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 Iceland

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 Europe

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: Madeira, Canary Is., Cape Verde, etc.

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

Note Emperor Islands, seamounts

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

 

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More explosive environments