chapter iv volcanism
TRANSCRIPT
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Volcanism
Chapter 4
Course notes are available on the web at:
http://www.colorado.edu/GeolSci/courses/GEOL1010-3/
First Midterm is tomorrow, covers first four chapters in the
Course notes. Mostly multiple choices, true false, etc.
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The Nature of Volcanic EruptionsFactors determining the violence orexplosiveness of a volcanic eruption
Composition of the magma
Temperature of the magma
Dissolved gases in the magma
These three factors control the viscosityof magma (which controls the nature ofan eruption)
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The Nature of Volcanic Eruptions
Viscosityis a measure of a materials resistanceto flow (higher viscosity materials flow withgreat difficulty; water has very low viscosity)
Factors affecting viscosity Higher temperature - magma is less viscous (or
more fluid)
Composition - higher silica (SiO2) content -magma is more viscous (rhyolite)
Lower silica content = lower viscosity or morefluid-like behavior (e.g., mafic lava such as basalt)
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The Nature of Volcanic Eruptions
Factors affecting viscosity continued
Dissolved Gases
Gas content affects magma mobility Gases expand in a magma as it nears the
Earths surface due to decreasing pressure
The violence of an eruption is related to how
easily gases escape from magma
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The Nature of Volcanic Eruptions
Summary
Fluid basaltic lavas generally produce
quiet eruptions (Hawaiian volcanos gurgle)
Highly viscous lavas (rhyolite or andesite)
produce more explosive eruptions
(Yellowstone or St Helens go boom)
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Materials extruded from a volcano
Lava Flows
Basaltic lavas are much more fluid
Types of basaltic flows
Pahoehoe lava (- twisted or ropey texture) Aa lava (rough, jagged blocky texture)
Dissolved Gases
1-6% of a magma by weight
Mainly water vapor and carbon dioxide
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A Pahoehoe lava flow
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A typical aa flow
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QuickTime an d a Cinepak decompressor are needed to see this picture.
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QuickTime an d a Cinepak decompressor are needed to see this picture.
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QuickTime an d a Cinepak decompressor are needed to see this picture.
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QuickTime an d a Cinepak decompressor are needed to see this picture.
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Materials extruded from a volcanoPyroclastic materialsFire fragments
Types of pyroclastic debris
Ash and dust - fine, glassy fragments
Pumice - porous rock from frothy lava
Lapilli - walnut-sized material
Cinders - pea-sized material
Particles larger than lapilli
Blocks - hardened or cooled lava
Bombs - ejected as hot lava
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A volcanic bomb
Bomb is approximately 10 cm long
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Volcanoes
General Features
Opening at the summit of a volcano
Crater - steep-walled depression at the
summit, generally less than 1 km diameter
Caldera - a summit depression typically
greater than 1 km diameter, produced by
collapse following a massive eruption
Ventopening connected to the magma
chamber via a pipe
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Volcanoes
Types of Volcanoes
Shield volcano
Broad, slightly domed-shaped
Composed primarily of basaltic lava
Generally cover large areas
Produced by mild eruptions of large volumesof lava
Mauna Loa on Hawaii is a good example
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A size comparison of the three
types of volcanoes
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Olympus Mons
Caldera
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VolcanoesTypes of Volcanoes continued
Cinder cone
Built from ejected lava (mainly cinder-sized)
fragments
Steep slope angle
Rather small size Frequently occur in groups
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QuickTime an d a Cinepak decompressor are needed to see this picture.
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Sunset Cratera cinder cone near
Flagstaff, Arizona
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VolcanoesTypes of volcanoes continued
Composite cone (Stratovolcano)
Most are located adjacent to the Pacific
Ocean (e.g., Fujiyama, Mt. St. Helens)
Large, classic-shaped volcano (1000s of ft.
high & several miles wide at base)
Composed of interbedded lava flows and
layers of pyroclastic debris
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A composite volcano
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. . composite volcano (prior to
eruption)
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Mt. St. Helens following the
1980 eruption
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St Helens
Eruption
Sequence
How would
ScientistsMonitor this
Process?
Seismometers
Tilt Meters
INSAR
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St Helens Base Surge (on the scale of an atomic bomb)
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Volcanoes
Composite cones continuedMost violent type of activity (e.g., Mt.
Vesuvius)
Often produce a nue ardente
Fiery pyroclastic flow made of hotgases infused with ash and otherdebris
Move down the slopes of a volcanoat speeds up to 200 km per hour
May produce a lahar, which is avolcanic mudflow
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A nue ardente on Mt. St. Helens
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QuickTime an d a Cinepak decompressor are needed to see this picture.
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Location of Montserrat
- a historical Nuee Ardente
Town of St Pierre on Montserrat after Nuee Ardente
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Town of St Pierre on Montserrat after Nuee Ardente
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Other volcanic landforms
Calderas (form by collapse of evacuated
magma chamber)
Steep-walled depressions at the summit
Size generally exceeds 1 km in diameter
Pyroclastic flows (explosive mix of rock,
gas and heat)
Only with felsic & intermediate magma
Consists of ash, pumice, and other
fragmental debris
Material propelled from vent at high speed
C ld f fill d b C k
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Caldera of Mt Mazama now filled by Crater Lake
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Mt Mazama Eruption and Caldera Collapse
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Other volcanic landforms
Fissure eruptions and lava plateaus
Fluid basaltic lava extruded from crustal
fractures called fissures
e.g., Columbia River Plateau
Flood basalts cover huge areas
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Flood Basalt erupted
from fissures - Snake
River Plain, southern
Idaho
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Volcanic landforms
Lava Domes
Bulbous mass of congealed lava
Most are associated with explosive
eruptions of gas-rich magma
St H l L D
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St Helens Lava Dome
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Volcanic landforms
Volcanic pipes and necks
Pipes are short conduits that connect a
magma chamber to the surface Volcanic necks (e.g., Devils Tower in
Wyoming and Ship Rock in New Mexico)are resistant vents left standing after
erosion has removed volcanic cone
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Formation of a volcanic neck
Shiprock in Northern New Mexico
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Shiprock in Northern New Mexico
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Plutonic igneous activity
Types of intrusive igneous features
Dikea sheetlike injection into a fracture (cutsacross pre-existing rocks)
Silla thick, sheetlike injection into a bedding
plane (lies parallel to bedding)
Laccolith
Similar to a sill
Lens or mushroom-shaped mass
Arches overlying strata upward
S i h P k d R di ti Dik ( th CO)
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Spanish Peaks and Radiating Dikes (southern CO)
Spanish Peaks
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Spa s ea s
Dike Outcrop
Spanish Peaks Digital Elevation
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Spa s ea s g a eva o
Spanish Peaks Dikes Volcanic Necks
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p
Dikes
Spanish Peaks Dikes - Digital 3D Elevation
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Spanish Peaks Dikes Digital 3D Elevation
Dikes
Volcanic Necks
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Some intrusive igneous structures
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Injection of a laccolith
QuickTi me and a Video decompressor are needed to see this picture.
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A sill in the Salt River Canyon, AZ
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Plate tectonics and igneous activity
Global distribution of magmatism is not
random
Most volcanoes are located within or near
ocean basins Basalt common in both oceanic and
continental settings
Granite is rare in oceans, mostly found incontinents
Di ib i f j l
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Distribution of major volcanoes
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Plate tectonics and igneous activity
Igneous activity along plate margins
Spreading centers
Great volumes of volcanic rock produced alongoceanic ridges
Mechanism of spreading
Lithosphere pulls apart and thins
Less pressure results in partial melting in mantle
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Plate tectonics and igneous activityIgneous activity along plate margins
Subduction zones
Descending plate partially melts Magma slowly moves upward
Rising magma can form either
An island arc if in the ocean (Aleutians)
A volcanic arc if on a continentalmargin(Sierra Nevada Mtns - California)
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Volcanism in subduction zones
QuickTime and a Video decompressor are needed to see thi s picture.
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Plate tectonics and magmatism
Intraplate volcanism
Associated with plumes of heat in mantle
Form localized volcanic regions in theoverriding plate called a hot spot
Produces basaltic magma sources in oceanic
crust (Hawaii and Iceland)
Produces granitic magma sources incontinental crust (Yellowstone Park)
V l i i
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Volcanism on a tectonic
plate moving over a hot spot
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End of Volcanism
(Chapter 4)These notes are available on the web at:
http://www.colorado.edu/GeolSci/courses/GEOL1010-03/
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