The United States currently has 169 potentially active volcanoes. Ranking behind Indonesia and Japan, the United States ranks third for historically active volcanoes based on written records. Per the USGS, “about 10 percent of the more than 1,500 volcanoes that have erupted in the past 10,000 years are located in the United States.”
Why Does the United States Have so Many Volcanoes?
Most of these volcanoes in the United States are located in the Aleutian Islands, the Alaska Peninsula, the Hawaiian Islands, and the Cascade Range of the Pacific Northwest.
Ring of Fire
Many of these volcanoes are located in Pacific Ring of Fire, where the Pacific Plate meets many surrounding tectonic plates. The Ring of Fire is the most active area of the world in terms of seismic and tectonic activity.
Volcanic activity over the Hawaiian Islands is the exception. Volcanic activity in Hawaii is not the result of plate boundaries meeting. The development of volcanic islands in Hawaii is theorized to be because of what geologists refer to as a “hot spot.” This hot spot is the result of a plume producing volcanoes that erode and become inactive as the Pacific Plate moves them away from the plume.
The Most Destructive Volcanic Eruption in the United States
On May 18, 1980, Mount St. Helens in Washington was the most destructive volcanic eruption in the history of the United States. A magnitude-5+ earthquake combined with a debris avalanche removed the cryptodome and abruptly unleashed pressure of the volcano.
USGS geologist Don Swanson photographed and filmed the eruption of Mount St. Helens from about 9:30 a.m. to 12:30 p.m. on that day. Swanson filmed the volcanic eruption from a fixed-wing surveillance aircraft using a Bell & Howell hand-wound 16mm movie camera.
The resulting volcanic activity killed 57 people, destroyed 250 homes, 47 bridges, 15 miles (24 km) of railways, and 185 miles (298 km) of highway. Mount St. Helen’s summit was reduced from 9,677 ft (2,950 m) to 8,363 ft (2,549 m).
World’s Largest Active Volcano
The United States is also home to the world’s largest active volcano. A shield volcano, Mauna Loa has an altitude of 4 km (13,100 ft) above sea level. The submarine part of Mauna Loa descends an additional 5,000 m (16,400 ft) to the sea floor. From base to summit, Mauna Loa is 17,000 m (56,000 ft) high.
The name stems from Hawaiian and means “Long Mountain.”
Mauna Loa is one of five volcanos on the island of Hawaii. Learn more: Geography Facts About the World’s Largest Active Volcano
Mauna Loa has erupted 33 times since 1843 and has last erupted between March and April of 1984. Typically erupting every six years on average, Mauna Loa has been in its quietest period of volcanic activity in recorded history.
Largest Volcanic Eruption in the 20th Century
On June 6 1912, the eruption from a vent that formed Alaska’s Novarupta marked the largest eruption in the 20th century. An estimated estimated 15 cubic kilometers of magma erupted over a 60 hour period. This is 30 times the eruption of Mount St. Helens in 1980.
The vent filled the Ukak River valley with hot ash. Even almost a hundred years later, Novarupta retains heat from the eruption and small fumaroles (volcanic gas vents) can be found at its base. The lava dome is 235 ft (380 m) wide and 211 ft (65 m) high.
Despite its intensity, the eruption that led to Novarupta led to no loss of life and little property damage due to its isolation from human settlements.
Pyroclastic flows (fast moving currents of hot ash and gases) from the eruption formed the Valley of the Thousand Smokes.
The venting at Novarupta caused the summit of Mount Katmai, 6 miles (10 km) away to the east, to collapse as magma was drained from underneath it. The collapse of the summit formed a lake-filled caldera which is about 1.9 mi (3 km) wide and 2000 ft (600 m) deep.
It wasn’t until 1916 that researchers funded by National Geographic were able to trek to the remote location to see the aftermath. Botanist Robert F. Griggs described the view:
“The sight that flashed into view…was one of the most amazing visions ever beheld by mortal eye. The whole valley as far as the eye could reach was full of hundreds, no thousands–literally tens of thousands–of smokes curling up from its fissured floor…It was as though all the steam engines in the world, assembled together, had popped their safety valves at once and were letting off surplus steam in concert.
Our feeling of admiration [for the Valley] soon gave way to one of stupefaction. We were overawed. For a while we could neither think nor act in a normal fashion.”