Notes on Mount St. Helens Volcano

Mount St. Helens (Lawetlat’la to the Indigenous Cowlitz tribe and Loowit or Louwala-Clough to the Klickitat) is an active volcano in Skamania County, Washington, in the Pacific Northwest of the United States.

It is 52 miles (83 kilometres) northeast of Portland and 98 miles (158 kilometres) south of Seattle.

Lord St Helens, a colleague of explorer George Vancouver who researched the region in the late 18th century, gave Mount St. Helens its English name.

The volcano is part of the Cascade Volcanic Arc, which is part of the Pacific Ring of Fire.

Mount St. Helens is most known for its May 18, 1980, massive eruption, which was the bloodiest and most economically damaging volcanic event in United States history.

57 people were killed, and 200 dwellings, 47 bridges, 15 miles (24 kilometres) of railway, and 185 miles (298 kilometres) of roadways were damaged.

A major debris avalanche created a lateral eruption[6] that decreased the elevation of the mountain’s top from 9,677 ft (2,950 m) to 8,363 ft (2,549 m), producing a 1 mile (1.6 km) wide horseshoe-shaped crater.

The volume of the debris avalanche was 0.6 cubic miles (2.5 km3). The 1980 eruption wreaked havoc on the terrestrial ecosystems surrounding the volcano.

Aquatic ecosystems in the area, on the other hand, profited significantly from the ash, allowing life to flourish fast. Many lakes in the vicinity had restored to normal by six years following the eruption.

The volcano has continued volcanic activity after its 1980 eruption until 2008.

Geologists expect that subsequent eruptions will become more devastating since the lava domes in the area require greater pressure to explode.

Despite this, Mount St Helens is a famous trekking destination that is climbed all year.

Mount St. Helens National Volcanic Monuments was designated in 1982 by U.S. President Ronald Reagan and the United States Congress.

Event Synopsis

On May 18, 1980, a magnitude-5+ earthquake followed by a debris avalanche released the confining pressure at the volcano’s summit by destroying the cryptodome.

This sudden pressure drop caused water temperature in the system to flash to steam, which then expanded explosively, launching a hydrothermal blast laterally across the landslide scar.

Because the upper section of the volcano was eliminated, the pressure on the magma system beneath the volcano was reduced.

A diminishing pressure wave travelled down the volcanic conduit to the underground magma reservoir, where it began to rise, form bubbles (degas), and explode explosively, resulting in a 9-hour long Plinian eruption.

Preliminary Activity

The earliest signs of activity at Mount St. Helens were a series of minor earthquakes on March 16, 1980.

The volcano erupted for the first time in nearly 100 years on March 27, following hundreds of further tremors.

Steam explosions blasted a 60- to 75-m (200- to 250-ft) broad hole through the summit ice cap, covering the snow-clad southeast region with black ash.

Within a week, the crater had expanded to nearly 400 m (1,300 ft) in diameter, with two massive crack networks crisscrossing the whole summit region.

The rate of eruptions increased from roughly one per hour in March to about one per day by April 22, when the initial period of activity ended.

Smaller eruptions started on May 7 and lasted until May 17.

By that time, the volcano had been shaken by over 10,000 earthquakes, and the north flank had extended outward around 140 m (450 ft) to produce a conspicuous bulge.

The bulge extended outward—nearly horizontally—at a steady pace of around 2 m (6.5 ft) every day from the beginning of the eruption.

The volcano’s extreme deformation was a solid indication indicating molten rock (magma) had climbed high into the volcano.

In reality, behind the surface bulge was indeed a cryptodome which had infiltrated into the volcano’s edifice but had yet to explode.

Conclusion:

The May 18 catastrophe killed 57 humans and countless animals, and the lateral concussive blasts blew down trees over a 200-square-mile (500-square-kilometre) region.

Mount St. Helens’ volcanic cone had been totally blasted away by the conclusion of the event, leaving a wedge crater with a height of 8,363 feet in lieu of its 9,677-foot (2,950-metre) top (2,549 metres). There were other eruptions until 1986, and a lava dome was built occasionally in the crater. Seismicity happened again between 1989 and 1991, as well as in 1995 and 1998 (including several minor explosions).