Chapter Four Volcanism and Extrusive Rocks VOLCANOES AND

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 VOLCANOES: ACTIVE CURRENTLY OR ERUPTED RECENTLY e. g. , EXTINCT NOT ERUPTED FOR 10, 000 YRS OR SO 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) • CAUSES FOR VOLCANISM: – COMPONENT GASES (WATER VAPOR, CO 2) – 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 u. Pahoehoe (pay-HOY-hoy) and a a (AH-AH) • Andesitic Lava – more viscous than basaltic and erupts • Rhyolitic lava- highly viscous and erupts explosively u. Pyroclastic materials w Tephra- volcanic dust, ash, cinders (lapilli) and volcanic bombs u. Welded tuff (fussed tephra particles) w Pyroclastic flows or Nuees ardents (noo-Ay AR-dant) u. Lahars (hot volcanic mudflows) w 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 u. Crater and Caldera • Volcanic Domes u. Produce great amount of solid volcanic fragments • Composite (Stratovolcanoe) u. Alternating layers of pyroclast and solid lava • Pyroclastic Cones (Cinder Cones) u. Loose pyroclastic materials around a vent

Types of Volcanoes • Shield volcanoes – 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 • 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 u. Avoid area… zoning u. Structural/strategic defense- lava wall, warning system, evacuation plan, measuring and predicting

Ash flow eruption

. 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, Pyroclastic occurrence Effective means of dealing with volcanic hazards