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Current eruption at Etna volcano, Sicily, Italy. Copyright 2002 by Ben Kennedy.
Volcanoes
A volcano is a weak spot vent
in the crust or surface of the
Earth through which magma
and associated gases and
ash erupt (come to the
surface).
Popocatepetl Mexico - 2001.
A volcano is also the form or
structure that is produced by
the ejected material (usually
conical).
Volcanic activity is
constructive.
Photograph copyrighted and provided by Daniel Hatcher.
Magma or Lava?
Magma is a molten mixture of
rock-forming substances, gases
and water from the mantle.
When magma reaches the
surface, it is called lava.
Lava released during volcanic activity builds up
the surface of the Earth
How do volcanoes form?
• A volcano forms when molten rock erupts or flows as
lava from an opening in earth’s surface and builds up
a volcanic cone.
• These openings are called vents.
• Volcanoes release molten rock, ash, and poisonous
gases. All these products result from melting in the
mantle or in the crust.
Like earthquakes, most volcanoes
occur at plate boundaries.
• Plate boundaries
– Divergent: where
two plates move
apart
– Convergent: where
two plates move
towards each other
– Transform: where
two plates move
past one another
http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part12.html
Asia
North
America
South
America
Australia
The RING OF FIRE
http://pubs.usgs.gov/publications/text/fire.html
The RING OF
FIRE
The Ring of Fire is a ring of
volcanoes around the
Pacific Ocean that result
from converging plate
boundaries.
Most of the Earth's
volcanoes are located
around the Pacific Ring
of Fire because that is
the location of most of
the Earth's subduction
zones.
There are about 600 active volcanoes on land
and more beneath the sea.
• Like earthquakes, most volcanoes occur at plate boundaries.
• Along a mid-ocean rift, where two plates move apart, divergent
boundary, a ridge of underwater volcanoes is formed. Examples:
Volcanoes in Iceland and the Azores Islands
• At convergent boundaries, where an oceanic plate collides with a
continental plate. Examples: Pacific Northwest and the Andes
Mountains in South America.
• At convergent boundaries, where an oceanic plate collides with
another oceanic plate creating an island arc. Examples: Japan, New
Zealand, Caribbean Islands, Philippines, Aleutian Islands.
• Some form at “hot spots” far from plate boundaries. Examples:
Hawaiian Islands, Yellowstone
Types of Volcanoes
• The type of eruption determines the type of
volcano.
• Thin, liquid lava spreads far from the vent,
building up a gentle slope.
• Thicker, more viscous lava erupts more violently
and produces more ashes and cinders that pile
up to form a steep cone-shape.
Types of Volcanoes
• Shield Volcanoes
– Kilauea, Hawaii
• Strato Volcanoes
– Mt. St. Helens, Washington
• Rhyolite Caldera Complexes
– Yellowstone
• Monogenetic Fields
– San Francisco volcanic field
• Flood Basalts
– Columbia River Basalt province,
Washington to Oregon
• Mid-ocean ridges
– Atlantic ridge
http://mordor_unknown_world.tripod.com/id4.html
Features of Volcanoes
• Magma Chamber
– The pocket beneath the
volcano where magma
collects
• Pipe
– A long tube through which
magma moves from the
magma chamber to
Earth’s surface
Features of Volcanoes
• Vent
– The opening through
which molten rock and
gas leave a volcano
– Usually there is one
central vent at the top of
the volcano
– Often times there are
additional vents that
open on the volcanoes
sides
Features of Volcanoes
• Lava flow
– The area covered by
lava as it pours out of a
volcano’s vent
• Crater
– A bowl-shaped area that
forms around a
volcano’s central
opening
– Lava collects there
Features of Volcanoes
• Liquid magma in the
asthenosphere is less dense
than the rock in the lithosphere
above it, so it flows upward
through cracks in the rock
– The magma is stored in the
magma chamber
• It continues upward (through
the pipe) until it reaches the
surface (exiting through a vent)
or it is trapped beneath layers
of rock.
Features of Volcanoes
A conduit or pipe feeds magma to
the surface.
Near the surface, gas expands and
fragments the lava into ash.
Some magma passes through dikes
across rock layers to feed vents.
Some magma intrudes parallel to
layers to make sills.
http://volcano.und.nodak.edu/vwdocs/vw_hyperexchange/parts.html
Types of lava:
1. Pahoehoe: hot, thin, flowing, forms
ropes
2. Aa: cooler, much thicker, slow,
forms jagged, sharp, blocks.
Types of lava:
3. Blocky lava: Cool and stiff, it does not travel
far from the volcano. It usually oozes out after
an explosive eruption.
4. Pillow lava: The result of lava erupting
underwater. The water causes rapid cooling
and results in bubble like pillows.
Types of lava:
5. Pyroclastic Flow
The expulsion of ash, cinders, and
bombs from a violent volcanic explosion
Volcanic blocks: Solid chunks blasted from the volcano chunks of the volcano top.
Volcanic bombs: Large blobs of magma that harden in the air tear-drop shapes larger
than 64mm in diameter.
Lapilli (La pill ee): Meaning “little stones” - pebble-like pieces
from
2mm to 64mm.
Volcanic Ash: Particles less than 2mm in diameter - forms
when
gases force magma to explode into tiny pieces.
Types of lava:
• The more silica a lava contains, the thicker
and more lightly-colored it will be.
• Rhyolite -
How magma affects a volcanic
eruption.
• There are three main factors that
determine whether a volcano extrudes
magma violently or quietly.
• The factors are:
– magma’s composition—silica content
– temperature
– amount of dissolved gases it contains.
Quiet Eruption
In general, the hot , basaltic magmas flow easily
and gently.
• dark-colored
• contains a lot of water
• rich in iron and magnesium
• cools to form igneous rocks such as basalt.
• lava is thin and fluid
The islands of Hawaii and Iceland were formed by
many lava flows.
Explosive Eruption
Thicker, cooler magmas
•
•
•
•
contain more silica
more difficult to force through the vent
may even plug up the vent
causes gases to collect in bubbles and pockets
that increase in size and pressure
• increasing pressure ejects the molten rock from
the volcano in a violent explosion.
Ex. Mt. St. Helens, 1980
Composite Volcanoes
•Composite volcanoes are
made up of alternating layers of
ash, cinders, and lava.
Composite volcanoes switch
between quiet eruptions of
flowing lava and violent
eruptions of thick gas-rich
lava.
This type of volcano has the
most powerful eruptions of all.
•Their magma is rich in silica
and therefore thick. Gases are
trapped in the magma, causing
eruptions that alternate between
flows and explosive activity that
produces cinders and ash.
•Composite volcanoes are
typically thousands of meters
high, with steep slopes.
When Mount St. Helens
erupted on 18 May
1980,the top
1,300 ft. disappeared
within minutes.
Mount St. Helens is a stratovolcano.
The blast area covered
an area of more than
150 sq. miles and
sent thousands of
tons of ash into the
upper atmosphere
Image taken on 10/16/94 from the Space Shuttle.
Mt. St. Helens
• Mt. St. Helens before 5/18/80
• Mt. St. Helens after 5/18/80
Shield Volcanoes
•
Shield Volcanoes look like a warrior’s shield. These broad, slightly domeshaped volcanoes are the world’s largest.
•
Volcanoes with broad, gentle slopes and built by eruption of fluid basalt lava
are called shield volcanoes.
•
Basalt lava tends to build enormous, low angle cones because it flows across
the ground easily.
•
The largest volcanoes on Earth are Shield Volcanoes.
Cinder Cones
• A cinder cone volcano is
built up from ashes,
cinders, and rocks that
burst from Earth during
violent eruptions.
• The rocks fall back to
Earth near the opening.
• At the same time, hot ash
covers a larger
surrounding area.
• Cinder cones erupt for a
very short amount of time.
Most cinder cones have a bowlshaped crater at the summit and
rarely rise more than a thousand
feet or so above their surroundings.
Craters and Calderas
• Craters: A funnel-shaped pit at the top of the
central vent of a volcano.
•Caldera: A large hole
formed when the
magma chamber
empties and collapses.
Lava Plateau
• Instead of forming mountains, some
eruptions form high, level plateaus.
– First, lava flows out of several long cracks in an
area.
– The thin, fluid lava travels far before cooling and
solidifying.
– Again and again, floods of lava flow on top of
earlier flows.
– After millions of years, these layers form high
plateaus.
– Example: Columbia Plateau (pictured here)
Volcanic Neck
• Formed when magma hardens in a
volcanic pipe. The softer rock around the
pipe wears away, exposing the hard rock
of the volcanic neck.
Batholith
• A batholith is a mass of rock formed when
a large body of magma cools inside the
crust. Example: Pike’s Peak
Dome Mountain
A dome mountain is formed when rising magma
is blocked by horizontal layers of rock. The
magma forces the layers of rock to bend upward
into a domed shape. Eventually the rock above
the dome wears away leaving it exposed.
• Not all of Earth’s volcanoes form at the edges of
the plates.
• Sometimes a volcano forms when a narrow
column of hot molten rock breaks through the
lithosphere in the middle of a plate. This narrow
column of magma is called a hot spot.
• A hot spot does not move. However, the plate
above it moves forming a chain of volcanoes.
• The Hawaiian Islands were
formed by a hot spot under the
Pacific Plate.
• Today, only the island of Hawaii,
located directly over the hot
spot, has active volcanoes.
Volcanic Activity
• Volcanoes are rather
unpredictable . Some
erupt regularly, others
have not erupted in
modern history.
• Scientists classify
them as active,
dormant or extinct.
Active
Volcanoes
• An active volcano is one that erupts wither
continually or periodically such as Mount
Katmai in Alaska and Mount St. Helens in
the Cascade Range.
Dormant Volcano
A volcano that has been known to erupt
within modern times but is now inactive is
classified as a dormant volcano. Mount
Rainier in Washington state are example
of dormant volcanoes in the United States.
Extinct Volcano
A volcano not known to have erupted within modern
history is classified as an extinct volcano.
They have been worn away almost to the level of their
magma chamber.
Scientists can be wrong. Mount St. Helens was
considered to be dormant but erupted after long
periods of inactivity.
Hot Spring
• A pool formed by
groundwater that
has risen to the
surface after being
heated by a nearby
body of magma
Geyser
• A fountain of water
and steam that
builds up pressure
underground and
erupts at regular
intervals
Geothermal
Energy
• Energy from water and steam that
has been heated by magma
• Geothermal energy is produced by
drilling a well into the ground where
thermal activity is occuring.
• Once a well has been identified and
a well head attached, the steam is
separated from the water, the water
is diverted through a turbine engine
which turns a generator.
• Usually the water is injected back
into the ground to resupply the
geothermal source.
Predicting Eruptions
• different from predicting a flood or an
earthquake
• volcanoes usually shows signs of
"awakening" before they erupt
• give geologists time to monitor the
precursory events
Predicting Eruptions
Geologists use sensitive instruments to monitor an active
volcano to determine if magma is approaching the surface.
Signs that a volcano is about to erupt include:
• Changes in tilt caused by magma movement underground
using tiltmeters & laser-ranging devices
• increasing temperature of hot springs near the volcano
• changes in the shape of the volcano or surrounding land
• an increase in hot gas and ash from the vent
• small earthquakes around the volcano
Photograph by Gudmundur E. Sigvaldason, Nordic Volcanological Institute, Reykjavik, Iceland