Volcanism and Extrusive Rocks Chapter Four Volcanism and Extrusive Rocks

VOLCANOES AND VOLCANISMS VOLCANO: LANDFORM THAT RESULTS FROM THE ACCUMULATION OF LAVA AND ROCK PARTICLES AROUND AN OPENING (OR VENT) IN EARTH’S SURFACE   VOLCANISM: SET OF GEOLOGICAL PROCESSES THAT MARK THE ASCENT OF MAGMA TO THE EARTH’S SURFACE ~ 2000 YRS ------- 600 VOLCANOES ERUPTED 50 VOLCANOES/YEAR VOLCANOES ARE WINDOWS INTO THE EARTH WATER AND AIR ----- CAME DUE TO VOLCANIC ERUPTIONS HYDROSPHERE AND ATMOSPHERE

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 Lava flows and pyroclasts pile up to form volcanoes

Classification of volcanoes ACTIVE EXTINCT ACTIVE CURRENTLY NOT ERUPTED FOR OR 10,000 YRS OR SO ERUPTED RECENTLY e.g., DORMANT VOLCANO: NOT ERUPTED RECENTLY (~1000 YRS OR SO) LIKELY WILL ERUPT IN THE FUTURE VOLCANOES PRESENT IN ALL CONTINENTS EXCEPT AUSTRALIA ALL MAJOR OCEAN BASINS INDONESIA HAS 76 ACTIVE VOLCANOES JAPAN HAS 60 & USA HAS 53 ACTIVE VOLCANOES

Lava tube formation

Causes for Volcanism GAS IN VOLCANIC MAGMA (1-9% IN MOST CASES) COMPONENT GASES (WATER VAPOR, CO2) RISING MAGMA RELEASES GASES EXPLOSIVE PRESSURE BY GAS MAGMA VELOCITY: -RESISTANCE TO FLOW -HEAT REDUCES VISCOSITY -VISCOSITY INCREASES WITH SILICA CONTENT -EXPLOSIVENESS INCREASES WITH VISCOSITY

Types of Lava . Basaltic Lava- low viscosity and non-explosive Pahoehoe (pay-HOY-hoy) and a a (AH-AH) Andesitic Lava – more viscous than basaltic and erupts Rhyolitic lava- highly viscous and erupts explosively Pyroclastic materials Tephra- volcanic dust, ash, cinders (lapilli) and volcanic bombs Welded tuff (fussed tephra particles) Pyroclastic flows or Nuees ardents (noo-Ay AR-dant) Lahars (hot volcanic mudflows) Water/air pollution, global climate change

Basaltic lava flow

Basaltic pillow lavas

Basaltic pillow lavas-contd.

Basaltic pillow lavas-contd.

Tephra Layers

Pyroclastic Flows

Lahars

Gas emissions

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

Volcanic Landforms Determined by the composition of Lava Shield volcanoes…gentle slope, effusive eruption- non-explosive Crater and Caldera Volcanic Domes Produce great amount of solid volcanic fragments Composite (Stratovolcanoe) Alternating layers of pyroclast and solid lava Pyroclastic Cones (Cinder Cones) Loose pyroclastic materials around a vent

Types of Volcanoes Shield volcanoes Cinder cones Broad Gently sloping Composed of solidified lava flows Cinder cones Small Steep-sided Composed of a pile of loose cinders

Lava-plateau formation

The Gorda ridge

Submarine volcano, infiltration

Submarine volcano, eruption

Volcanic domes, gas escape

Volcanic domes, explosion

Mount Mazama

Mount Mazama-contd

Mount Mazama-contd.

Mount Mazama-contd.

Mount Mazama-contd.

PYROCLASTIC MATERIAL TEPHRA: CLASSIFIED BY SIZE RECORDERS IN LAKES SMALL: VOLCANIC DUST & ASH- 1 MICRON MEDIUM: CINDERS OR LAPILLI (>1-62 MICRON) LARGE: VOLCANIC BOMBS (>64 MICRON) RECORDERS IN LAKES VOLCANIC MUDFLOWS-LAHAR SECONDARY VOLCANIC EFFECTS: ACIDIC PRECIPITATION GLOBAL CLIMATIC CHANGES- REDUCED SUNLIGHT REDUCED TEMPERATURE

PRODUCTS OF VOLCANISM OCCURRENCE: MAFIC LAVA (BASALT)-MOST COMMON ANDESITIC LAVA – INTERMEDIATE FELSIC LAVA – LEAST COMMON

ERUPTIVE STYLES ERUPTIVE STYLES: EFFUSIVE ERUPTION: NONEXPLOSIVE, BASALTIC CENTRAL VENT ERUPTIONS PRODUCE SHIELD VOLCANOES PRODUCE CALDERAS AND FLANK ERUPTIONS FISSURE ERUPTIONS ON LAND PRODUCE LAVA PLATEAUS OR FLOOD BASALTS SUBAQUEOUS ERUPTIONS (MAINLY OCEAN) SUBGLACIAL ERUPTIONS (MAINLY UNDER GLACIERS)

Volcanic Hazards Life threatening situation- explosive ejection of pyroclastic materials Water and air pollution Forest fire Global climate change Mitigation Avoid area… zoning Structural/strategic defense- lava wall, warning system, evacuation plan, measuring and predicting

Ash flow eruption

Extrusive Rocks and Gases . Extrusive Rocks and Gases Scientific Investigation of Volcanism Rocks, gases and events from observed eruptions compared to similar lavas elsewhere to infer past activity Rock Composition Rhyolite - high silica; light color Basalt - low silica; dark color Andesite - intermediate silica and color

Chapter Summary - 4 Benefits of Volcanic Eruption/Environmental Effects Active, Dormant, and Extinct Volcano Pyroclastic Volcanoes Definition of Viscosity Relationship between Viscosity, Temperature, Si content in magma Basaltic, Andesitic, Rhyolitic Lava – Most volcanic eruptions occur in what type of lava Classification of Tephra Definition of Lahar Contents of Volcanoes Volcanoes, Pyroclastic occurrence Effective means of dealing with volcanic hazards