Volcanoes

Located along the Pacific Ring of Fire, the Western US contains many volcanoes. As can be seen in the table below, Alaska has the most volcanoes, most of which are found along the Aleutian Island Arc as a result of Pacific plate subduction. California, Washington, and Oregon contain the Cascade Range’s volcanoes, which result from the subduction of the Juan de Fuca plate. All of the Cascade and Aleutian volcanoes are considered to be either active or dormant, which means they are capable of becoming active. Volcanic eruptions in the Cascades have been occurring for over 500,000 years, and have taken place at an average rate of two eruptions per century for the past 4000 years (Figure 10.15)

The vast majority of volcanoes formed by subduction are stratovolcanoes, also known as composite volcanoes. Stratovolcanoes are built from the eruption of andesitic magma, which forms as the mantle above the subducting plate melts. This magma is viscous because of its high silica content, making it capable of trapping gasses and producing explosive eruptions. Stratovolcanoes eventually become layered with lava flows, pyroclastics, rock, and volcanic ash fragments ejected by an explosive eruption. They tend to be tall and cone-shaped, as can be seen in the picture of Augustine Volcano (Figure 10.16), located 300 kilometers (186 miles) southwest of Anchorage, Alaska. When this volcano erupted in 1986, ash from an 11-kilometer (7-mile) high ash plume drifted over the Cook 304 Inlet and grounded air traffic in Anchorage.

Figure 10.15: The eruptive history of the Cascade volcanoes.

Katmai National Park in Alaska is the site of the most powerful volcanic eruption in US history. In 1912, 13 cubic kilometers (3.1 cubic miles) of ash erupted from a vent on the side of Mt. Katmai. The ash completely filled a valley, which became known as the Valley of Ten Thousand Smokes for its many smoking fumaroles. The top of Mt. Katmai collapsed 1200 meters (3900 feet), creating an enormous caldera. After the explosive eruption, viscous rhyolitic lava pushed up from the vent, creating a volcanic dome, now known as Novarupta, which means “new volcano” (Figure 10.17).

Figure 10.16: Augustine Volcano.

Figure 10.17: Novarupta and associated ashfalls.

Mt. Saint Helens is the most active volcano in the Cascade Range. Its most spectacular eruption occurred on May 18, 1980, when an earthquake shook the side of the mountain, triggering the largest landslide ever recorded. An explosive blast issued from the side of the mountain, reaching speeds up to 480 kph (300 mph) as it flattened forests in its path. Avalanches of hot ash and pumice called pyroclastic flows sped quickly down the side of the volcano and spread for miles. The volcano’s rapidly melted glaciers created lahars that took out 27 bridges and 200 homes. Despite evacuations in the weeks leading up to the eruption, 57 people died during the explosion. It was the deadliest and most costly volcanic event in US history. Mt. Saint Helens continues to be active, and it has built a lava dome inside its crater.

The Long Valley Caldera of California (Figure 10.18) was the site of a colossal eruption 760,000 years ago. The eruption created the Bishop Tuff ash layer (deposits of which cover an area nearly 2200 square kilometers [850 square miles] in size), and is estimated to have released 600 cubic kilometers (145 cubic miles) of ash, compared to the 1.2 cubic kilometers (0.3 cubic miles) of ash that was emitted by Mt. Saint Helens. There is still no good explanation for the basaltic and rhyolitic magmas that underlie the area, since there is no associated hotspot or subducting plate. There are many related volcanic features in the area, including Mammoth Mountain (a ski resort that sits atop a lava dome complex), Panum Crater (a rhyolitic lava dome), and numerous hot springs such as Hot Creek, which is located on a resurgent dome in the center of the caldera.

Figure 10.18: Extent and location of the Long Valley Caldera.

The Mojave Desert is dotted with scoria cones and their associated basaltic lava flows. Scoria cones, also known as cinder cones, form as basaltic lava Volcanoes spews into the air and falls back onto the ejection site. The lava cools so quickly that volcanic gases become trapped in it as bubbles called vesicles. Scoria cones are small, steep, and short-lived. Amboy Crater and the Cima volcanic field of the Mojave National Preserve are excellent examples, among which the youngest scoria cone is only 10,000 years old. The Boring Lava Field, located near Portland, Oregon is an extinct lava field with 32 scoria cones (Figure 10.19). Scoria cone landscapes may appear ominous, but their effects are local, and they rarely cause a hazard to humans.

Figure 10.19: Beacon Rock is part of the Boring Lava Field in Oregon. It is the central core of a cinder cone whose outer layers were stripped away 57,000 years ago by the Missoula Floods.

The USGS constantly monitors volcanoes in Alaska, California, Hawai’i, Oregon, and Washington, as well as the Yellowstone area in Wyoming. Although the exact timing of a volcanic eruption is difficult to predict, there are many signs that a volcano is becoming active. Volcanologists on the watch for eruptions look for earthquake swarms, changes in the level of the land, changes in hydrothermal activity, and changes in the gases emitted by the suspect volcano. Because ash plumes can destroy jet engines in less than a minute, the USGS also issues warnings for aviation. Luckily, the most active volcanoes in the United States are either in Alaska, far from population centers, or in Hawai’i, where shield volcanoes erupt without significant explosions or loss of life.