In the western
United States lies the
Cascade Volcanic Arc. It includes nearly 20 major volcanoes, among a total of over 4,000 separate volcanic vents including numerous
stratovolcanoes,
shield volcanoes,
lava domes, and
cinder cones, along with a few isolated examples of rarer volcanic forms such as
tuyas. Volcanism in the arc began about 37 million years ago, however, most of the present-day Cascade volcanoes are less than 2,000,000 years old, and the highest peaks are less than 100,000 years old. It formed by
subduction of the
Gorda and
Juan de Fuca plates at 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) down-dip from the deformation front. Further down-dip, 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. Geological evidence indicates that great earthquakes may have occurred at least seven times in the last 3,500 years, suggesting a return time of 400 to 600 years. There is also evidence of accompanying tsunamis with every earthquake, as the prime reason they know of these earthquakes is through "scars" the tsunami left on the coast, and through
Japanese records (tsunami waves can travel across the Pacific).
The
1980 eruption of Mount St. Helens was the most significant to occur in the contiguous 48
U.S. states in recorded history (
VEI = 5, 0.3 cu mi, 1.2 km
3 of material erupted), exceeding the destructive power and volume of material released by the 1915 eruption of
California's
Lassen Peak. The eruption was preceded by a two-month series of
earthquakes and
steam-venting episodes, caused by an injection of
magma at shallow depth below the mountain that created a huge bulge and a fracture system on
Mount St. Helens' north slope. An earthquake at 8:32 a.m. on May 18, 1980, caused the entire weakened north face to slide away, suddenly exposing the partly molten, gas- and
steam-rich
rock in the volcano to lower pressure. The rock responded by exploding into a very hot mix of pulverized
lava and older rock that sped toward
Spirit Lake so fast that it quickly passed the avalanching north face.
Alaska is known for its volcanism, having more than 50 volcanoes which have erupted since about 1760. Volcanoes can be found not only in the mainland but also in the
Aleutian Islands.
The most recent activity in the American portion of the Ring of Fire occurred in early 2009 when
Mount Redoubt in Alaska became active and finally erupted late in the evening of March 22, the eruption currently ongoing.
