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