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

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Shield Volcano:

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Mountain with broad sloping sides and circular base

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Largest volcano

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Usually not very explosive

Types of Volcano

Cinder Cones

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Steep sides; generally small

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Form when small pieces of magma pile up around the vent

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Often form around larger volcanoes

Types of Volcanoes

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Composite Volcanoes

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Formed from hardened chunks of lava from very violent eruptions

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Cone shaped but are larger than cinder cones

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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 700 o C) 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!