Mt. St. Helens

Front 1

The Cascades volcanic arc lies along the leading edge of the

Back 1

North American Plate

Front 2

Is the Cascades volcanic arc intra‐oceanic or continental‐margin?

Back 2

Continental margin

Front 3

The Cascades volcanic arc extends from ____________ _______________ in the south, through _____________, and then ___________________ before terminating in southern
_________________

Back 3

northern California,  Oregon, Washington,  Canada

Front 4

List below at least two prominent volcanoes that lie within the Cascades arc from each of the following states: California, Oregon, Washington

Back 4

California: Mt Shasta and Lassen Peak, Oregon: Mt. Hood and Crater Lake, Washington: Mt. St.
Helens and Mt. Rainier

Front 5

Most of the present day volcanoes in the Cascades volcanic arc are younger than

Back 5

2 millionyears

Front 6

The most recent eruption of a volcano within the Cascades arc occurred in  

Back 6

1980

Front 7

The Mount St. Helens eruption had a VEI of

Back 7

VEI 6

Front 8

The visible cone of Mt. St. Helens prior to May 18, 1980 had formed in the last

Back 8

~2,200years

Front 9

What were the first signs of a potential eruption at Mount St. Helens?

Back 9

earthquake activity

Front 10

What kind of an event is produced when hot magma encounters ground water?

Back 10

steam-blast explosion

Front 11

Volcanic tremors are different from the distinct short jolts characteristic of earthquakes along
major faults.  They are described as a ______________________

Back 11

continuous rhythmic shaking of the land surface.

Front 12

. The volcanic tremors that occurred On March 31, 1980 were interpreted to signal what?

Back 12

The movement of magma or gases beneath the volcano

Front 13

About 10,000 earthquakes were recorded through mid‐may with the greatest concentration occurring in a small area less than ~2.57 km beneath a bulge on the north flank of Mt. St. Helens.  In fact, the bulge near the summit was more than ~137 meters (~450 feet) higher than it was in the summer of 1979 and was growing horizontally in a northward direction at a rate of ~1.5 meters/day (~5 feet/day).  In addition to the growing bulge, new cracks and fractures were
also evident along the north flank.  How was all of this activity interpreted?

Back 13

It was interpreted to signify that magma was being forced into a sub‐volcanic magma chamber located beneath the growing bulge

Front 14

On Sunday, May 18, 1980, Mt. St. Helens erupted explosively.  What triggered the eruption?

Back 14

A magnitude 5.2 earthquake with a focus located about 1 mile beneath the volcano

Front 15

The collapse of the north flank of the volcano produced a landslide‐debris avalanche.  How is this avalanche ranked in history

Back 15

It is the largest ever recorded

Front 16

The landslide‐debris avalanche traveled at speeds of

Back 16

177 to 249 km/hour

Front 17

Did the landslide‐debris avalanche have enough momentum to travel up and over a ridge ~350
meters (1,150 feet) high?

Back 17

yes

Front 18

The landslide‐debris avalanche traveled down the North Fork of the Toutle River and filled it to a
depth of

Back 18

45.7meters

Front 19

The collapse of the north flank of Mt. St. Helens unloaded the summit and triggered the expansion of high temperature‐high pressure steam trapped in voids and cracks, and gases dissolved in the dacitic magma that had produced the northward bulge.  The result of this sudden and catastrophic expansion of steam and magmatic gases was a northward‐directed

Back 19

lateral blast

Front 20

The northward directed lateral blast formed a _____________ _______ that devastated a fan‐shaped sector some 597.7 square kilometers in area.

Back 20

pyroclastic flow

Front 21

Though the pyroclastic flow created by the lateral blast occurred seconds after the landslide‐debris avalanche did it have enough speed to overtake the avalanche?

Back 21

yes, it overtook the landslide‐debris avalanche

Front 22

What is the maximum estimated speed of the pyroclastic flow created by the lateral blast?

Back 22

.~1078 km/hour

Front 23

The devastation from the lateral blast reached as far as _____ kilometers from the volcano.

Back 23

31km

Front 24

The area affected by the lateral blast can be subdivided into three roughly concentric areas.  
What are these three concentric areas called?

Back 24

The three concentric zones are the direct blast, channelized blast, and seared zones

Front 25

Was there anything left standing within the direct blast zone?

Back 25

No, everything that was standing was carried away or vaporized by the pyroclastic flow.

Front 26

What happened to trees within the channelized blast zone?

Back 26

They were all knocked down.

Front 27

What happened to trees within the seared zone?

Back 27

They were singed brown by the hot gases of the pyroclastic flow.

Front 28

The eruption column following the lateral blast climbed to what height?

Back 28

~19.3 km

Front 29

Was lightening produced during the eruption?  If so, then what were its effects?  

Back 29

Yes, vegetation and trees were set afire.

Front 30

Following the lateral blast, what process produced pyroclastic flows as the eruption column climbed to ~19.3 km?

Back 30

Parts of the eruption column collapsed, generating several pyroclastic flows that moved down the flanks of the volcano.

Front 31

Minutes after the May 18 eruption destructive lahars were produced.  What specific events initiated the development of these lahars?   

Back 31

Pyroclastic flows traveling over the glacier and snowfields near the summit of Mt. St. Helens

Front 32

The early lahars likely were the products of wet or dry rock avalanches?

Back 32

The early lahars were all produced by wet rock avalanches.

Front 33

The lahar with the greatest volume entered and flowed down the North or South Fork Toutle River?

Back 33

It flowed down the North Fork Toutle River

Front 34

In the two week s following the May 18 eruption, how far had ash spread?

Back 34

It had drifted around the globe

Front 35

The eruption that began on May 18, 1980 was over by

Back 35

May 19th

Front 36

The May 18 eruption lowered the highest point on the summit of Mt. St. Helens by

Back 36

400 meters

Front 37

How many lives were lost due to the May 18, 1980 eruption at Mt. St. Helens?

Back 37

57

Front 38

What was the total cost of the damage produced by the May 18 eruption?

Back 38

2.74 billion