Earth Sciences 240A - Lecture 21 - Subduction volcanoes
Currently active volcanoes
Adjacent to Juan de Fuca Plate
Previously active volcanoes
No subducting plate
Most active
volcano in
Mount
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
Major eruption ~ 6800 ya
Caldera collapse (10 km wide)
Formed
Estimated volume ejected: 40 km3
Santorini
(and the legend of Atlantis)
North of
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
Slowly heated as
magma rose
Broke crater
walls; huge lahar
Viscous magma piled at dome
Side wall blew
Ships in harbor sunk
First witnessed nuée ardente
“Pelean
eruption”
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