Download Izalco volcano, El Salvador

Santiaguito
Volcano
2009
Bellringer# 31
• Read “Volcanoes and Humans” page 747
• Answer the question in your journal.
Chapter 21 S.2 Volcanoes
• Volcanic eruptions are windows to the Earth’s
interior.
• They eject lava, therefore they provide
opportunities to observe the processes that
occur deep beneath the Earth’s surface.
Izalco volcano, El Salvador
What is a volcano exactly?
• A volcano is an opening in Earth’s crust
through which magma has reached Earth’s
surface.
• crust: an opening in the surface of Earth
through which volcanic material passes
Mount St. Helens
May 18, 1980
Kilauea in Hawaii 2008
Factors Affecting Eruptions
• The primary factors that determine whether a
volcano erupts violently or quietly include:
– Magma composition (what it is made of)
– Magma temperature
– The amount of dissolved gases in the magma
Viscosity
• Viscosity is a substance’s resistance to flow.
– Magma from an explosive eruption is going to be
thousands of times more viscous than magma
that is extruding quietly.
• Magma that has been heated flows more
easily.
• Viscosity of lava depends on its silica
content.
– Rhyolitic lavas are very viscous and do not flow
easily.
– Basaltic lavas are more fluid. (less silica)
Dissolved Gases
• These gases trapped in magma provide the
force to eject molten rock from the vent.
• Vent- an opening to the surface.
• When magma move towards the surface, the
pressure is reduced and dissolved gases are
released suddenly.
• Highly viscous magmas slow the movement of
expanding gases.
– Gases collect in bubbles and pockets that increase
in size and cause eruptions like Mount St. Helens.
Volcanic Material
• Lava is not the only
material extruded from
volcanoes.
– Broken rock
– Lava bombs
– Fine ash
– Dust
• All volcanic eruptions
emit large quantities of
gas.
Hawaii 2008
Lava Flows
• Basaltic lavas flow at
a rate of 10-30 meters
per hour. (low silica)
• The movement of
rhyolitic lava is too
slow to be visiable.
(lots of silica)
Gases
• Magmas contain dissolved gases.
• They are held in the molten rock by confining
pressure. (just like soda)
• When the pressure is reduced, the gases begin to
escape.
• The gases in volcanoes have helped form the
atmosphere.
• Hawaiian eruption example:
– 70% water vapor, 15% carbon dioxide, 5% nitrogen,
5% sulfur, and small amounts of chlorine, hydrogen
and argon.
Santiaguito
Volcano
2009
Pyroclastic Material
• Pyroclastic material is
the name given to
particles produced by
volcanic eruptions.
• The fragmanets ejected
during eruptions range
in size from very fine
dust and volcanic ash to
pieces that weigh
several tons.
• Begins- when a
fissure or crack
forms and then
magma comes out of
the fissure.
• Repeated eruptions
and long periods of
inactivity build the
volcano.
• Crater- a steep
walled depression at
the summit.
Anatomy of a Volcano
VOLCANIC STRUCTURE
• The central opening is called a vent.
• There is usually only one vent but there can be
several smaller vents. (secondary cones)
• The shape of a volcanic structure will vary
according to the nature of the eruption and the
magma type
Types of Volcanoes
• The three main
volcanic types
are :
– Shield volcanoes
– Cinder cones
– Composite cones
Shield Volcano
• Produced by the
accumulation of fluid
basaltic lavas.
• Shape: broad slightly
domed, resembles a
warrior’s shield.
• Most form out of the
deep ocean floor to
form islands.
• Ex: Hawaiian Islands
and Iceland
• Largest on Earth
Mauna Loa Hawaii
Cinder Cone
• Made from ejected
fragments the size of
cinders, harden in the air.
(pyroclastic)
• Result from gas rich
basaltic magma.
• Shape: steep sides
• Only erupts once.
• Erupts for weeks or a few
years. Magma solidifies
when it is over.
• Size: small 30-300 meters
• Ex: SP Crater in AZ,
Mount Etna
Capulin Volcano, New Mexico
Sunset Crater, Arizona
Composite Cone
• Earth’s most beautiful
and dangerous
• Also called a
stratovolcano.
• Most are around the
Pacific Ocean
• Made of lava and
pyroclastic material
• Tallest volcanoes
• Shape: steep summit,
sloping flanks
• Ex: Fujiyama in Japan,
Mount Shasta in CA,
Mount Shasta, California
Mount Rainier, Washington 2005
Dangers of Composite Cones
• Pyroclastic flows, hot gases, glowing ash, large
rock fragments.
• The flow can race down the slope at 200km per
hour.
• Mudflows called lahars happen when the debris
becomes saturated with water.
– Rain or ice melting
• Lahars can happen with out an eruption.
Mount St. Helens erupted often between 1980 and 1986. An explosive
eruption on March 19, 1982, sent pumice and ash 9 miles (14 kilometers)
into the air, and resulted in a lahar (the dark deposit on the snow) flowing
from the crater into the North Fork Toutle River valley. Part of the lahar
entered Spirit Lake (lower left corner) but most of the flow went west
down the Toutle River, eventually reaching the Cowlitz River, 50 miles
(80 kilometers) downstream.
Mount St. Helens erupted often between 1980
and 1986. An explosive eruption on March 19,
1982, sent pumice and ash 9 miles (14
kilometers) into the air, and resulted in a lahar
(the dark deposit on the snow) flowing from
the crater into the North Fork Toutle River
valley. Part of the lahar entered Spirit Lake
(lower left corner) but most of the flow went
west down the Toutle River, eventually
reaching the Cowlitz River, 50 miles (80
kilometers) downstream.
USGS Photograph taken on March 21, 1982, by
Tom Casadevall.
Other Volcanic Landforms
• Calders- a large depressionin a volcano. They
form two ways.
– The collapse of the top of a composite volcano
after a violent eruption
– The collapse of the top of a shield volcano after the
magma chamber is drained.
• Ex: Mount Mazama erupted to form Crater
Lake, Oregon.
Crater Lake is the
world's seventh deepest
lake. At it's greatest
depth it measures
1,932 feet
Over 6,000 years ago Mount Mazama
erupted. Before the explosion the
mountain was 12,000 feet high; when it
was over it had been replaced by a 1,900foot deep crater. Crater Lake, famed for
its intense blue waters, was made a
National Park in 1902. Volcanic activity
occurred some time after the Mount
Mazama explosion, creating Wizard
Island in the middle of the lake.
• Necks and Pipes- most volcanoes are fed
magma through conduits, called pipes
connecting a magma chamber to the surface.
• Form because of erosion of cinder cones.
• A neck is what is left over.
• Best known pipes are the diamond bearing
pipes of S. Africa
Diamonds are formed in the Earth's mantle,
Ship Rock,
New Mexico
and are brought up to the surface in the
volcanic rock called kimberlite .
Lava Plateaus
• The greatest
amount of volcanic
material comes
from fissures.
• No cone is formed.
The lava flows over
a wide area of land.
• Ex: Columbia
Plateau
• 1.6 km thick
Basalt lava flows exposed in
canyons hundreds of feet deep
occur throughout the Snake River
Plain and Columbia Plateau.
Large landslides tend to form
where the basalts are underlain
by unconsolidated sediments.
• More than 800 active
volcanoes in the world.
• There was no way to
understand why volcanoes
are located where they are
before the 1960s.
• Scientists understand now
because of the plate
tectonic theory.
Paricutin, Mexico 2008
Top 10
1. Mount Mazama/Crater
Lake, Oregon
2. Mount Etna, Sicily
3. Mount Vesuvius, Italy
4. Mount Tambora, Indonesia
5. Mount Krakatau, Indonesia
6. Mount Pelee, Martinique
7. Parícutin, Mexico
8. Mount St. Helens,
Washington
9. Nevada del Ruiz, Colombia
10. Mount Pinatubo, Philippines
Where do Volcanoes Occur?
• Most volcanoes occur at convergent plate
boundaries.
– 75% of the active volcanoes on Earth are located in an
area known as the Ring of Fire.
– The Ring of Fire is located along the edges of the
Pacific ocean, where oceanic tectonic plates are
colliding with continental plates.
• Underwater volcanoes occur at divergent plate
boundaries.
– As plates move apart at divergent boundaries,
magma rises to fill the gap.
– This magma creates the volcanic mountains that
form ocean ridges.
– Iceland is a volcanic island on the Mid-Atlantic
ridge that is growing outward in opposite
directions.
• Volcanoes occur at hotspots.
– Some volcanoes occur in the middle of plates.
– Mantle plumes are mushroom shaped trails of hot
rock that rise from deep inside the mantle, melt as
they rise, and erupt from volcanoes at hot spots at
the surface.
– The plumes remain in the same place as the
tectonic plate moves, creating a trail of volcanoes.
– The Hawaiian Islands are an example of this type
of volcanic activity. (intraplate volcanism)
Homework
• Chapter 21 Section 2 Review Questions
• Page 746
• Do #1-8
• You do not have to write the question.