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Volcanoes
and
Other Igneous Activity
Origin of magma
• Magma originates when essentially solid
rock, located in the crust and upper mantle,
melts
Origin of magma
• Factors:
– Role of heat
• Earth’s natural temperature increases with depth
(geothermal gradient) is not sufficient to melt rock
at the lower crust and upper mantle
• Additional heat is generated by
– Friction in subduction zones
– Crustal rocks heated during subduction
– Rising, hot mantle rocks
Origin of magma
• Factors:
– Role of heat
– Role of pressure
• Increase in confining pressure causes an increase in
melting temperature
• Drop in confining pressure can cause decompression
melting
– Lowers the melting temperature
– Occurs when rock ascends
Origin of magma
• Factors:
– Role of heat
– Role of pressure
– Role of volatiles
• Primarily water
• Cause rock to melt at a lower temperature
• Play an important role in subducting ocean plates
Origin of magma
• Factors:
–
–
–
–
Role of heat
Role of pressure
Role of volatiles
Partial melting
• Igneous rocks are mixtures of minerals
• Melting occurs over a range of temperatures
• Produces a magma with a higher silica content than
the original rock
Where do volcanoes form?
• Volcanoes form at:
– Hot Spots (10% of all volcanic activity)
Where do volcanoes form?
• Volcanoes form at:
– Hot Spots (10%)
– Spreading Centers (80% of all volcanic activity)
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
Plate tectonics and
igneous activity
• Global distribution of igneous activity is not
random
– Most volcanoes are located on the margins of
the ocean basins (intermediate, andesitic
composition)
– Second group is confined to the deep ocean
basins (basaltic lavas)
– Third group includes those found in the
interiors of continents
Plate tectonics and
igneous activity
• Plate motions provide the mechanism by
which mantle rocks melt to form magma
– 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)
Plate tectonics and
igneous activity
• Plate motions provide the mechanism by
which mantle rocks melt to form magma
– 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
Plate tectonics and
igneous activity
• Plate motions provide the mechanism by
which mantle rocks melt to form magma
– Intraplate igneous activity
• Activity within a rigid plate
• Plumes of hot mantle material rise
• Form localized volcanic regions called hot spots
– the Hawaiian Islands and the Columbia River Plateau
Lithospheric Plates
Volcanoes of the World
Volcanic Eruptions
• Factors that determine the violence of an
eruption
– Composition of the magma
– Temperature 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
Volcanic Eruptions
• Factors affecting viscosity
– Temperature
• hotter magmas are less viscous
Volcanic Eruptions
• Factors affecting viscosity
– Temperature
– Composition (silica content)
• High silica – high viscosity (e.g., rhyolitic lava)
• Low silica – more fluid (e.g., basaltic lava
Volcanic Eruptions
• Factors affecting viscosity
– Temperature
– Composition
– Dissolved gases (volatiles)
•
•
•
•
Mainly water vapor and carbon dioxide
Gases expand near the surface
Provide the force to extrude lava
Violence of an eruption = how easily gases escape
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
Gas escape from
magma
Easy
Eruptive style
Peaceful
increasing
increasing
increasing
High
Difficult
Explosive
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
Materials associated with
volcanic eruptions
• Lava flows
– Molten rock that has
flowed out onto the Earth’s
surface
– Types of basaltic lava
• Pahoehoe lava (resembles
braids in ropes)
• Aa lava (rough, jagged
blocks)
Pillow basalts
Materials associated with
volcanic eruptions
• Gases
– One to 5 percent of magma by weight
– Mainly water vapor and carbon dioxide
Materials associated with
volcanic eruptions
• Pyroclastic materials
–
–
–
–
–
Ash and dust – fine, glassy fragments
Pumice – from "frothy" lava
Cinders – "pea-sized"
Lapilli – "walnut" size
Particles larger than
lapilli
• Blocks
• Bombs
Bomb is approximately 10 cm long
Materials associated with
volcanic eruptions
• Nuée Ardente (or pyroclastic flow)
– 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
• Lahar
– volcanic mudflow
A nueé
ardente on
Mt. Saint
Helens
A lahar along the Toutle River
near Mt. St. Helens
Volcanoes
• General features
– Conduit, caries gas-rich magma to the surface
– Vent, the surface opening (connected to the
magma chamber via a pipe)
– Crater, steep-walled depression at the summit
• Caldera (a summit depression greater than 1 km
diameter)
– Parasitic cones
– Fumaroles
Ash and tephra
Explosive eruption
Volcanic bombs
Side vent
Eroded cone
Lava cone
Sills
Pyroclastic flow
(nuée ardente)
Mud
flows
(older)
Sequential
ash and lava
layers
Lava flow
Fracturing
Dikes
Old lava dome
Lavas
Sedimentary
rocks
Laccolith
Cinder cones
Lava pavement
(cracked/broken)
Metamorphic
rocks
Chimney
Contact
metamorphism
Granite
intrusion
(older/cold)
Magma chamber
Types of volcanoes
• Shield volcano
–
–
–
–
–
Low Viscosity, Low Volatiles, Large Volume
Broad, slightly domed
Primarily made of basaltic (fluid) lava
Generally large size
Hawaiian Islands
Types of volcanoes
• Cinder cone
–
–
–
–
–
Low Viscosity, Medium Volatiles, Small Volume
Built from ejected lava fragments
Steep slope angle
Rather small size
Frequently occur
in groups
Sunset
Crater,
Flagstaff,
Arizona
Types of volcanoes
• Composite cone (or stratovolcano)
–
–
–
–
High Viscosity, High Volatiles, Large Volume
Most are adjacent to the Pacific Ocean
Large in size
Interbedded lavas and pyroclastics
Mt. St. Helens – a typical
composite volcano
Mt. St. Helens following the
1980 eruption
Mt. St. Helens
New Dome
Vent
1980-1986
Dome
Rock
Glacier
http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16721
Mt. St. Helens, October 1, 2004
http://www.nasa.gov/vision/earth/lookingatearth/mshelenslidar.html
Types of Volcanoes
• Composite cone (or stratovolcano)
– Often produce nuée ardente
– May produce a lahar - volcanic mudflow
Eruptions
in the
Cascades
Ranges
A size comparison of the three
types of volcanoes
Other volcanic landforms
• Calderas
– High Viscosity, High Volatiles, Very Large Volume
– Steep walled depression at the summit formed by
collapse
– Size exceeds one kilometer in diameter
Other volcanic landforms
• 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
Formation of Crater Lake
Crater Lake, Oregon
Other volcanic landforms
• Fissure eruptions and lava
plateaus
– Low Viscosity, Low
Volatiles, Very Large Volume
– Basaltic lava extruded from
crustal fractures
– Incredibly large volumes of
lava pour out of fissures over
2-3 million years
– Can affect global climate
Formation of Flood Basalts
The Columbia River basalts
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
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
Formation of a volcanic neck
~ End ~