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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
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, 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
eruptionnon-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
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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