ESS 106 - Living with Volcanoes: Mt St Helens 1
In the wake of the Mt. St. Helens eruption and the ash that fell across the northwest, locals called the falling ash this term, relating it to a meteorological phenomenon commonplace in winter.
"Snow fall that never melted"
What were the financial costs from the MSH eruption?
$2.7 billion in damages, 200 miles of roads destroyed, 15 miles of railway destroyed, 27 bridges lost, ash fall led to widespread agriculture losses and damaged transportation and utility systems, timber loss. (Shipping in Columbia river was also influenced)
What is mass wasting?
(A time when Catholics feel that they spent an hour of their Sunday sitting on uncomfortable benches listening to an uninspiring homily) A sedimentary process where material falls/flows downhill, such as landslides and mudflows.
How quickly did the bulge grow?
1.5 meters (5 feet) per day 140 vertical meters (460 feet) of swelling
What was the maximum column altitude and around what time did it get there?
12 miles high at around noon
What was the body count from the Mt St Helens eruption?
35 confirmed deaths, 22 missing, 128 survivors. (asphyxiation caused 17 deaths)
How many individual pyrocastic flows were there in the Mt St Helens eruptions?
5-15 individual flows
When did Mt. St Helens "run out of steam"?
5:15 pm May 18th
List the progression of events of the May 18th eruption.
8:32 AM - 5.1 magnitude earthquake 1 mile below the volcano Flank fails and slumps away --large blocks of the mountain slid off, one after the other Rapid decompression of cryptodome --caused lateral blast Quickly followed by vertical eruption column
How much material was deposited by the lahars?
95 million cubic yards of material deposited in the Columbia and Cowitz rivers
In class we observed a sample of the soil and sediment from the eruption divided into three layers A, B, and C. From bottom to top, what were the compositions of these layers?
A (bottom) basal with tree fragments and abundant eroded material B (middle) normally graded with fine materials depleted C (top) fine grained ash with wavy or laminate features
What is a "fire fountain"?
A kind of explosive volcanic behavior where lava is thrown up into the air, making for great photography but not much of a regional threat
What is a hummock?
A large block of material directly from the volcano that later became displaced and embedded in the soil after the eruption.
What is a lahar?
A large mud flow with lots of debris, such as rocks, trees, etc
A steam eruption from a volcano is known as a what?
A phreatic event
How large of an area did the debris avalanche from Mt. St. Helens cover and what was the average thickness?
Around 24 square miles with an average depth of 45m (450 ft) but up to 180m (600ft) in some places.
How long did the vertical eruption last?
Around 9 hours
At Mt. St. Helens around noon, the ash column began to do what?
Collapse and turn into pyroclastic flows down the mountain
What was the total damage caused by the lahars?
Destroyed 43 bridges and 200 homes
What was one immediate response to the increase in sediment in the Columbia river?
Dredging of the material
In what direction did the vertical ash column drift and why?
Drifted eastward due to easterly winds
What were the major precursors to the May 18th eruption of Mt St Helens?
Earthquakes (beginning March 16th 1980, up to 10,000 recorded in Mid-May) Steam (preatic) explosions (March 27th thru April) Melting of Snow (heat source) New Fractures on Flank THE BULGE
What volcanic activity followed the Mt St Helens explosion in 1980?
Effusive eruptions leading to the building of the lava dome (1980-1986)
A lava dome (like the one in Mt St Helens) is made by what kind of lava?
Effusive lava
What kind of volcanic behavior was most common for St Helens pre-1980?
Explosive (ash) eruptions (Other behavior included effusive lava eruptions)
True or False: There is no possibility of Mt St Helens regaining its old cone shape because the dome is not growing anymore
False! The dome could build up the volcano to its prior form in just a few more decades
True or False: There were a few survivors found in the direct blast zone of Mt. St. Helens when it erupted
False. Sadly, the blast was far to powerful and fast for anything to survive. However, there were a few survivors found in the standing-dead zone, on the edges of the tree-down zone
What is one long term response?
Filtering out the fine coarse sediment as it erodes into the river
In what cardinal direction(s) did the lahar from Mt. St. Helens flow?
First followed the drainages north but then turned west and towards the Columbia river
What are the techniques volcanologists use to study volcanoes and predict eruptions?
Ground Deformation Remote Sensing (Satellites) Seismicity Geophysical Measurements Hydrology Gas
In a pyroclastic flow, do heavy materials or light materials fall out first?
Heavy! (explaining the differences in fine grains vs coarse grain locations in the soil)
What kind of a topography was left behind in the aftermath of the Mt. St. Helens eruption?
Hummocky topography
Mt. Shasta in California, another cascade volcano, has what kind of topography similar to that now surrounding MSH?
Hummocky topography (around 30 miles from Mt Shasta, only at the base for MSH)
How are lava tubes formed?
In a deep lava flow the outside cools before the inside does, forming a tube as the inner lava flows out of the hardened shell
Mt St Helens had a pretty good conical shape (formed 2200 years ago) prior to its cataclysmic eruption in 1980. What does this shape tell us about the volcano?
It had frequent and relatively recent eruptions in its history in order to maintain that conical shape
Hummocky topography is indicative of what?
Large landslides, slumping, or the failing of flanks in the mountain (mass wasting)
Lava tubes are indicative of what kind of volcanic behavior?
Lava flows (non-explosive volcanism)
What problems arose from the ash fall?
Machines, such as engines, which rely on fine precision were damaged by the fine ash (which is essentially glass)
When did Mt St Helens erupt?
May 18th, 1980
Why was it tough for scientists to predict what would happen at Mt St Helens leading up to the eruption?
Mt St Helens possessed many different features indicative of many different kinds of volcanism (such as effusive lava flows and lava tubes) while lying on an oceanic-continental subduction zone where almost any kind of magma is fair game
On May 18th, 1980, was anything able to survive the lateral blast of the volcano?
Not really. Ground soil, animals, trees, everything was ripped up and blown into shreds that then became part of the blast.
What primarily composed the pre-1980 composites around Mt. St. Helens?
Pyroclastic deposits (welded together ash)
On a scale from the-video-we-saw-in-class to gifrickinhugeic, how large was the lahar from Mt. St. Helens?
REALLY REALLY REALLY BIG
How high did the eruption column reach and how fast did it get there?
Reached a height of 10 miles in 15 minutes
In terms of volume of material, speed, and direction, what made the lateral blast so devastating?
Speed and direction (there was not that much material when compared to the subsequent lahar flows and vertical eruption column)
What was in the initial debris avalanche in the Mt. St. Helens eruption?
The flanks of the volcano mixed with lots of rock, vegetation, bits of trees, etc.
How did scientists know that there were about four slumping flanks in the Mt. St. Helens eruption, utilizing hummocks?
The hummocks had multiple layers of material (Mt. St. Helens was a stratovolcano, after all) which corresponded to layers and striations found in the crater of the volcano.
What was the single most surprising and unexpected aspect about the eruption of Mt. St. Helens?
The lateral balst
What structures were remobilized into forming the large lahars?
The slumping of the northern flanks were remobilized into the lahar
What was one immediate economic consequence of the Mt. St. Helens eruption?
The stalling of trade down the Columbia river and specifically Portland, Oregon
How much material of the cone was removed by landslides and the blast of Mt. St. Helens?
The upper 1,312 feet of cone was removed
What is the "standing-dead" zone?
The zone where large trees remained standing yet were completely singed by the hot gases of the volcano.
When did the lahars begin and end for Mt. St. Helens on May 18th?
They began around 8:50 AM and continued on throughout the eruption
Where did most of the pyroclastic flows go?
To the north (spirit lake, etc.)
Looking at the aftermath of the blast, scientists were able to look at how this geological feature affects how the blast progresses.
Topography (the downed trees would indicate the direction of the blast. Understanding this could be useful for instrument placement)
In what direction(s) did the pyroclastic flows extend from Mt. St. Helens?
Trick question, all directions! It just fell down the mountain
True or False: Places like Tacoma and Seattle are built upon lahar deposits
True
True or False: There is no film or video recording of the Mt. St. Helens eruption
True (sadly)
True or False: the evacuation of dangerous areas leading up to the eruption of MSH was voluntary
True, making it easier for people to shrug it off, but close to the eruption it was closed to people going into the danger zone
How thick were the pyroclastic flow deposits from MSH?
Up to 300 ft in some places
Where is Mt St Helens located?
Washington state
How many states, even in some small portion, received ash from the eruption?
Washington, Idaho, Oregon, Montana, and Wyoming (5 states)
What is a pyroclastic flow?
When pyroclastic material (ash, bombs, etc.) is ejected into the atmosphere and some of that material flows at ground level
