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VOLCANISM
•80% of the earths crust is composed of volcanic rocks
•Volcanism: the process by which magma from the interior of the earth rises through the crust, emerges onto the surface as lava, and cools into volcanic rock
Types of lava (based on silica content)
•felsic, andesitic, mafic
Types of Volcanic Deposits
•Basaltic: Flood Basalts
Different Layers of Basalt
Pyroclastic Deposits
•Air fall or tephra
•Ejecta: ash, tuffs and breccias, bombs
Explosive vs. Gentle Eruptions
•Pyroclastic Flows
•Surge – directed blast of hot material
•nuée ardante (glowing cloud)
•Mudflow
•lahar
Landforms and Eruptive Styles
1. Shield Volcanoes
i. gentle eruptions consisting of basaltic lavas
ii. low silica and low viscosity
iii. fissure eruptions and flood basalts
iv. example: Mauna Loa
Volcanic Domes
i. explosive eruptions consisting of felsic volcanism
ii. high silica and high viscosity
iii. ash flows
iv. example: Mount St. Helens (1980)
Plate Tectonics and Volcanism (cont)
1. Spreading zone volcanism: basalts
2. Convergence zone volcanism: Ring of Fire
i. OC-OC convergence: basalts and occasional andesites
ii. OC-CC convergence: ash and rhyolite
3. Interplate volcanism
4. Hot Spot Activity