Chapter 4 Volcanism and Extrusive Rocks

Atmosphere

Originally created from gases released by magmas

Hydrosphere

Produced by condensation of volcanic water vapor

Biosphere

Positively and negatively influenced by volcanism

Lava flows

Weather to produce fertile soils

Ash

Weather to produce fertile soils

Violent eruptions

Can destroy nearly all life in their paths

Ash

Can trigger rapid climate changes and contribute to mass extinctions

Volcanism

Occurs when magma makes its way to the Earth's surface

Volcanoes

Landforms formed by the extrusion of lava

Lava

Produced when magma reaches Earth's surface

Explosive eruptions

Produces rapidly cooled rock fragments called Pyroclasts, size ranges from dust (ash) to boulders (blocks and volcanic bombs)

Effusive eruptions

Calm oozing of magma out of the ground produces lava flows

Mt. Merapi, Indonesia, 2010

Example of explosive eruption

Hawaii, 2011

Example of effusive eruption

Creation of New Land

Lava flows build up volcanic islands like Hawaii

Geothermal Energy

Underground heat generated by igneous activity

Effect on Climate

Very large eruptions can result in measurable global cooling resulting in crop failures and famines

Dissolved gases in magma

Control the violence of eruptions

Viscosity

A fluid's resistance to flow, controlled by silica content, lava temperature, and amount of dissolved gases

Mafic Lava Flows

Low viscosity and flows easily

Pahoehoe

Type of mafic lava flow with a smooth, billowy, or ropy surface

A'a

Type of mafic lava flow with a rough, jagged surface

Flood Basalts

Very low viscosity and flows very easily from erupting fissures

Columnar Jointing

Parallel mostly six-sided vertical columns formed from effusive eruptions

Submarine Lava Flows

Pillow structure formed as lava flows into water

Intermediate and Felsic Lava Flows

Thicker viscous lavas that flow short distances

Pyroclastic Materials

Include dust, ash, cinders or lapilli, blocks and bombs

Pyroclastic Flows

Mixture of gas and pyroclastic debris that flows rapidly down slope

Shield volcanoes

Broad landforms with gently sloping sides, composed of solidified lava flows, cover large area

Cinder cones

Small, steep-sided landforms composed of a pile of loose cinders

Composite volcanoes

Moderately to steeply sloping cone-shaped volcanoes composed of alternating layers of lava and tephra

Distribution of Composite Volcanoes

Mainly located around the Pacific Ocean (Ring of Fire) and Mediterranean Sea

Rhyolite

High silica; light-colored extrusive rock

Basalt

Low silica; dark-colored extrusive rock

Andesite

Intermediate silica and color extrusive rock

Glassy

Extrusive rock texture with no mineral crystals, e.g., Obsidian

Fine-grained

Extrusive rock texture with most crystals < 1 mm, e.g., Basalt

Porphyritic

Extrusive rock texture with larger crystals in a matrix of much finer crystals or glass, e.g., Andesite

Vesicular/Frothy

Extrusive rock texture with trapped gas bubbles, e.g., Vesicular basalt, Pumice

Fragmental

Extrusive rock texture with particles blasted apart by explosive eruptions, e.g., Dust and ash, Cinders, Blocks and bombs

Lava Domes

Extremely high viscosity, degassed, felsic lavas (often glassy, for example, obsidian)

Vent

Opening through which lava erupts

Crater

Basin-like depression over the vent at the summit of the volcano

Caldera

Volcanic depression much larger than the original crater, having a diameter of at least 1 km

Volcanic catastrophes

Examples include Mt. St. Helens (Washington), Pompeii (Italy), Krakatoa and Tambora (Indonesia), Crater Lake (Oregon)

Pyroclastic flows

Account for the largest number of deadly events, e.g., Pompeii

Volcanic gases

Include carbon dioxide, responsible for events like Nyos Cameroon

Volcanic mudflows (Lahars)

Example from Armero Colombia

Volcano status

Approximately 1,500 potentially active volcanoes worldwide, categorized as active, dormant, or extinct

Hawaiian Volcano Observatory

An example of a volcano monitoring institution

Shield Volcanoes (Extraterrestrial)

Found on Venus, Mars, and Io, e.g., Mons Olympus

Lava domes (Extraterrestrial)

Found on Venus, Moon

Flood eruptions (Extraterrestrial)

Very fluid and create extensive flat lava plains, e.g., Moon, Mars, Venus, Mercury

Volcanic Activity at Divergent Boundaries

Involves decompression melting, effusive eruptions of basaltic magmas, and formation of most of the sea floor, e.g., Mid-oceanic ridges, Iceland

Volcanic Activity at Convergent Boundaries

Involves explosive composite volcanoes and viscous andesitic lavas

Within-Plate Volcanic Activity

Involves mantle plumes (Hot Spots) like Hawaii and Yellowstone, and basaltic magma/lava

Flood eruptions (Other Eruption Types)

Very fluid, extremely large in volume, and create extensive lava plateaus

Submarine eruptions (Other Eruption Types)

Nearly always basaltic, e.g., Mid-ocean ridge eruptions, Pillow basalts, Columbia river flood basalts