chapter four volcanism and extrusive rocks. volcanoes and volcanisms volcano: landform that results...
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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