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

Anatomy of a volcano

Types of lava (based on silica content)

•felsic, andesitic, mafic

Types of Volcanic Deposits

•Basaltic: Flood Basalts

Different Layers of Basalt

Types of Volcanic Deposits (cont)

•Basaltic: Pahoehoe,

Aa,

Pillow lava

•Rhyolitic

•Andesitic

Textures

glassy, vesicular (related to cooling rates and gas content)

Explosive vs. Gentle Eruptions

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)

Cinder-Cones Volcanoes

i. pyroclastic eruptions

Composite Volcanoes (stratovolcano)

i. alternating lava and pyroclastic eruptions

Craters

i. central depression at the summit of most volcanoes

Calderas (collapsed craters)

Other Volcanic Features

1. diatremes

2. fumaroles, hot springs, geysers

Plate Tectonics and Volcanism

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

Benefits of Volcanism

• atmosphere, oceans, land masses, soil, geothermal uses