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

ACTIVE

Classification of volcanoes

VOLCANOES: EXTINCT ACTIVE CURRENTLY OR ERUPTED RECENTLY e.g., 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  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

– 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  Avoid area… zoning  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 • Volcanoes, Pyroclastic occurrence • Effective means of dealing with volcanic hazards