Download Volcano Notes 2012

GEOL: CHAPTER 5
Volcanoes and Volcanism
Learning Outcomes
• LO1: Understand volcanism and
volcanoes
• LO2: Identify the types of volcanoes
• LO3: Identify other volcanic landforms
• LO4: Identify the distribution of volcanoes
Learning Outcomes, cont.
• LO5: Understand the relationship between
plate tectonics, volcanoes, and plutons
• LO6: Understand volcanic hazards,
volcano monitoring, and forecasting
eruptions
Intro to Volcanoes
• Created much of Earth's atmosphere and
hydrosphere
• Create oceanic crust
• Create volcanic islands
• Profound effects of major eruptions
– Tambora, 1815
– Krakatoa, 1883
– Pinatubo, 1991
Intro to Volcanoes, cont.
• Volcanism: process by which magma rises
and issues onto Earth’s crust
• 550 active volcanoes: erupted in historic
time
• Dormant volcanoes: haven’t erupted in
historic time, but could in the future
• Extinct/inactive volcanoes
• Volcanoes on Io, Triton
Volcanic Gases
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50%-80% water vapor
Carbon dioxide
Nitrogen
Sulfur dioxide and hydrogen sulfide
Fumaroles
Can cause explosive eruptions
Can cause fatalities
Lava Flows
• Usually move slowly
• Lava tube:
– Margins and upper surface solidify
– Liquid lava flows rapidly through tube for long
distances
– Leaves cave with skylights
Lava Flows, cont.
• Pahoehoe
– Ropy surface like taffy
– Less viscous
• Aa
– Jagged and angular
– More viscous
Lava Flows, cont.
• Pressure ridges:
– Gas escaping from a flow hurls lava globs
which form spatter cones
• Columnar joints:
– Stationary lava flow cools and contracts
– Joints open: polygonal columns
Columnar joints in a basalt lava flow at Devil’s Postpile National
Monument in California. The rubble in the foreground is collapsed columns.
Surface view of the columns from (b). The straight lines and polish resulted
from abrasion by a glacier that moved over this surface.
As lava cools and contracts, three-pronged cracks form that
grow and intersect to form four- to seven-sided columns,
most of which are six-sided.
Stepped Art
Fig. 5-5, p. 89
Lava Flows, cont.
• Pillow lava:
– Nonexplosive eruptions of mafic lava
– Upper part of oceanic crust
– Lava chills beneath water
Pyroclastic Materials
• Ejected materials
• Volcanic ash: <2 mm
– Ash fall from atmosphere
– Ash flow close to surface
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Lapilli: 2 mm to 64 mm
Bombs and blocks: > 64 mm
Bombs: streamlined
Blocks: angular
Volcanoes
• Hill or mountain that forms around a vent
where lava, pyroclastic materials, and
gases erupt
• Craters: less than 1 km diameter
Calderas
• Huge, from voluminous eruptions
• Magma chamber drains
• Summit collapses into partially vacated
magma chamber
• Crater Lake in Oregon
Shield Volcanoes
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Low, rounded profiles
Slopes from 2-10 degrees
Composed of mafic, low-viscosity flows
Basalt lava flows
Kilauea volcano and Mauna Loa on Hawaii
Cinder Cones
• Small and steep-sided
• Pyroclastic materials accumulate around
vent
• Slope angles to 33 degrees
• Often form on flanks of larger volcanoes,
or in calderas: final stage of basaltic
volcanism
Composite Cones
• Also called stratovolcanoes
• Pyroclastic layers and lava flows of
intermediate composition
• Lahars: volcanic mudflow, rain or eruption
• Steep-sided near top
• Typical in continental and island volcanic
arcs at convergent boundaries
• Violent and dangerous eruptions
Lava Domes
• Also called volcanic domes and plug
domes
• Viscous felsic or intermediate magmas
• Slow eruptions caused by great pressure
below
• Violent, destructive eruptions
• Nuee ardente: fast-moving cloud of
pyroclastic materials and gases
Basalt Plateaus
• Fissure eruptions from long cracks
• Low-viscosity basalt
• Covers vast areas, often with repeated
eruptions
• Columbia River basalt plateau
• Currently occur in Iceland
Pyroclastic Sheet Deposits
• Vast areas covered by felsic volcanic
rocks, a few to several hundred meters
thick
• Erupted from long fissures
• Some welded tuff
• May occur during the origins of calderas
Volcano Distribution
• Most in well-defined zones or belts
• Ring of Fire: circum-Pacific belt
– Central and South America volcanoes
– Philippines, Indonesia, Japan, New Zealand
– Aleutian Islands in Alaska
– Cascade Range, northern California to
southern British Columbia; Mount St. Helens
1980 eruption killed 57 people
• Mediterranean
Igneous Activity at
Divergent Plate Boundaries
• Mafic magma emplaced as vertical dikes
and gabbro plutons in lower oceanic crust
• Some erupts as lava flows and pillow
basalts to form upper oceanic crust
• Mid-Atlantic Ridge emerges and erupts in
Iceland
• East African Rift system
Igneous Activity at
Convergent Plate Boundaries
• Many large active volcanoes in circumPacific Ring of Fire and Mediterranean
• Composite cones near leading edges of
overriding plates, oceanic or continental
• Intermediate to felsic composition
• Some lava domes
• Mount St. Helens eruption, 1980, in the
Cascade Range
Intraplate Volcanism
• Mantle plumes can create hot spots far
from a divergent or convergent boundary
• Mafic lava builds shield volcanoes
• Hawaiian Islands built as the Pacific Plate
moves over a hot spot
Volcanoes as Hazards
• Volcanic explosivity index
– 0 (gentle) to 8 (cataclysmic)
– Based on volume ejected, height of plume
– Doesn’t account for volume of lava, property
damage, and fatalities
– Tambora, 1815: VEI = 7
• Eruption duration: weeks to years
VEI 4-7
VEI 3-5
VEI 1-3
Plinian
Vulcanian
Strombolian
Stepped Art
Fig. 5-16, p. 99
Forecasting Eruptions
• Only a few volcanoes are monitored
• Tiltmeters: change in slope
• Geodimeter: laser measures horizontal
distances
• Gas emissions
• Groundwater and hot springs
• Volcanic tremor
• Eruptive history