VOLCANOES

1. Increase Temperature

2. Decrease Pressure

3. Alter Composition (to decrease melting temperature)

Melting the Mantle

1. Mid Ocean Ridges (decompression)

2. Hotspots (high temperature + decompression)

3. Subduction zones (add water, decrease melting temperature)

Where Does Melting Occur?

Magma: result of melting rocks

Lava: magma that flows out over the surface

Pyroclastic rocks:Rock fragments resultingfrom explosive volcanism, depositedby sedimentary processes.

Volcanic Products

Pu'u O'o, Hawaii

Fire Fountain

1. Viscosity Depends on SiO2 content

“Mafic” magmas: poor in SiO2, rich in Mg, Fe

low viscosity

“Felsic” magmas: rich in SiO2, poor in Mg, Fe

high viscosity

Controls on Eruptive Style

2. Volatiles H2O and CO2

“Dry” mantle rocks generate mafic magma “Wet” mantle rocks generate felsic magma

Volatiles decrease magma viscosity when dissolved

Controls on Eruptive Style

1. Fissure eruption2. Shield volcano3. Pillow basalts4. Domes & Cones5. Strato-volcano

Subaerial eruption,low viscosity magma

Submarine eruption,low viscosity magma

Subaerial eruption,high viscosity magma

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Eruption Style

Fissure eruption:Krafla, Iceland

Hexagonal cooling joints in basalt: Devil's Postpile, California

Hexagonal cooling joints in basalt,e.g. Giant’s Causeway, Antrim

Sunset Crater, Arizona

Bonito Flow

Cinders

Cerro Negro, Nicaragua1947 eruption