Earth Sciences
Lecture 19 Volcanoes II
Introduction
Avg. densities: Mantle: 3.3; Hottest mantle melt: 2.9
Volcanic activity locations
Mid-ocean ridges
Subduction
zones/rifts
Hot spots
Eruptive environments
Early classification attempt (Never satisfactory; abandoned)
Hawaiian (+ Icelandic)
Strombolian
Vulcanian
Pelean
Current
classification (Abbott)
Non-explosive
Icelandic
Hawaiian
Low to moderate explosivity
Strombolian
Vulanian
High explosivity
Vesuvian
Plinian
Pelean
Caldera type
Oceanic spreading
centers
80% of all volcanic activity
Peridotite mantle: 1200-1300oC;
Max 30-40% partial melt (basalt)
Magma
>50%: sheeted dikes (ophiolite sequence)
May pause in ‘chambers’
Submarine eruptions
Critical water depth: 100m
Below: Pressure too great for explosive action
Pillow lavas and
‘black smokers’; Seamounts
MORB: Tholeiitic:
Ca-px, Ca-plag, mt, (ol); low in Na, K
Above
‘hydrovolcano’: Surtsey
Surtseyan Eruption (not in text)
Icelandic type, But started as submarine
Surtsey: off sw
coast
Grew 1963-67
Above sea level: cinder cones
Icelandic Type
Basalt injection along rift fractures
Steps in a fissure eruption
Lava fountaining
Lava plus gas
Spatter cones
Quiet flows
System plugs and process starts
again
Laki, 1783 (the
exception to the rule)
Initial flow (for 50 days)
5000 m3 per second (=50% Amazon)
Erupted for 8 months; Total 14.73 km3 lava
Largest land eruption in human
history
High gas content (blasted to 15 km
height); high SO2, F; ‘blue fog’ over
Killed 9000 humans, 75% sheep, 50%
cattle (mass starvation)
Hawaiian Eruptive
Environment
Activity from hot spot over plume; [Avg.
life of plume: ~100 my]
Similar activity in Mid-Atlantic:
Alkali basalt (not MORB); Higher
Na, K, Si, Al
From: richterite,
kaersutite, mica in deep mantle
Toward top mixes
with ‘MORB’
Sporadic magma progress up
monitored
Hawaiian Eruption
Oldest ~ 75 mya; Aleutian Trench
Current
Lava fountaining
Quiet flows (pahoehoe, aa)
Non-stop in recent years
Deaths
Soldiers of King Keoua; gas in valley
Next
More explosive
environments