Earth Sciences 240A - Lecture 21 - Subduction volcanoes

 

Cascade Range

Currently active volcanoes

Adjacent to Juan de Fuca Plate

Previously active volcanoes

No subducting plate

Mount St. Helens

Most active volcano in North America (other than Alaska)

Mount St. Helens Eruption Warning

1975: 3 USGS geologists

Report noting eruption history

Most likely to erupt before 2000

1978

Detailed hazard map

Paths for flows, ash clouds, lahars

Most widely read paper in 1980!!

20-27 March 1980

First earthquake (M 4.2); Rapid increase in quake numbers

27 March

First eruption of steam + ash

Result of magma reaching groundwater

Great tourist attraction!

Decreased total quakes; increased large quakes

April – May 1980

Tilt meter data: Bulging side

Bulge quickly became visible: Growth ~ 1 m/day

12 May

Bulge at +150 m

Evacuation ordered

18 May 1980

Earthquake 5.1

Bulge moved downslope

Uncovered magma chamber

Plinean eruption: 20 km height; 9 hour duration

Lateral burst

Hot pyroclastic flows: 1080 km/h initial flow; 100 km/h (300-370oC) subsequent flows

Lahars, ash flows widespread

Subsequently

Many small eruptions as dome reconstructs; traced by earthquakes

Will erupt until Juan de Fuca gone

Total 1980 volume ejected: ~ 1 km3

Mount Mazama

Major eruption ~ 6800 ya

Caldera collapse (10 km wide)

Formed Crater Lake; Wizard Island (cinder cone) later

Estimated volume ejected: 40 km3

Santorini (and the legend of Atlantis)

North of Crete in Aegean Sea

Thera: Remnants of Santorini volcano

Home of the Minoans until 1500 BC

Devastating eruption in 1500 BC

Minoan culture wiped out

Total volume ejected: ~ 30 km3

Mt. Pelée, 1902

St. Pierre main city (French colony)

Lake in central crater

Slowly heated as magma rose

Broke crater walls; huge lahar

Viscous magma piled at dome

Side wall blew

St. Pierre destroyed

Ships in harbor sunk

First witnessed nuée ardente

Pelean eruption”

Mt. Pinatubo, 1991

Dormant for pervious 600 years

Biggest eruption in past 100 years

Early eruption very gas-rich

15-20 million tons SO2 into atmosphere

Approx. 5 km3 volume rock ejected

Caused global temp drop of ~ 1oC 1992-93; destroyed much ozone

 

Friday:  Subduction volcano film