Chapter FourChapter FourVolcanism 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 ERUPTED50 VOLCANOES/YEARVOLCANOES ARE WINDOWS INTO THE

EARTHWATER AND AIR ----- CAME DUE TO

VOLCANIC ERUPTIONSHYDROSPHERE 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 volcanoesVOLCANOES:ACTIVE EXTINCTACTIVE CURRENTLY NOT ERUPTED FOROR 10,000 YRS OR SOERUPTED RECENTLYe.g.,

DORMANT VOLCANO:NOT ERUPTED RECENTLY (~1000 YRS OR SO)LIKELY WILL ERUPT IN THE FUTURE

VOLCANOES PRESENT IN ALL CONTINENTS EXCEPT AUSTRALIAALL MAJOR OCEAN BASINSINDONESIA HAS 76 ACTIVE VOLCANOESJAPAN 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, 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 DomesProduce 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)

• Life threatening situation- explosive ejection of pyroclastic materials

• Water and air pollution• Forest fire• Global climate change• Mitigation

Avoid area… zoningStructural/strategic defense- lava wall, warning system,

evacuation plan, measuring and predicting

Volcanic Hazards

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