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Chapter 10 Notes
Volcanoes and other Igneous Activity
10.1 Volcanoes and Plate Tectonics
 More than 800 active volcanoes on Earth
Origin of Magma
 Molten rock beneath Earth’s Surface
 Complex mixture made of melted mineral crystals, dissolved gasses, and water
 Magma forms in the crust and upper mantle when solid rock partially melts.
 Formation depends on heat, pressure and water content.
Heat
 Depth of 100km temperature is 1400 – 1600 degrees Celsius (Asthenosphere –
near melting point)
 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
 Pressure increases with depth inside Earth
 Increasing pressure raises the melting point/Decreasing pressure lowers melting
point
 If pressure drops enough decompression melting occurs
 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
 Most volcanoes form along divergent and convergent boundaries.
 Some form far from plate boundaries above “hot spots” in the crust
Divergent Boundary Volcanism
 Plates pull apart and mantle rock rises to fill the gap
 As rock rises, decompression melting occurs and forms magma
 Magma erupts along axis of spreading center
 Example – Mt. Kilimanjaro
Convergent Boundary Volcanism
 Slabs of Oceanic crust are pushed into the mantle
 Increase in temperature and pressure
 Magma formed slowly migrates upward forming volcanoes
 Can form at convergent boundaries where 2 oceanic plates meet
 Forms chain of volcanoes on the ocean floor
 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
 Can form at convergent boundaries of continental plate and oceanic plate
 Result in continental volcanic arc
Intraplate Volcanism.
 Kilauea volcano in Hawaii is Earth’s most active volcano
 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
 Hot spot – small volcanic region a few hundred km across
 More than 40 hot spots are known
 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
 Magma that has reached the surface is called lava
 Lava cools and hardens to form rock
Viscosity
 Substances resistance to flow
 Temperature and chemical composition determine viscosity
 Chemical composition has effect of type of eruption.
 More silica in magma the greater its viscosity
 More viscous = explosive eruption
Dissolved Gases
 Gases trapped in magma provide force to propel molten rock out of vent
 Vent – an opening to the surface
 Gases are mostly water vapor and carbon dioxide
 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
 Eruptions also emit large amounts of gases
 Lava Flows
 Silica content and temperature affect lava flow
 Pahoehoe – hotter faster moving basaltic lava
 Aa – Cooler slower moving basaltic lava
Gases
 Magma contains varied amount of gases
 As pressure is reduced the gases begin to escape
Pyroclastic Materials
 Particles produced in volcanic eruptions
 2 -64 mm called lapilli or cinders
 Larger than 64 mm called blocks when hardened lava or bombs when ejected as
glowing lava
Types of Volcanoes
 3 main types
Anatomy of a Volcano
 Volcano begins when fissure develops in the crust
 Magma is forced toward the surface
 Magma collects in a pocket beneath the surface called a magma chamber
 Gas rich magma rises form the chamber through a circular pipe
 Reaches surface at a vent
 Repeated eruptions eventually build a mountain called a volcano
 Crater is at the summit of a volcano, deep walled depression
 Form of volcano depends on composition of magma
Shield Volcano
 Produced by the accumulation of fluid basaltic lavas
 Most have grown from the deep ocean floor to form islands
 EX – Hawaiian Islands and Iceland
Cinder Cones
 Formed by ejected lava fragments that harden in the air
 Fragments range form fine ash to bombs but most are lapilli or cinders
 Usually product of a single eruption
Composite Cones
 Most beautiful and potentially dangerous
 Also called stratovolcanoes
 Large, nearly symmetrical volcanic mountain composed of layers of both lava and
pyroclastic deposits
 Gas – rich magma
 Most explosive eruptions
 Most located in the Ring of Fire
Other Volcanic Landforms
 Volcanic mountains are not the only landforms that result form volcanic activity
Calderas
 Depression in a volcanic mountain after end of activity
 Ex – Crater Lake
Volcanic Necks
 Landform made of magma that hardened in a volcano’s pipe and later was
exposed by erosion
Lava Plateaus
 Volcanic landform produced by repeated eruptions of very fluid, basaltic lava
 Erupts through long cracks called fissures
 Lava spreads out over a wide area
Volcanic Hazards
 Volcanic soil is good for farming
 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
 Most magma cools and hardens within the earth
Classifying Plutons
 Structures that result form the cooling and hardening of magma beneath Earth’s
surface
 Types of Plutons – sills, laccoliths and dikes ( classified by size, shape and
relationship to surrounding rock)
Sills and Laccoliths
 Magma intrudes between rock layers close to the surface
 Sill – magma flows between parallel layers of sedimentary rock
 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
 Surface exposure of more than 100sq. km
 Much larger than a pluton
 Formed by plutons that clump together as they rise
 Core of many mountain ranges
 Ex – Sierra Nevada