Volcanism and Extrusive Rocks

Volcanism and Earth’s Systems• Atmosphere originally created from gases released by

magmas

• Hydrosphere produced by condensation of volcanic water vapor; chlorine and sulfate in oceans from dissolved gases

• Biosphere both positively and negatively influenced by volcanism– Lava flows and ash weather to produce fertile soils– Violent eruptions can destroy nearly all life in their paths– Large amounts of ash and volcanic gases in atmosphere can trigger rapid

climate changes and contribute to mass extinctions

Volcanic Eruptions

• Lava is produced when magma reaches Earth’s surface

• Explosive eruptions can produce rapidly cooled rock fragments called pyroclasts– Size range from dust (ash) to boulders (blocks

and volcanic bombs)

• Calm oozing of magma out of the ground produces lava flows

• Pyroclastics and lava flows form extrusive igneous rocks

• Lava flows and pyroclasts pile up to form volcanoes

Living with Volcanoes• Mythology, religion and volcanoes

– Hawaii - Pele– Iceland - Loki

• Growth of volcanic islands (Hawaii)

• Geothermal energy– Natural steam harnessed as clean

energy resource

• Climatic effects– Very large eruptions can result in measurable global

cooling: Mt Pinatubo cooling

– Resulting crop failures and famines

• Volcanic catastrophes– Mt. St. Helens, Pompeii, Krakatoa,

Tambora, Crater Lake

The people of Pompeii; mummified in 5-8 meters of hot ash in A.D. 79

The smoldering city of Pierre, Martinique

Eruptive Violence and Physical Characteristics of Lava

• Violence of eruptions controlled by:– Dissolved gases in the magma

• Water vapor, carbon dioxide, sulfur dioxide, etc.

– Ease/difficulty of gases escaping to atmosphere

• Viscosity- a fluid’s resistance to flow– Silica content

• Higher silica contents produce higher viscosities

– Lava temperature• Cooler lavas have higher viscosities

– Amount of dissolved gases• The more dissolved gases, the more fluid the lava

Extrusive Rocks and Gases

• Scientific Investigation of Volcanism– Rocks, gases and events from observed

eruptions compared to similar lavas elsewhere to infer the nature of past activity

• Rock Composition– Rhyolite - high silica; light color

– Basalt - low silica; dark color

– Andesite - intermediate silica and color

Extrusive Textures

• Texture - appearance of a rock with respect to size, shape and arrangement of its grains

• Glassy - glass without mineral crystals

– Obsidian

• Fine-grained - most crystals < 1 mm

– e.g., Basalt

• Porphyritic - larger crystals in a matrix of much finer crystals or glass– e.g., Andesite

Extrusive Textures

• Vesicular/Frothy - trapped gas bubbles

– Vesicular basalt

– Pumice

• Fragmental - particles blasted apart by explosive eruptions

– Dust and ash (<2 mm)

– Cinders (2-64 mm)

– Blocks and bombs (>64 mm)

Volcanic Landforms

• 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

Types of Volcanoes

• Shield volcanoes– Broad

– Gently sloping

– Composed of solidified lava flows of low viscosity

– Flows often contain lava tubes

• Cinder cones – Small

– Steeply sloping

– Composed of a pile of loose cinders: pyroclastic

Types of Volcanoes• Composite volcanoes

– AKA stratovolcanoes– Moderately to steeply sloping– Constructed of alternating layers of pyroclastic

debris and solidified lava flows– Composed primarily of intermediate

composition volcanic rocks (i.e., andesite)– Most common type of volcano at convergent

plate boundaries (e.g., Pacific Ring of Fire)

• Volcanic domes– Extremely high viscosity, degassed, felsic lavas

(often glassy, e.g., obsidian)

Lava dome

Mt Fuji: Stratovolcano

Where do we see deep earthquakes? What is happening there?

The ‘Ring of Fire’

Other Eruption Types• Flood eruptions

– Very fluid (basalts)

– Extremely large in volume

– Create extensive lava plateaus

– Eruption times correspond with largest mass extinction events

• Submarine eruptions– Nearly always basaltic

– Mid-ocean ridge eruptions

– Pillow basalts

Columbia river flood basalts

Pillow basalts

Volcanism on Other Planets

• Shield volcanoes– Venus, Mars, Io

• Lava domes– Venus, Moon

• Flood eruptions– Very fluid (basalts)– Extremely large in volume– Extensive flat lava plains

(Moon, Mars, Venus, Mercury?)

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Olympus Mons on Mars

Lava domes on Venus

Lava channel in volcanic plains on Moon

Volcanic eruption on Io