Mount St. Helens is not the only volcano that has rumbled recently. Minor eruptions on Mount Asama in central Japan began almost a month ago and are continuing. The first explosion there, on September 1, emitted about 100,000 tons (90,000 metric tons) of material, mainly ash.
"The head of the magma body [rose] and reached a few tens of meters below the crater rim on September 16," said Kozo Uto, who is the deputy director of the Institute of Geology and Geoinformation in Tsukuba, Japan.
Volcanic activity is closely linked with plate tectonics. Both Mount St. Helens and Mount Asama are located on continental boundaries. As huge slabs of rock collide in the Earth's outer shell, they can produce magma and trigger a chain of events that may end with eruptions.
"The volcanoes in Japan are very similar to the ones we have [on the West Coast of] the United States," said Pierson, the U.S. Geological Survey scientist.
By contrast, Hawaii's volcanoes form in the middle of tectonic plates and are caused by unusually hot areas deep within the Earth.
One of those volcanoes, Kilauea, which has been erupting continuously since 1983, has shown increased activity recently. And hundreds of earthquakes have been recorded in recent months under Mauna Loa, the world's largest volcano, prompting fears that it could erupt for the first time in 20 years.
Still, some scientists say all the volcanic activity is not exceptional.
"There is not a terribly unusual number of volcanoes active right now," said Steve Malone, a seismologist at the University of Washington in Seattle. "There are always volcanoes active in the Pacificsome weeks more than others."
While other volcanoes in the Cascade region of Washington State are more than a million years old, the 8,364-foot (2,549-meter) Mount St. Helens is very young, perhaps a few hundred thousand years old. With a history of large and very explosive eruptions, it's been far more active than other volcanoes.
The May 18, 1980, eruption blasted away the top 1,300 feet (400 meters) of the volcano. A cycle of smaller explosive eruptions ensued. By October the magma had lost its gas and the power to cause explosions. As thick lava continued to ooze out and harden, a dome-shaped mass of rock was formed on top of the volcano.
Today the dome is 925 feet (281 meters) tall and more than 2,000 feet (610 meters) wide. Additional molten rock could cause it to expand further.
The frequency of the new earthquakes, ranging in magnitude from 2.0 to 3.3, has gradually increased, cranking up to a level not seen since 1986. They are occurring at depths less than a mile (1.6 kilometers) below the lava dome, suggesting magma from an old event is the cause.
"We had magma come up to shallow levels in 1998, but it didn't reach the surface," Pierson said. "So we think it could be a relatively small batch of 1998 magma that is doing the pushing now."
New measurements show the lava dome in the volcano's crater has moved 2.5 inches (6.4 centimeters) to the north since Monday. (In the two months before the May 18, 1980, eruption, the mountain's northern flank bulged outward more than 450 feet/137 meters.)
Tests of gas samples above the volcano have not shown unusually high levels of carbon dioxide or sulfur. If such gases are not escaping from the mountain in high levels, it suggests fresh magma has not been welling up. In the absence of new magma, any additional eruption would be not be major.
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