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Report: Giant Rocks Caused Mount St. Helens Eruption
by Donna Millsap - The Oregon Herald
  Wednesday September 5, 2018 - 6:01 AM
 
PORTLAND, Oregon — Scientists from Oregon State University scientists are saying that Mount St. Helens is out of line with other major volcanoes on the Cascade Range.

University geophysicist Adam Schultz believes a giant subsurface rock formation diverted magma, causing Mount St. Helens to form west of the Cascadia Arc.

"It seems like what we call the Spirit Lake batholith is probably the reason why Mount St. Helens actually pops up far to the west of where you would anticipate it to be," Schultz said.

Researchers put together different imaging studies to gain a better understanding to what may be below the surface of the volcano. They compared older studies what show certain structure, density and temperature — with newer ones that show subsurface electrical conductivity.

This report will be published this week in "Nature Geoscience."

"Either method by itself can lead to a level of uncertainty, but when you layer them together as we have done in this project you get a much clearer picture of what lies below," Schultz said.

Mount St. Helens formed because of the Juan de Fuca plate off the coast of Washington and Oregon went under the North America plate. The plate went past crustal blocks with marine sediments that "slammed into the continent, where they accreted," Schultz said.

"This material is more permeable than surrounding rock and allows the magma to move through it," Schultz said. "The big batholith acts kind of like a plug in the crust and diverted magma that normally would have erupted in line with the other major Cascade volcanoes, resulting in St. Helens forming to the west of the Cascadia Arc, and Mount Adams slightly to the east."

BACKGROUND: On May 18, 1980, a major volcanic eruption occurred at Mount St. Helens, a volcano located in Skamania County, in the State of Washington. The eruption was the most significant volcanic eruption to occur in the contiguous 48 U.S. states since the much smaller 1915 eruption of Lassen Peak in California. It has often been declared as the most disastrous volcanic eruption in U.S. history. 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 volcano that created a large bulge and a fracture system on the mountain's north slope.

An earthquake at 8:32:17 a.m. PDT (UTC-7) on Sunday, May 18, 1980, caused the entire weakened north face to slide away, creating the largest landslide ever recorded. This allowed the partly molten, high-pressure gas- and steam-rich rock in the volcano to suddenly explode northwards toward Spirit Lake in a hot mix of lava and pulverized older rock, overtaking the avalanching face.

An eruption column rose 80,000 feet (24 km; 15 mi) into the atmosphere and deposited ash in 11 U.S. states. At the same time, snow, ice and several entire glaciers on the volcano melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River, nearly 50 miles (80 km) to the southwest. Less severe outbursts continued into the next day, only to be followed by other large, but not as destructive, eruptions later that year. Thermal energy released during the eruption was equal to 26 megatons.


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