studies indicate that the locked zone is fully locked for 60 kilometers (about 40 miles) downdip from the deformation front. Further downdip, there is a transition from fully locked to aseismic sliding.

Unlike most subduction zones worldwide, there is no oceanic trench present along the continental margin in Cascadia. Instead, terranes and the accretionary wedge have been uplifted to form a series of coast ranges and exotic mountains. A high rate of sedimentation from the outflow of the three major rivers (Fraser River, Columbia River, and Klamath River) which cross the Cascade Range contributes to further obscuring the presence of a trench. However, in common with most other subduction zones, the outer margin is slowly being compressed, similar to a giant spring. When the stored energy is suddenly released by slippage across the fault at irregular intervals, the Cascadia subduction zone can create very large earthquakes such as the magnitude 9 Cascadia earthquake of 1700.

Cascadia subduction zoneMain article: Cascadia subduction zone

Area of the Cascadia subduction zone, including Cascade volcanoes (red triangles)The Cascade Volcanoes were formed by the subduction of the Juan de Fuca, Explorer and the Gorda Plate (remnants of the much larger Farallon Plate) under the North American Plate along the Cascadia subduction zone. This is a 680 mi (1,094 km) long fault, running 50 mi (80 km) off the west-coast of the Pacific Northwest from northern California to Vancouver Island, British Columbia. The plates move at a relative rate of over 0.4 inches (10 mm) per year at a somewhat oblique angle to the subduction zone.

Because of the very large fault area, the Cascadia subduction zone can produce very large earthquakes, magnitude 9.0 or greater, if rupture occurred over its whole area. When the "locked" zone stores up energy for an earthquake, the "transition" zone, although somewhat plastic, can rupture. Thermal and deformation studies indicate that the locked zone is fully locked for 60 kilometers (about 40 miles) downdip from the deformation front. Further downdip, there is a transition from fully locked to aseismic sliding.

Unlike most subduction zones worldwide, there is no oceanic trench present along the continental margin in Cascadia. Instead, terranes and the accretionary wedge have been uplifted to form a series of coast ranges and exotic mountains. A high rate of sedimentation from the outflow of the three major rivers (Fraser River, Columbia River, and Klamath River) which cross the Cascade Range contributes to further obscuring the presence of a trench. However, in common with most other subduction zones, the outer margin is slowly being compressed, similar to a giant spring. When the stored energy is suddenly released by slippage across the fault at irregular intervals, the Cascadia subduction zone can create very large earthquakes such as the magnitude 9 Cascadia earthquake of 1700.

Major catastrophic eruptions
1980 Eruption of Mount St. Helens
The 1980 eruption of Mount St. Helens was one of the most closely studied volcanic eruptions in the arc and one of the best studied ever. It was a Plinian style eruption with a VEI=5 and was the most significant to occur in the lower 48 U.S. states in recorded history. An earthquake at 8:32 a.m. on May 18, 1980, caused the entire weakened north face to slide away. An ash column rose high into the atmosphere and deposited ash in 11 U.S. states. The eruption killed 57 people and thousands of animals and caused more than a billion U.S. dollars in damage.

1914–17 Eruptions of Lassen Peak
On May 22, 1915, an explosive eruption at Lassen Peak devastated nearby areas and rained volcanic ash as far away as 200 miles (320 km) to the east.

A huge column of volcanic ash and gas rose more than 30,000 feet (9,100 m) into the air and was visible from as far away as Eureka, California, 150 miles (240 km) to the west. A pyroclastic flow swept down the side of the volcano, devastating a 3-square-mile (7.8 km2) area. This explosion was the most powerful in a 1914–17 series of eruptions at Lassen Peak.

2350 BP Eruption of Mount Meager
Mount Meager produced the most recent major eruption in Canada, sending ash as far away as Alberta.

The eruption was similar to the 1980 eruption of Mount St. Helens, sending an ash column approximately 20 km high into the stratosphere. This activity produced a diverse sequence of volcanic deposits, well exposed in the bluffs along the Lillooet River, which is defined as the Pebble Creek Formation. The eruption was episodic, occurring from a vent on the north-east side of Plinth Peak. An unusual, thick apron of welded vitrophyric breccia may represent the explosive collapse of an early lava dome, depositing ash several meters in thickness near the vent area.

7700 BP Eruption of Mount Mazama
The 7,700 BP eruption of Mount Mazama was a large catastrophic eruption in the U.S. state of Oregon. It began with a large eruption column with pumice and ash that erupted from a single vent. The eruption was so great that most of Mount Mazama collapsed to form a caldera and subsequent smaller eruptions occurred as water began to fill in the caldera to form Crater Lake. Volcanic ash from the eruption was carried across most of the Pacific Northwest as well as parts of southern Canada.

13100 BP Eruption of Glacier Peak
About 13,000 years ago, Glacier Peak generated an unusually strong sequence of eruptions depositing volcanic ash as far away as Wyoming.

Other eruptions
Silverthrone Caldera
Most of the Silverthrone Caldera's eruptions occurred during the last ice age and was episodically active during both Pemberton and Garibaldi Volcanic Belt stages of volcanism. The last eruption from Mount Silverthrone ran up against ice in Chernaud Creek. The lava was dammed by the ice and made a cliff with a waterfall up against it.

Mount Cayley
Mount Cayley last erupted about 20,000 years ago. It contains several complex features which probably represent multiple eruptions under different conditions and are difficult to classify.

Mount Garibaldi
Mount Garibaldi was last active about 10,700 to 9,300 years ago from a cinder cone called Opal Cone. It produced a 15 km long broad dacite lava flow with prominent wrinkled ridges. The lava flow is unusually long for a silicic lava flow.

Mount Baker
During the mid-19th century, Mount Baker erupted for the first time in several thousand years. Fumarole activity remains in Sherman Crater, just south of the volcano's summit, became more intense in 1975 and is still energetic. However, an eruption is not expected in the near future.

Glacier Peak
Glacier Peak last erupted about 200–300 years ago and has erupted about six times in the past 4,000 years.

Mount Rainier
Mount Rainier last erupted between 1824 and 1854, but many eyewitnesses reported eruptive activity in 1858, 1870, 1879, 1882 and in 1894 as well. Mount Rainier has created at least four eruptions and many lahars in the past 4,000 years.

Mount Adams
Mount Adams was last active about 1,000 years ago and has created few eruptions during the past several thousand years.

Mount Hood
Mount Hood was last active about 200 years ago, creating pyroclastic flows, lahars, and a well-known lava dome close to its peak called Crater Rock. Between 1856 and 1865, a sequence of steam explosions took place at Mount Hood.

Newberry Volcano
A great deal of volcanic activity has occurred at Newberry Volcano, which was last active about 1,300 years ago. It has one of the largest collections of cinder cones, lava domes, lava flows and fissures in the world.

Medicine Lake Volcano
Medicine Lake Volcano has erupted about 8 times in the past 4,000 years and was last active about 1,000 years ago when rhyolite and dacite erupted at Glass Mountain and associated vents near the caldera's eastern rim.

Mount Shasta
Mount Shasta last erupted in 1786 and has been the most active volcano in California for about 4,000 years, erupting once every 300 years. The 1786 eruption created a pyroclastic flow, a lahar and three cold lahars, which streamed 7.5 miles (12 km) down Shasta's east flank via Ash Creek. A separate hot lahar went 12 miles (19 km) down Mud Creek.