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VOLCANOES
Volcanoes
Mount St. Helens
Formation of volcanoes
• Under certain conditions, small amounts of
mantle rock can melt, forming magma.
• The magma rises upward through the crust,
erupting at the surface as a volcano.
• Magma rises because it is less dense than the
solid rock around it.
How a volcano erupts
• Similar to soda bottle after it has been shaken
• Magma is under pressure and contains
dissolved gases (CO2 and water vapor)
How a volcano erupts
• As magma approaches surface, the lower
pressure allows gases in magma to expand
rapidly.
• Eruption occurs when gases bubble out
through a crack in crust, propelling magma to
the surface.
Structure of a volcano
Crater
Vent
Magma collects in a pocket
called a magma chamber
before an eruption.
A vent is an opening in the
ground where magma escapes
A crater is a bowl
to theshaped
surface.
pit at Pipe
theOften
ofONE
most
central
volcanoes.
vent at top
A pipe
istop
a narrow,
vertical
of volcano,
channel through
whichsometimes
magma the
side.
rises to Earth’s surface.
Magma
Chamber
Tracy Saxby, Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/imagelibrary/).
Caldera
• After eruption, magma chamber and main
vent may empty of magma…hollow shell.
• Collapses inward, creating a huge depression
called caldera.
VHP Photo Glossary: Caldera. Aerial view of Aniakchak Caldera, Alaska; ...
volcanoes.usgs.gov
Eruptions
• Volcanoes erupt explosively or quietly,
depending on the characteristics of magma.
• Viscosity = the resistance to flow.
High viscosity = thick, resists flowing
Low viscosity = thin, flows easily
Viscosity of Magma
• 3 factors that determine viscosity of magma:
– Temperature
– Water content
– Silica content
Temperature
• Higher temperature = lower viscosity
• Magma will flow more easily
Water Content
• Greater water content = lower viscosity
• The presence of water in magma helps it flow
more easily.
Silica Content
• High silica content = high viscosity
• Silica is made up of silicon and oxygen that
have strong bonds.
• The silica in magma acts like glue, preventing
the magma from flowing easily.
Quiet Eruptions
•
•
•
•
Very HOT, low silica magma
Lava flow = stream of low viscosity lava
Lava flows can travel great distances
Pahoehoe = HOT, fast moving lava, ropelike
surface
• Aa = cooler, slow-moving lava, crumbly
appearance
Explosive Eruptions
• High silica magma
• Thick magma can clog pipe, causing enormous
pressure to build up. Trapped steam adds to
pressure.
• EXPLODES – lava and hot gases are hurled
outward
The June 12, 1991 eruption column from Mount Pinatubo taken from the east side of Clark Air Base. U.S. Geological Survey Photograph taken on June 12,
1991, by Dave Harlo
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Lava from Explosive Eruption
• Lava cools quickly shattering into pieces
• Range in size:
– Dust and ash
– Cinders (pebble sized)
– Bombs (several cm to size of small car)
Ash and volcanic rocks from the Mount Merapi volcano cover deserted houses in Central Java.
Photograph by C. Heliker on January 26, 1988
Tarko Sudiarno/AFP/Getty Images
Location
• Most volcanoes occur…
– Along plate boundaries
– Hot spots
Plate Boundaries
• Volcanoes occur most often at convergent
plate boundaries where oceanic plate is
subducting into the mantle.
• Example: Ring of Fire
volcano.oregonstate.edu
Hot Spots
• Hot spot = region where hot rock extends
from deep within the mantle to the surface.
• Hot spot stays in place while plate moves over
it…results in a chain of volcanoes over time.
volcano.oregonstate.edu
Three Major Types of Volcanoes
• Shield volcano
• Cinder cone
• Composite volcano
Shield Volcano
•
•
•
•
Low viscosity lava
Quiet eruptions
Wide, flat volcano
Example: Hawaii
Photograph by D. Little (date unknown). View of the NNW flank of Mauna Loa Volcano from the south side of Mauna Kea Volcano, Hawai`i; both are shield volcanoes.
Cinder Cone
•
•
•
•
Explosive eruption
Composed of ash and cinders
Steep sides, small
Example: Mt. Etna, Italy
Photograph by J. Lowenstern
Composite Volcano (Stratovolcano)
• Explosive eruptions that produce a
combination of lava and ash
• Steep sides, tall
• Example: Mt. Shasta, California
Mount Shasta and Shastina, California. USGS Photograph taken by Lyn Topinka, 1984
Other igneous features
• Sometimes magma does not reach the
surface, but hardens in the crust.
• Batholith = largest of igneous intrusions, often
form core of mountain range (Sierra Nevada)
• Sill = hardened lava squeezed into a crack that
is parallel to existing rock layers.
• Dike = hardened lava squeezed into a crack
that cuts across rock layers.
http://www.tulane.edu/~sanelson/geol212/intro&textures.htm
Other igneous features
• Volcanic neck = magma hardens in a volcano’s
pipe
By occecid
Devils Tower (Lakota: Mato Tipila, which means “Bear Tower”) is a monolithic igneous intrusion or volcanic neck located in the Black Hills near Hulett and Sundance in Crook County,
northeastern