Download Volcanoes and Other Igneous Activity

Volcanoes
and
Other Igneous Activity
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Volcanoes:
Content Standards Addressed
6th
1
Plate tectonics accounts for important features of Earth's surface and
major geologic events. As a basis for understanding this concept:
1a
Students know evidence of plate tectonics is derived from the fit of
the continents; the location of earthquakes, volcanoes, and mid-ocean
ridges; and the distribution of fossils, rock types, and ancient climatic
zones
1d
Students know that earthquakes are sudden motions along breaks in
the crust called faults and that volcanoes and fissures are locations
where magma reaches the surface.
1e
Students know major geologic events, such as earthquakes, volcanic
eruptions, and mountain building, result from plate motions
1f
Students know how to explain major features of California geology
(including mountains, faults, volcanoes) in terms of plate tectonics.
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Origin of magma
• Magma originates when essentially solid
rock, located in the crust and upper mantle,
melts
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How Magma Rises
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Where do volcanoes form?
• Volcanoes form at:
– Hot Spots (10% of all volcanic activity)
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Where do volcanoes form?
• Volcanoes form at:
– Hot Spots (10%)
– Spreading Centers (80% of all volcanic activity)
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Where do volcanoes form?
• Volcanoes form at:
– Hot Spots (10%)
– Spreading Centers (80%)
– Convergent Plate
Boundaries
(10% of all volcanic
activity)
• Ocean–Continental
• Ocean – Ocean
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Plate tectonics and
igneous activity
• Intraplate igneous activity
– Activity within a rigid plate
– Plumes of hot mantle material rise
– Form localized volcanic regions called hot
spots
• the Hawaiian Islands & the Columbia River Plateau
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Plate tectonics and
igneous activity
• Divergent plate boundaries
– Produces the greatest volume of volcanic rock
•
•
•
•
Lithosphere pulls apart
Less pressure on underlying rocks
Partial melting occurs
Large quantities of fluid basaltic magma are
produced
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Plate tectonics and
igneous activity
• Convergent plate boundaries
– Descending plate partially melts & magma
slowly rises upward
– Rising magma can form
• Volcanic island arcs in an ocean (Aleutian Islands)
• Continental volcanic arcs (Andes Mountains)
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Tectonic Settings and Volcanic Activity
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Plate Boundary Features
Volcanic Eruptions
• Factors that determine the violence of an
eruption
– Temperature of the magma
– Composition of the magma
– Dissolved gases in the magma
• The above three factors actually control the
viscosity of a given magma which in turn
controls the nature of an eruption
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Viscosity, Temperature and
Water Content of Magmas
Rock Type
SiO2 content
Basalt
Andesite
Rhyolite
45-55%
55-65%
65-75%
800 – 1,000 ºC
600-900 ºC
Magma
1,000 – 1,250ºC
temperature
Viscosity
Low
increasing
High
Gas escape from
magma
Easy
increasing
Difficult
Eruptive style
Peaceful
increasing
Explosive
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Plate-Tectonic Setting of
Volcanoes Revisited
• Why more volcanic
activity at spreading
centers?
– Low SiO2 content
– High temperature
– Low pressure as plates pull
apart
• Fluid basaltic lavas
generally produce quiet
eruptions
• Why less volcanic activity
at subduction zones?
– High SiO2 content
– Lower temperatures
– Higher pressures
• Highly viscous lavas
produce more explosive
eruptions
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Volcanic Explosivity Index
Plume
Height
VEI
Description
Volume
Classification
How often
Example
0
non-explosive
< 100 m
1000s m3
Hawaiian
daily
Kilauea
1
gentle
100-1000 m
10,000s m3
Haw/Strombolian
daily
Stromboli
2
explosive
1-5 km
1,000,000s m3
Strom/Vulcanian
weekly
Galeras, 1992
3
severe
3-15 km
10,000,000s m3
Vulcanian
yearly
Ruiz, 1985
4
cataclysmic
10-25 km
100,000,000s m3
Vulc/Plinian
10's of years
Galunggung, 1982
5
paroxysmal
>25 km
1 km3
Plinian
100's of years
St. Helens, 1981
6
colossal
>25 km
10s km3
Plin/Ultra-Plinian
100's of years
Krakatau, 1883
7
super-colossal
>25 km
100s km3
Ultra-Plinian
1000's of years
Tambora, 1815
8
mega-colossal
>25 km
1,000s km3
Ultra-Plinian
10,000's of years
Yellowstone, 2 Ma
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Source: http://volcano.und.edu/vwdocs/eruption_scale.html
Volcano: Any
landform that
releases lava,
gas, or ashes or
has done so in
the past
Refers to
both the
VENT and
the CONE
Many shapes and sizes due to magma
chemistry and origin
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Volcano Types
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Volcanoes can be classified into
6 major types
• Volcanoes can be
classified into 6 major
types
Based on their
size, shape, and
origin
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Features Of The Two General
Categories Of Volcanoes
Central Vent
Volcanoes




central vent
summit crater
flank eruptions
fissure eruptions
Large-scale Volcanic
Terrains
 no central vent
 network of source material
 generally massive
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Comparison Of The Six Types Of Volcanoes
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Shield volcano
• Low silica, low gas magma originates in the
mantle = fluid, basaltic lava
• Broad, gentle slopes.
• Phreatomagmatic eruptions occur when lava
contacts water.
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SHIELD VOLCANOES ARE A TYPE
OF CENTRAL VENT VOLCANO.
Mauna
Loa
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Cinder Cones
• high lava fountains on the
vents of shield volcanoes.
Composed of
pyroclastic debris.
Cinders formed by
high gas content.
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Sunset
Crater,
Flagstaff,
Arizona
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Mt. Fuji
Aniakchak Caldera
Stratovolcanoes And Rhyolite
Caldera Complexes Are Central
Vent Volcanoes.
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Composite cone or stratovolcano
– Alternating andesitic lava flows and layers of
explosively ejected pyroclastics.
– Intermediate magma = viscous lava that is
difficult to erupt.
– Explosive eruptions due to buildup of gases.
– Often produce nuée ardente
– May produce a lahar - volcanic mudflow
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Mt. St. Helens
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Mt. St. Helens
New Dome
Vent
1980-1986
Dome
Rock
Glacier
http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16721
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Mt. St. Helens, October 1, 2004
http://www.nasa.gov/vision/earth/lookingatearth/mshelenslidar.html
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Eruptions
in the
Cascades
Ranges
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Calderas
• High-silica, high-gas
magmas.
• Massive explosions
(most explosive of all
types).
• Collapse, producing
an “inverse volcano”,
or Caldera (Spanish
for cauldron).
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Calderas
• Famous (or infamous)
collapsed calderas:
– Long Valley, California
(Mammoth)
– Crater Lake (Mount
Mazama), Oregon
– Yellowstone, Wyoming
– Krakatau, Indonesia, 1883
– Santorini and the Lost
Continent of Atlantis
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How Calderas Form
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Large-scale volcanic terrains lack
a central vent
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Monogenetic Fields
San Francisco Volcanic Field
• Poorly understood.
• Multiple maar vents and
cinder cones.
• Erupt at different times.
• Grow laterally.
• Usually a single magma
source
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Monogenetic Fields
• Large Igneous Provinces
– Fed by massive mantle plumes
– Caused by flood basalts
– Discharge over
Columbia River Basalts
time through long
fissures (cracks).
– Create large
plateaus.
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The Columbia River basalts
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Monogenetic Fields
• Mid-ocean Ridges
– develop at Spreading
Centers
– Basaltic flow creates
global network of
interconnected ridges
submarine volcanism
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Other volcanic landforms
• Lava Domes
– Bulbous mass of congealed lava
– Most are associated with explosive eruptions of
gas-rich magma
– One is currently developing in Mt. St. Helens
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Other volcanic landforms
• Volcanic pipes and necks
– Pipes are short conduits that connect a magma
chamber to the surface
– Volcanic necks (e.g., Ship Rock, New Mexico)
are resistant vents left standing after erosion has
removed the volcanic cone
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Formation of a volcanic neck
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Damage is
caused by:
• Lava flows
•Pyroclastic flows.
• Lahars (mudflows).
• Ash falls.
• Volcanic bombs.
• Massive lethal, violent
explosions.
• Earthquakes.
• Landslides.
• Open cracks and
chasms.
• Phreatomagmatic
eruptions.
• Climate change.
Volcanic Hazards
Threaten Human
Communities
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Lava flows
Molten rock that has flowed out onto the Earth’s
surface
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Pahoehoe lava
(braids or ropes)
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Aa lava
(rough, jagged)
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Pillow basalts
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Gases
• One to 5 percent of magma by weight
• Mainly water vapor and carbon dioxide
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Pyroclastic materials
Ash and dust – fine, glassy fragments
Pumice – from "frothy" lava
Cinders – "pea-sized"
Lapilli – "walnut" size
Particles larger than lapilli
Blocks
Bombs
weblink
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TYPES OF PYROCLASTIC
DEBRIS
ASH
PUMICE
VOLCANIC BOMBS
LAPILLI
VOLCANIC BLOCK
WELDED TUFF
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Nuée Ardentes
Fiery pyroclastic flow made of hot gases infused
with ash
Flows down sides of a volcano at speeds up to 200
km (125 miles) per hour & 800º C
Examples:
– Mt. Vesuvius
– Mt. Shasta
– Mt. St. Helens
Mayon Volcano, Philippines, in 1984
Source
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Mount Unzen, Japan. Note
the broad paths of the
pyroclastic flows across flat
cultivated land. source
Lahars (mudflows)
How to make a lahar:
Add water + volcanic material = lahar
Water sources:
Melt glaciers
Displace lakes
Rain
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Intrusive igneous structures
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Intrusive Igneous Features
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~ End ~
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