The Earth Story

Over the crater

Astronaut Jeff Williams captured this panoramic video while the International Space Station moved over Mt. St. Helens in Washington State. In 1980, the north side of the volcano was blown off in a lateral blast eruption, leaving a gap on the north edge of the crater. In this frame you can clearly see the broken crater wall, along with channels carving downward into the newly-deposited ash layers. Spirit Lake, along with trees that arrived in the lake during the avalanche associated with the eruption, can be viewed at the upper left of the video. Inside the crater a small lava dome, made of the same magma that caused the 1980 explosion, began extruding in 2004 and can be clearly made out. Along the edge of the dome, a new glacier is forming.

-JBB

Video Credit: Jeff Williams https://twitter.com/Astro_Jeff/status/765613298905604096

Lateral Blast

Today, May 18th, is the 36th anniversary of the eruption of Mt. St. Helens in Washington State. This photograph of the volcano was taken earlier this month by astronaut Tim Peake from the International Space Station and although much of the surrounding landscape has recovered, the area damaged during the blast is still clearly outlined.

Mt. St. Helens’s 1980 eruption was a rare type known as a lateral blast. Most volcanic eruptions send material upward, in line with the direction of the plumbing system below. When those volcanoes produce pyroclastic flows and surges, they occur because of factors like the density and temperature in the ash cloud above. Mt. St. Helens was a different beast; the north side of the volcano collapsed, releasing stored pressure on the side of the volcano rather than at the top and sending a blast of debris out horizontally.

Both the location of the volcano in the United States (easily accessible to USGS scientists) and the unique style of eruption made the St. Helens eruption one of the best studied in the world. It is now recognized that lateral blasts, like St. Helens in 1980, occur globally about once every decade on average. They tend to be driven, as in the St. Helens case, by sector collapses and avalanches along one side of the volcano, or by the collapse of lava domes that similarly fall off to one side. In the case of St. Helens, the lateral blast did damage far beyond the limits of the evacuation zone since all the force of the eruption was in a single direction, so understanding the mechanics of eruptions like this one is important for understanding future volcanic hazards.

Mt. St. Helens, like many volcanoes, has continued its life since the 1980 eruption. Geologic investigation even shows that 1980 was not the first lateral blast in St. Helens’s lifetime; a smaller one occurred about a thousand years ago on this same damaged north slope. About a decade ago, the same magma that fed the 1980 eruption appeared at the summit and began slowly building a lava dome; since the magma had mostly degassed that eruption wasn’t explosive.

In the plumbing system beneath the volcano, earthquake pulses every few years show that molten rock continues entering the system, keeping it active. It could be years, decades, or even centuries until the next eruption; at present scientists don’t understand the conditions required to bring a volcano at the surface to life. Ongoing research suggests that these systems are complicated, constantly churning new batches of molten rock through their plumbing systems, mixing old and new batches together, and eventually this process creates the conditions required for a future eruption.

-JBB

Image credit: Tim Peake/ESA/NASA https://flic.kr/p/G2qATr

Recent St. Helens Science: http://www.sciencedirect.com/science/article/pii/S0377027306001879 http://pubs.usgs.gov/gip/msh/lateral.html http://www.wired.com/2016/05/mount-st-helens-recharging-magma-stores-setting-off-earthquake-swarms/

Lateral blast

These photos were taken by photographer John V. Christiansen at 8:32 a.m. local time on May 18, 1980. They were later featured in a 1981 edition of National Geographic magazine.

The shots were taken from Mount Adams, looking across at Mt. St. Helens just over 50 kilometers away.

They capture perfectly the beginning of the lateral blast eruption. Pressure had been building for weeks inside of Mt. St. Helens, deforming the north side of the volcano into a bulge. An earthquake and an avalanche on the morning of May 18th ripped open that bulge and released the pressure on one side of the mountain. The mountain exploded; not upwards like most volcanoes, but sideways, sending an enormous cloud of superheated rock, gases, and large pieces of the mountain out sideways, sandblasting everything in their path.

-JBB

Image credit: National Geographic

1980 Mount St. Helens Eruption

It’s been 35 years since Mount Saint Helens erupted at 8:32am on Sunday, May 18, 1980. The warning signs of an imminent eruption had been apparent since March, so the U.S. Geological Survey was monitoring the volcano extensively. The eruption is considered the most studied of the 20th century.

The volcanoes in the Cascade Mountain range formed by the subduction of the Juan de Fuca, Explorer, and Gorda Plates under the North American Plate. They are part of the Ring of Fire. Helens is a relatively young and active volcano prone to explosive eruptions. It has erupted approximately once every 100 years since 1400C.E. Before 1980, it had been 130 years since the last one.

The first real sign of danger came on March 16, 1980, when a 4.2 magnitude earthquake was recorded on a newly established system of seismometers meant to help monitor the Cascades. Three days later, a 4.0 was recorded. After that the quakes beneath the volcano begin to occur in swarms; hundreds were recorded every day.

On March 27, the first steam eruption occurred. The rising plug of magma heated the groundwater above causing an explosion of steam and ash that blasted a 60 to 75 meter (200 to 250 feet) wide crater into the mountain summit. Steam eruptions became a daily occurrence through April 22; then they stopped for a short time, and resumed on May 7.

During this time the north flank of the mountain bulged out 140 meters (450 feet). The deformation was evidence that a cryptodome had formed - a body of magma that rises into the volcano but doesn’t erupt. The bulge grew nearly horizontal at a rate of about 2 meters (6.5 feet) per day. As it grew and the steam eruptions continued, the earthquake count reached 10,000.

At 8:32am on May 18, after 4 days without any steam eruptions, a 5.1 magnitude earthquake struck. As it did, the bulge and northern flank of the summit unexpectedly plunged down from the mountain in the largest landslide in recorded history. The landslide took part of the cryptodome with it. Cryptodomes are extremely hot and under very high pressure, so the sudden removal immediately depressurized the volcano’s magmatic system and triggered powerful, explosive eruptions that went out laterally through the opening instead of straight up. The lateral blast caught up to the landslide and accelerated the debris to at least 480 km/hr (300 mi/hr) and knocked down virtually every tree in the dense forest within the blast zone.

Soon after the initial blast, an eruption column of tephra (airborne rock fragments & ash) rose out of the summit crater. Within 15 minutes it reached a height of more than 24 km (15 miles). The eruption lasted 9 hours and, along with the prevailing wind, sent 520 million tons of ash eastward over the entire US in about 3 days. The ash cloud circled the globe in about 15 days.

Helens had glaciers at its summit, which the blast quickly melted. The melted water, along with groundwater released in the landslide, mixed with hot ash and magma to form lahars - a mixture of water and volcanic debris that move rapidly downstream. They destroyed some bridges and homes.

56 people died during the eruption, including volcanologist David Johnston and volunteer observers. They were located at observation points that were thought to be relatively safe. See our post from yesterday (http://on.fb.me/1IKvssE) for more information.

Photo Credit: Austin Post, scanned photograph by USGS http://bit.ly/1bUxjOz

References: http://1.usa.gov/1PoRRBt http://on.doi.gov/1dSXTHU http://on.doi.gov/1r7ZG2O http://bit.ly/1L8akw3

This day in geology. May 18, 1980, Mount St. Helens erupted with fury. A plume of ash, steam, and debris was sent to a height of 80,000 feet. The mountains north face completely collapsed in one of the largest landslides in recorded history. It lost 1300 feet of altitude and nearly 2/3 cubic mile of material. The ash cloud circled the earth in 18 days and provided for spectacular sunsets. -NF

http://vulcan.wr.usgs.gov/Volcanoes/MSH/ Video of eruption: http://www.youtube.com/watch?v=bgRnVhbfIKQ http://en.wikipedia.org/wiki/1980_eruption_of_Mount_St._Helens

Lateral blast These photos were taken by photographer John V. Christiansen at 8:32 a.m. local time on May 18, 1980. They were later featured in a 1981 edition of National Geographic magazine. The shots were taken from Mount Adams, looking across at Mt. St. Helens just over 50 kilometers away. They capture perfectly the beginning of the lateral blast eruption. Pressure had been building for weeks inside of Mt. St. Helens, deforming the north side of the volcano into a bulge. An earthquake and an avalanche on the morning of May 18th ripped open that bulge and released the pressure on one side of the mountain. The mountain exploded; not upwards like most volcanoes, but sideways, sending an enormous cloud of superheated rock, gases, and large pieces of the mountain out sideways, sandblasting everything in their path. -JBB Image credit: National Geographic