Download 3A8 Week 05 Lecture 13-Volcanoes Part One

VOLCANOES
Melting the Mantle
1. Increase Temperature
2. Decrease Pressure
3. Alter Composition
(to decrease melting temperature)
Where Does Melting Occur?
1. Mid Ocean Ridges
2. Hotspots
(decompression)
(high temperature + decompression)
3. Subduction zones
(add water, decrease
melting temperature)
Volcanic Products
Magma: result of melting rocks
Lava: magma that flows out over the surface
Pyroclastic rocks:
Rock fragments resulting
from explosive volcanism, deposited
by sedimentary processes.
Pu'u O'o, Hawaii
Fire Fountain
Controls on Eruptive Style
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
Eruption Style
1.
2.
3.
4.
5.
Fissure eruption
Shield volcano
Pillow basalts
Domes & Cones
Strato-volcano
}
Subaerial eruption,
low viscosity magma
Submarine eruption,
low viscosity magma
}
Subaerial eruption,
high viscosity magma
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
Cinders
Bonito
Flow
Cerro Negro, Nicaragua
1947 eruption