Volcanism and Extrusive Rocks Physical Geology

Volcanism and Earth’s Systems Atmosphere originally created from gases released by magmas Hydrosphere produced by condensation of volcanic water vapor 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 Resulting crop failures and famines Volcanic catastrophes Mt. St. Helens, Pompeii, Krakatoa, Tambora, Crater Lake https://www.youtube.com/watch?v=e-TMtRh8AIs

Crater Lake, Oregon

Eruptive Violence and Physical Characteristics of Lava Violence of eruptions controlled by: Amount of Dissolved gases in the magma Water vapor, carbon dioxide, sulfur dioxide, etc. The more dissolved gases, the more fluid the lava 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

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 Texture terms: Glassy - glass without mineral crystals ex: Obsidian Fine-grained - most crystals < 1 mm ex: Basalt Porphyritic - larger crystals in a matrix of much finer crystals or glass ex: 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 Cinder cones Broad Gently sloping Composed of solidified lava flows Flows often contain lava tubes Cinder cones Small Steeply sloping Composed of a pile of loose cinders

Types of Volcanoes Composite volcanoes Volcanic domes Also called 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) ex: Cascade volcanos Volcanic domes Extremely high viscosity, degassed, felsic lavas (often glassy, e.g., obsidian)

Other Eruption Types Flood eruptions Submarine 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?) Insert Fig. 22.18 here Insert Fig. 22.27A here Olympus Mons on Mars Volcanic eruption on Io Insert Fig. 22.14A here Lava domes on Venus Lava channel in volcanic plains on Moon