Volcanoes
Earth’s Creators and Destroyers
Structure of the Earth
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The earth is composed of layers:
Inner Core – solid
Outer Core – liquid
Mantle – solid
Upper Mantle – plastic
Crust – solid
Diagram of Earth’s Layers
Tectonic Plates
• The lithosphere is the Earth’s hard,
outermost shell that is divided into a
mosaic of 16 major slabs, or tectonic
plates.
• These plates float on the upper mantle.
• As the plates move about they spread
apart, collide, or slide past each other
Why Plates Move: Convection Cells
Earth’s Tectonic Plates
Where Volcanoes Occur
• Volcanoes occur most frequently at plate
boundaries.
• Some volcanoes, like those that form the
Hawaiian Islands, occur in the interior of
plates at areas called hot spots.
• The greatest number of volcanoes occur on
the ocean floor along spreading ridges.
• Over 80% of those on land occur at edges of
continents, or subduction zones, where one
plate dives, or subducts, under another plate.
Why Volcanoes Occur
• Temperatures in the mantle are hot
enough to melt rock into magma.
• Less dense than the solid rock around it,
magma rises and some of it collects in
magma chambers.
• As the magma rises, pressure decreases
allowing trapped gasses to expand and
propel the magma through openings in the
Earth’s surface causing an eruption.
• Erupted magma is called lava.
How Volcanoes Erupt
• Eruptions are described as explosive or
effusive (loosely flowing).
• How explosive an eruption is depends on
the magma’s chemical composition and
gas content, which affect the magma’s
stickiness, or viscosity.
• If magma is fluid, gases can escape
rapidly and lava flows; if magma is
viscous the gases can not escape and
pressure builds inside the magma until
the gases escape, sometimes violently.
Magma vs. Lava
• Magma is molten rock beneath
the surface.
• Lava is erupted magma. There
are 2 types of lava:
– A a (ah ah) is largely solidified
rock that gets pushed forward.
– Pahoe hoe (pah hoy hoy) is flowing
“liquid” with a ropy, billowy
surface.
Lava – Pahoe hoe
Lava - Aa
Types of Lava Flows
• Lava flows are superheated streams of
molten rock that flow at 1 – 50 mph.
• Pyroclastic flows are avalanches of hot
ash, rock fragments, and gases that flow
at speeds greater than 100 mph.
• Landslides are avalanches of rock, snow
and ice on slopes of volcanoes (loosened
and tumbling due to seismic activity).
• Lahars (mud flows) are a mixture of
volcanic ash and water (like wet concrete)
Volcano Terms
• A vent is an opening through which eruptions
take place.
• A crater is a basin like depression over a vent,
at the summit of a volcano
• A caldera is a depression larger than the
original crater (>1km. Diameter) that forms
when the summit is blown off, or when the
volcano collapses into the empty magma
chamber.
– Example: Crater Lake atop Mt. St. Helens.
Types of Volcanoes
• Repeated volcanic eruptions build
volcanic mountains of three basic types,
or shapes, depending on the composition
of the materials deposited by the
eruption.
• Shield volcanoes
• Stratovolcanoes
• Cinder cones
Shield Volcanoes
• Shield volcanoes are broad gently sloping
volcanic mountains slowly formed by layer
over layer of solidified lava.
• Shield volcanoes are formed by effusive
eruptions of fluid lava.
• These can become very large as the low
viscosity lava spreads widely and thickly.
• Examples: Kilauea, Hawaii and Mt. Etna,
Italy
Kilauea, HI
Kilauea
Mt. Etna, Italy
Mt. Etna
Mt. Etna
Stratovolcanoes (Composite)
• Stratovolcanoes are formed from both
explosive and effusive eruptions.
• Layers of tephra alternating with layers
of viscous lava flows create steep-sided,
often symetrical cones that can be very
large.
• Formed over long spans of time as periods
of 100,000+ yrs. separate periods of a
few years of intense activity.
• Examples: Aconcagua, Andes (22,825’)
and Mt. St. Helens
Aconcagua, Andes
Aconcagua
Mt. St. Helens
Mt. St. Helens
Cinder Cone Volcanoes
• Cinder cones are the smallest volcanoes
(< 500’), formed by explosive eruptions of
lava. Blown violently into the air, the
erupting lava breaks apart into fragments
called cinders that fall and accumulate
around the vent.
• Cinder cones are temporary geologic
features as they are easily eroded. They
have short life spans as gas causing
violent eruptions is quickly depleted.
• Example: Paricutin, Mexico
Paricutin, Mexico
1950
1944
1966
1946
1948
Evolution of Paricutin
Monitoring and Predicting
Eruptions
• Volcanic activity is monitored using
several observations:
• Land deformation
• Ash clouds
• Tremors- measured by seismic data
– Volcanic Tremors (VT)
– Rockfall (RF)
Active,or Recently Active, Volcanoes
Name
Bezymianmy
Type
Stratovolcano
Last Erupted
1993
Erebus
Kilauea
La Palma
Stratovolcano
Shield
Stratovolcano
1980
1995
1954
Mt. Etna
Mt. St. Helens
Nevada del Ruiz
Ol Doinyo Lengai
Shield
Stratovolcano
Stratovolcano
Stratovolcano
1993
1986
1991
1993
Paricutin
Pinatubo
Cinder cone
Stratovolcano
1952
1992