Similar to many volcanoes around the world , this simmering range is the product of a subduction zone, a tectonic collision where, in this case, a dense oceanic plate plunges beneath a more buoyant continental plate. As the slab descends, pressures and temperatures climb, and fluids percolate out of the slab, triggering the solid mantle rocks to melt. Less dense than its surroundings, this molten magma shoves its way upward through the crust, creating volcanoes.
Most of the Cascade volcanoes—and others around the world—take shape above the spots where the plunging slab descends to roughly 62 miles deep, where temperatures get high enough for magma to form. But the situation is different at Mount St. Standing tens of miles to the west of other volcanoes, the infamous peak perches a mere 42 miles above the subducting plate.
The iMUSH project kicked off in the summer of in part to try and solve this conundrum. For one part of the analysis, the researchers detonated a series of blasts and watched the waves roll in. Another set of instruments recorded every tremble around the peak—such as the rumble of ocean waves and earthquakes on the other side of the world—for two years. Other researchers tackled the system by studying the chemistry of the rocks themselves.
The results show that seismic waves creep along slowly in a zone east of Mount St. Helens, some 10 to 25 miles deep. Differences in minerals can influence the speed of seismic waves, but magma can be another source of this sluggishness. Perhaps rocks melt as expected near the rest of the Cascade volcanoes, the analysis suggests, but some diverts westward to squeeze through the subsurface and feed Mount St. The story from the rocks themselves fits with this picture. By melting samples of erupted rock under a variety of conditions in the lab, the team revealed that the sticky gas-rich magmas that give Mount St.
After the eruption, researchers may have even caught trembles from nearby this deep melt zone, as the earth adjusted to the draining of molten rock. For nearly a year after the blast, Moran says, tremors rumbled to the southeast of the peak. Subterranean shifts in magma can produce quakes around volcanoes, so knowing whether these tremors are in fact linked to Mount St. The choreographer of this magmatic dance is still being debated. Many scientists see clues in the surrounding landscape , which bears scars from millions of years of tectonic jostling that could help direct the modern flow of molten rock.
As the ocean between the two landmasses closed, seafloor sediments were scraped into a heap beneath the surface and squeezed into stone. The scientists sketched out structures from this merger using a method known as Magnetotellurics, which tracks the conductivity of rocks.
Sure enough, just beneath Mount St. Helens, a swath of such illumination marks the region where ancient marine sediments were turned into a particular rock type called metasedimentary. The analysis unveiled another surprise just to the east of the volcano: A vast area of low-conductivity rock sits just above where deep magma may pond.
The scientists believe this rock is a slug of now-cooled magma that formed millions of years before Mount St. Helens was born. The differences in the properties of this volcanic plug, known as a batholith, and the metasedimentary rocks of the suture zone may alter the stresses in the region and thus direct the magma flow.
The batholith limits magma from rising to the east of Mount St. A dense wall of rock beneath these metasediments, also revealed by the seismic array , may actually be part of this lost landscape, providing a westward stop for the flow of magma, says Jade Crosbie , a geophysicist with the USGS in Lakewood, Colorado, and part of the iMUSH team. While the iMUSH analyses help sharpen our view deep inside the planet, the picture remains far from complete, Moran says. Today, the remains of Siletzia can be seen only piecemeal at the surface, partially buried by flows of now solidified lava and soils studded with trees.
This leaves scientists debating where the suture zone—and its role in magmatic direction— precisely lies. As the researchers continue to sort through the sea of other data from iMUSH, many more questions dance in their heads. How does the system change over time? How quickly does the magma move?
How does such a vast zone of partly melted rock focus into a volcanic pinprick on the surface? Each potential answer helps shape our understanding of how and why volcanoes erupt, which can help researchers connect what happens at one volcano to the broader picture of volcanism around the world, says seismologist Helen Janiszewski of the University of Hawaii at Manoa.
In addition to the deep tragedy of sixty-one lives lost, several billion dollars in material damage was incurred, the effects of which will be felt for decades. There was also a psychological effect from the destruction of the once beautiful mountain that I had come to know through a number of ascents in my youth.
Those of us who are mountaineers in the northwest feel that we have lost an old friend. I was particularly stunned for it was on Mount St. Helens, about 60 miles from my home in Tacoma, that I first learned to climb and ski back in the 30s. I remember how fresh and warm the ground seemed as we started the aborted hike in the fog above Spirit Lake. Now it seems almost prophetic. One of my climbing partners, Wayne Smith, a physician from Chehalis, was carrying replacement thermometers for a weather shelter he had placed on the summit in one of his 60 ascents of the peak in recent years.
The nation too was numbed, humbled and awed by the press reports and photographs. But paradoxically, as a scientist I was excited, for this was a geological phenomenon unparalled in my lifetime.
But from the geological view, the eruption was not unusual, being only one of many in recent geologic time. Records reveal an eruption almost as large in , as well as lava and pyroclastic flows and much ash between and A. Before that there was a cataclysmic eruption four times as great in B. Vesuvius in A. Crandell and Donald R. Mullineaux in U. Geological Survey Bulletin C, published in This report also considers questions of geologic hazards from future eruptions and predicted a major outburst before the end of the century.
Geologists studying the phenomenon are convinced that this mountain will be rebuilt by ongoing natural forces, because it has been the site of so much activity over the past 30 to 40 centuries … at least one major eruption every years in the past 36, years.
These are paled, however, by the gigantic blowup in B. That eruption produced 46 times more ejecta than the Mount St. Helens event and deposited upwards of 20 feet of ash within miles of the vent, even throwing cobble-sized rocks as far northeast as the Province of Alberta.
As we look back over the history of Mount St. Helens, it is of interest to note the legend of the Cowlitz Indians that depicts this mountain as Loo-wit, a beautiful Indian princess who, to insure peace in the tribe, had been turned into this graceful mountain after her suitors warred to the death to woo her hand.
Was this legend born out of the cataclysm of ? A span of 26 years of eruptions was reported between and after which the princess lay dormant. On March 27, , a new crater opened on the mountain.
Dirty steam and ash stained the white snow of spring, the first outward sign of rebirth of the volcano. This was one week after a series of unusual seismic events were observed and believed to herald new lava movements at depth.
On through April and early May there were continued earth tremors of increasing magnitude, with clouds of ash and steam … some rising to 18, feet … emitted over a few seconds to a few hours at a time. White clouds meant phreatic water, steam resulting from the heating of downward percolating ground water. Darker clouds involved explosive bursts of solid material, actually pulverized clasts of rock tephra.
Increasingly, slides and avalanches occurred with ash-laden snow pouring down the slopes, as chocolate syrup off the sides of an ice cream sundae. Then the crater periodically heaved, seethed and erupted, deepening to feet and further cracking in its inner wall with more material sliding down.
This was followed by surface swelling of about five feet per day on the north side, as recorded by U. Geological Survey helicopter crews monitoring emplaced tilt meters and conducting precise laser distance measurements. By April 10, the upper flank had bulged out by more than feet. The scientists referred to this as the Forsyth Bulge, as it paralleled the Forsyth Glacier—see photos. Accompanying this was further cracking and inward flow of melt-water with such steam generation from dissipated ice and snow.
We now realize that the warnings were clear, because the entire side of the mountain was easing northward, with more cracks forming and ground temperatures increasing. On the seismographs, dramatically jagged lines of normal earthquakes gave way to smooth lines of the harmonic tremor, produced by the rising of lava from depth.
During the last week of April, an average of 33 quakes were reported per day. A few individuals sneaked to the summit and peered into the hissing crater, to return and blatantly report their reckless ventures to a press eager for information on the unfolding drama. The activity brought Dr. David Johnston to his Coldwater Ridge observation post, along with Reid Blackburn of the Vancouver Columbian, hired by the National Geographic Society to set up radio-triggered cameras and to take photos for the U.
Geological Survey. On May 17 it had brought Jim Fitzgerald with permission to do volcanological field work in the vicinity of Spirit Lake. He was to be joined there the following morning at eight A. Their lives were spared at A. Turning their car, they sped away at 90 mph as they watched the edge of the billowing cloud approach. Had they been in the center line of the mph blast, they would not have made it during those few seconds when the mountain literally tore herself apart.
At the same time, other observers turned and fled … but, as we know, not all were so lucky. Mount St. The eruption of Mount St. In , this mountain erupted explosively, killing 30, persons in the nearby town of Saint Pierre.
This is also the kind that characterized the eruption of Mount Laming- ton in New Guinea and of Bezymyannaya in Kamchatka in , mountains at far points in the so-called Pacific Ring of fire.
We now believe that Mount St. Mount Adams is in the background right. Forty years ago, after two months of earthquakes and small explosions, Mount St. Helens cataclysmically erupted. A high-speed blast leveled millions of trees and ripped soil from bedrock. The eruption fed a towering plume of ash for more than nine hours, and winds carried the ash hundreds of miles away.
Lahars volcanic mudflows carried large boulders and logs, which destroyed forests, bridges, roads and buildings. These catastrophic events led to 57 deaths, including that of David Johnston , a dedicated USGS scientist, and caused the worst volcanic disaster in the recorded history of the conterminous United States.
Had we known then what we know today about volcanoes, could the loss of life and economic damage caused by the Mount St. Helens eruption have been prevented or mitigated? Over the past 40 years, technology and the scientific study of volcanoes have made significant advances.
Better cooperation, monitoring and forecasting possibly could have allowed for earlier evacuations, hazard mitigation and reduced risk. But the truth is the eruption of Mount St. Helens sparked the advances in cutting-edge volcano science and monitoring that exist today. Mount St. Helens turned out to be the ideal laboratory to study volcanic activity. The eruption was the first large explosive eruption studied by scientists and observers using modern volcanology.
The volcano was also easily viewed and accessible. As a result, the eruption and its effects were heavily photographed from numerous vantage points. The debris avalanche opened the cone, and scientists were able to inspect its interior in a new and novel way.
The eruption jump-started interest in the study of explosive eruptions and monitoring efforts to improve warning systems that help mitigate hazards. The eruption underscored the importance of using as many monitoring tools as possible to track unrest and eruption activity. The north flank collapse and eruption at Mount St. We now know that type of terrain is evidence of a past flank collapse at that volcano about between , and , years ago that occurred without an eruption.
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