Transcript Volcanos - Anderson County Schools Home

Volcanoes
Learning Targets:
• Describe how plate tectonics influences
the formation of volcanoes
• Locate major zones of volcanism
• Identify the parts of a volcano
• Differentiate between volcanic landforms
Volcanism
• All the processes associated with the
discharge of magma, hot fluids, and
gases.
• In any given year, 60 volcanic eruptions
will occur.
Volcanism
• The majority of volcanoes are
found at plate boundaries.
• Most are found at convergent
boundaries or divergent
boundaries.
Convergent Volcanism
• Most volcanoes on land occur because
oceanic plates subduct under continental
plates.
• As the oceanic plate descends, magma
forms.
• Eventually the magma moves up because
it is less dense.
• These volcanoes have explosive
eruptions.
Convergent Volcanism
• Most convergent volcanoes are found in
two major geographic belts:
– Pacific Ring of Fire (Circum-Pacific)
– Mediterranean Belt
Pacific Ring of Fire
Mediterranean Volcanic Belt
Divergent Volcanism
• At divergent boundaries in the ocean, new
ocean floor is produced as magma fills up
the new gaps.
• These volcanoes are rarely explosive.
Hot Spots
• Some volcanoes form far away from plate
boundaries.
• These are called “hot spot” volcanoes.
• It is theorized that hot spots form in
unusually hot regions of Earth’s mantle
• Most are formed under the ocean and
form islands (Hawaii).
Learning Target
• Describe how plate tectonics
influences the formation of volcanoes.
• Where are the major zones of
volcanism? What is significant about
their locations?
Anatomy Of A Volcano
Anatomy of a Volcano
• Conduit: tube structure through which lava
travels to reach the surface
• Vent: an opening in the conduit to allow
lava to leave the conduit
• Crater: a bowl shaped depression formed
around the vent
• Caldera: a larger depression that can form
later
Types of Volcanoes
• Shield Volcano:
– Mountain with broad sloping sides and
circular base
– Largest volcano
– Usually not very explosive
Types of Volcano
Cinder Cones
– Steep sides; generally small
– Form when small pieces of magma pile up
around the vent
– Often form around larger volcanoes
Types of Volcanoes
• Composite Volcanoes
– Formed from hardened chunks of lava
from very violent eruptions
– Cone shaped but are larger than cinder
cones
– Very explosive generally
Learning Targets
• List the parts of a volcano and then
define each.
• Differentiate between volcanic
landforms.
Magma Formation
• Magma can be formed by:
– Melting the crust
– Melting in the mantle
• Four main factors are involved:
– Temperature
– Pressure
– Water Content
– Mineral content of crust
Magma Formation
• Temperature increases with depth in
Earth’s crust
• Pressure also increases with depth; due to
weight of overlying rocks
• Water content; rocks often have water in
them which changes melting point of
rocks; as water content increases, melting
point decreases
Magma Formation
• Mineral Content
– Different minerals have different melting
points
• Basalt has a high melting point
• Granite is lower
• Generally rocks with high iron and magnesium
melt at higher temperatures
Magma Formation
• Viscosity
– The physical property that describes a
material’s resistance to flow
• Cooler magma has a higher viscosity (it’s thicker);
it resists flowing
Formation of Magma
Types of Magma
• Balsaltic magma
– Forms when rocks in upper mantle melt
– Less than 50% silica
– Low viscosity
– Found in quiet eruptions
– Kilauea in Hawaii
Types of Magma
• Andesitic magma
– 50-60% silica
– Found along oceanic-continental subduction
zones
– Intermediate viscosity
– Intermediate explosivity
Types of Magma
• Rhyolitic magma
– More than 60% silica
– High viscosity
– Very explosive
Explosive Eruptions
• When lava is too viscous to flow freely,
pressure builds up until the volcano
explodes. This throws lava and rock into
the air.
• Tephra is the pieces of lava or pieces of
crust thrown into the air
Explosive Eruptions
• Pyroclastic flows
– Tephra that is rapidly moving mixed with hot
(over 700oC) suffocating gases
– Mt. Pelee in the Carbbean Sea killed 29,000.
– Death caused by suffocation or burned to
death
Learning Targets
• Explain how magma type influences
volcanic activity.
• Discuss the role of temperature, pressure
and dissolved gases in eruptions.
• Recognize classifications of material
ejected by eruptions.
Mount Rainer, Washington – 4392 m.
Mount Rainer is
potentially the most
dangerous volcano in
the Cascades
because it is very
steep, covered in
large amounts of ice
and snow, and near a
large population that
lives downhill.
Mount Erebus, Antarctica – 3794 m
-Southernmost historically active glacier
-Capped by an elliptical 500 x 600 m. wide. 110
m. deep summit crater with an active lava lake;
continuous activity since 1972
-Occasional strombolian eruptions eject lava
bombs onto crater rim
Devil’s Tower, Wyoming
http://volcano.und.nodak.edu/vwdocs/kids/legends.html
Caldera
•A large depression created
by the collapse of a
volcano
• Magma chamber drains
• Loss of structural support
• Collapse of overlying material
Volcanoes are good!
• Produced our atmosphere
• Volcanic soil very fertile
• Beautiful scenery
• Mineral resources
-Diamonds, copper, gold, lead, zinc,
etc.
-Pumice, BBQ rocks, Aquarium stones
• Geothermal heat
• Geologic time markers
• Eruptions make for a good story line!