Lecture 11 - Volcanic eruption case studies

Question 1

Where is the Cascade Volcanic Arc?

Answer 1

It stretches from northern California to the Washington/British Columbia border.

Question 2

Prior to 1980 why was the northern flank of Mt. St. Helens unstable?

Answer 2

A previous eruption in 1843 had deposited a pyroclastic flow there.

Question 3

What first indicated renewed unrest of Mt. St. Helens?

Answer 3

An earthquake swarm starting mid March 1980

Question 4

What happened to Mt. St. Helens’ north flank during March 1980?

Answer 4

It bulged due to pressure from expanding gases and magma.

Question 5

On what day did Mt. St. Helens erupt?

Answer 5

18th May 1980 (Sunday)

Question 6

What caused the collapse of the unstable north flank of Mt. St. Helens?

Answer 6

A Mw 5.1 earthquake and major landslide.

Question 7

What triggered the pyroclastic flows at Mt. St. Helens?

Answer 7

The release of pressure led to a lateral blast that blew the north side of the mountain out, triggering pyroclastic flows.

Question 8

What was the cause of the formation of the vertical plume at Mt. St. Helens?

Answer 8

The lateral blast “cleared the volcano’s throat”, driving formation of a vertical plume.

Question 9

How many people were killed by the eruption?

Answer 9

57 including 3 in the “no-go” zone

Question 10

Over what area was ash dispersed?

Answer 10

~60,000 km²

Question 11

How long were motorways closed for?

Answer 11

two weeks

Question 12

How many flights were cancelled due to the eruption?

Answer 12

around 1,000 flights

Question 13

How large was the “no-go” zone?

Answer 13

∼5 km from the volcano

Question 14

What was the reach of the lateral blast?

Answer 14

The lateral blast obliterated everything within ∼10–15 km of the volcano on its northern side, and seared certain areas as far as ∼25 km away.

Question 15

How many m³ of timber were damaged or destroyed by the blast?

Answer 15

9 million m³

Question 16

What created the lahars?

Answer 16

Melting of the volcano’s ice cap and mixing of pyroclastic material with river water generated massive lahars on all sides of the volcano.

Question 17

How many properties were destroyed by the lahars?

Answer 17

∼200

Question 18

What was the effect of volcanic debris and lahars on the Columbia river?

Answer 18

Volcanic debris carried downstream was dumped into the Columbia river, abruptly reducing the depth of its navigable channel from 40 ft to as little as 15 ft. This forced closure of the port of Portland — the fifth largest on the U.S. Pacific coast — upstream. It took several days to reopen the channel partially, a month to reopen it fully, and nearly a year to dredge it to its previous depth.

Question 19

What type of eruption was the 1980 Mt. St. Helens eruption?

Answer 19

Plinian-style eruption

Question 20

How long did Mt. St. Helens erupt for?

Answer 20

∼9 hours

Question 21

What elevation did the vertical plume reach?

Answer 21

∼20 km

Question 22

How many tonnes of ash were deposited over a large swath of the northwestern U.S?

Answer 22

∼500 million tonnes