Ch. 13 Volcanoes

Volcanoes

• Volcanic eruptions can be more powerful than the explosion of an atomic bomb.

• Many of these eruptions are caused by the movement of tectonic plates.

Volcanism• Volcanism-any activity that includes the

movement of magma toward or onto Earth’s surface

• Magma-liquid rock produced under Earth’s surface

• Lava-magma flowing onto Earth’s surface• Volcano-a vent or fissure through which

magma and gases are expelled

• Magma rises because it is less dense than surrounding rock.

• Bodies of magma become larger as they rise.– So hot that it melts some of surrounding rock– It is forced into cracks in surrounding rock, causing

large blocks of overlying rock to break off and melt

• Lava can build up as a cone or eventually form a mountain

Major Volcanic Zones

• Most volcanoes near convergent and divergent tectonic plate boundaries

• Ring of Fire– Major zone of active volcanoes– Encircles Pacific Ocean– Formed by subduction of plates along coasts

Subduction Zones• Subduction-one plate moves under another

• Deep trench forms on ocean floor along edge of continent where plate is subducted

• Line of mountains forms along edge of continent from continental lithosphere

Subduction Zones• Oceanic plate sinks into asthenosphere and melts

to form magma• Magma breaks through overriding continental

plate to Earth’s surface

• Island Arc-A string of volcanic mountains that form on overriding plate– Eventually islands become larger and join

• EX: Japan

Mid-Ocean Ridges• Largest amount of magma comes to surface here

• Forms underwater volcanoes

• Unnoticed by humans because deep in ocean

Hot Spots

• Volcanically active area away from plate boundary– Most form where mantle plumes reach lithosphere

• Mantle plume-column of solid, hot material from deep mantle

– Forms volcano where it breaks through crust– Lithosphere continues to drift slowly

• Volcano on surfaces moves away from mantle plume• Volcano activity stops• New volcano forms on lithosphere over mantle plume

Volcanic Eruptions• Two Types of Magma

– Mafic-rich in magnesium and iron, dark in color– Felsic-rich in feldspar and silica, light in color

– Oceanic Crust-Mafic– Continental Crust-Felsic and mafic

Types of Eruptions

• Viscosity, resistance to flow, of magma affects the force with which a particular volcano will erupt

• Magma with large amounts of trapped gases are more likely to produce explosive eruptions

Quiet Eruptions

• Oceanic volcanoes• Mafic magma

– Low viscosity– Gases can escape

– EX: Hawaii

Explosive Eruptions

• Continental volcanoes• Felsic Lava

– Cooler and stickier– Large amounts of trapped gases (H2O, CO2)– Explode and throw pyroclastic materials into air– EX: Mount St. Helens

Pyroclastic Materials• Fragments of rock• Can form when magma breaks into fragments

because of rapidly expanding gases

• Can form when fragments of erupting lava cool and solidify as they fly through the air

• Large clots of lava thrown out while red-hot, volcanic bombs

Types of Volcanoes• Lava and pyroclastic materials build up

around vent and form volcanic cones

• Funnel-shaped pit at top of volcanic vent is known as crater

• 3 types of volcanoes—pg. 328– Shield Volcano, Cinder Cone, Composite Volcano

Shield Volcano

Cinder Cone

Composite or Stratovolcano

Calderas• Caldera-large circular depression

– forms when the magma chamber below a volcano partially empties and causes the ground above to sink

– Eruptions that discharge large amounts of magma can also cause a caldera to form

• Calderas may later fill with water to form lakes

Predicting Volcanic Eruptions• Earthquake Activity

– Important warning sign– Increase in strength and frequency can signal

eruption is about to occur• Patterns in Activity

– Upward movement of magma may cause surface of volcano to bulge outward

– Knowledge of previous eruptions– Changes in composition and amount of gases

emitted by the volcano

Volcano Assignment

• Pg. 332#8, 10, 11, 13-16, 20, 22