10.1 Volcanoes and Plate Tectonics
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More than 800 active volcanoes on Earth
Origin of Magma
 Molten rock beneath Earth’s Surface
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Complex mixture made of melted mineral crystals, dissolved gasses, and water
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Magma forms in the crust and upper mantle when solid rock partially melts.
 Formation depends on heat, pressure and water content.
Heat
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Depth of 100km temperature is 1400 – 1600 degrees Celsius (Asthenosphere – near melting point)
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Additional heat needed comes form 3 sources o Friction from slabs of lithosphere sliding past each other in subduction zones o Mantle itself heats these subducting slabs o Hot mantle rock can rise and intrude into the cooler lithosphere, heating it
Pressure
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Pressure increases with depth inside Earth
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Increasing pressure raises the melting point/Decreasing pressure lowers melting point
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If pressure drops enough decompression melting occurs
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Solid hot rock rises if less dense which decreases pressure and forms pockets of magma
Water Content
 Rocks melting temperature lowered due to water content
 “wet” rock deep beneath surface has lower melting point
Volcanoes and Plate Boundaries
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Most volcanoes form along divergent and convergent boundaries.
 Some form far from plate boundaries above “hot spots” in the crust
Divergent Boundary Volcanism
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Plates pull apart and mantle rock rises to fill the gap
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As rock rises, decompression melting occurs and forms magma
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Magma erupts along axis of spreading center
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Example – Mt. Kilimanjaro
Convergent Boundary Volcanism
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Slabs of Oceanic crust are pushed into the mantle
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Increase in temperature and pressure
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Magma formed slowly migrates upward forming volcanoes
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Can form at convergent boundaries where 2 oceanic plates meet
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Forms chain of volcanoes on the ocean floor
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Eventually grow large enough to rise above ocean and form volcanic island arc
 Ring of Fire – long belt of volcanoes that circle much of the Pacific Ocean
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Can form at convergent boundaries of continental plate and oceanic plate
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Result in continental volcanic arc

Intraplate Volcanism.
 Kilauea volcano in Hawaii is Earth’s most active volcano
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Yellowstone National park is a volcanic region
Interaplate volcanism – volcanic activity that occurs within a plate
 Occurs where a mass of hotter than normal mantle material called mantle plume rises toward the surface
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Hot spot – small volcanic region a few hundred km across
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More than 40 hot spots are known
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Volcanic mountains of Hawaii have formed as the Pacific Plate moves over a hot spot.
10.2 The Nature of Volcanic Eruptions
Factors Affecting Eruptions
 Primary factors that determine whether a volcano erupts explosively or quietly include characteristics of the magma and the amount of dissolved gases in the magma
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Magma that has reached the surface is called lava
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Lava cools and hardens to form rock
Viscosity
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Substances resistance to flow
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Temperature and chemical composition determine viscosity
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Chemical composition has effect of type of eruption.
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More silica in magma the greater its viscosity
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More viscous = explosive eruption
Dissolved Gases
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Gases trapped in magma provide force to propel molten rock out of vent
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Vent – an opening to the surface
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Gases are mostly water vapor and carbon dioxide
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Reduced pressure allows dissolved gases to be released suddenly
Volcanic Material
 Explosive eruptions eject lava as well as broken rock, lava bombs, fine ash and dust
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Eruptions also emit large amounts of gases
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Lava Flows
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Silica content and temperature affect lava flow
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Pahoehoe – hotter faster moving basaltic lava
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Aa – Cooler slower moving basaltic lava
Gases
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Magma contains varied amount of gases
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As pressure is reduced the gases begin to escape
Pyroclastic Materials

Particles produced in volcanic eruptions
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2 -64 mm called lapilli or cinders

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Larger than 64 mm called blocks when hardened lava or bombs when ejected as glowing lava
Types of Volcanoes
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3 main types
Anatomy of a Volcano
 Volcano begins when fissure develops in the crust
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Magma is forced toward the surface
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Magma collects in a pocket beneath the surface called a magma chamber
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Gas rich magma rises form the chamber through a circular pipe
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Reaches surface at a vent
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Repeated eruptions eventually build a mountain called a volcano
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Crater is at the summit of a volcano, deep walled depression
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Form of volcano depends on composition of magma
Shield Volcano
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Produced by the accumulation of fluid basaltic lavas
 Most have grown from the deep ocean floor to form islands
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EX – Hawaiian Islands and Iceland
Cinder Cones
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Formed by ejected lava fragments that harden in the air
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Fragments range form fine ash to bombs but most are lapilli or cinders
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Usually product of a single eruption
Composite Cones
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Most beautiful and potentially dangerous
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Also called stratovolcanoes
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Large, nearly symmetrical volcanic mountain composed of layers of both lava and pyroclastic deposits
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Gas – rich magma
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Most explosive eruptions
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Most located in the Ring of Fire
Other Volcanic Landforms
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Volcanic mountains are not the only landforms that result form volcanic activity
Calderas
 Depression in a volcanic mountain after end of activity
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Ex – Crater Lake
Volcanic Necks
 Landform made of magma that hardened in a volcano’s pipe and later was exposed by erosion
Lava Plateaus
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Volcanic landform produced by repeated eruptions of very fluid, basaltic lava
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Erupts through long cracks called fissures
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Lava spreads out over a wide area
Volcanic Hazards
 Volcanic soil is good for farming
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Hazards include lava flows, volcanic ash, pyroclastic flows, and mudflows

Mudflows called lahars occurs when water soaked volcanic rock and ash slide rapidly down hill.

10.3 Intrusive Igneous Activity
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Most magma cools and hardens within the earth
Classifying Plutons
 Structures that result form the cooling and hardening of magma beneath Earth’s surface
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Types of Plutons – sills, laccoliths and dikes ( classified by size, shape and relationship to surrounding rock)
Sills and Laccoliths
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Magma intrudes between rock layers close to the surface
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Sill – magma flows between parallel layers of sedimentary rock
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Laccolith – lens shaped pluton that has pushed the overlying rock layers upward
Dikes

Magma moves into fractures that cut across rock layers
Batholith
 Largest bodies of intrusive rocks
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Surface exposure of more than 100sq. km
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Much larger than a pluton
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Formed by plutons that clump together as they rise
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Core of many mountain ranges
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Ex – Sierra Nevada