Download Chapter 7 Notes: Volcanoes Volcanoes and Plate Tectonics Volcano Magma

Chapter 7 Notes: Volcanoes
Volcanoes and Plate Tectonics
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Volcano: a weak spot in the crust where molten material or magma comes to the surface
Magma: a molten mixture of rock forming substances, gases and H2O from the mantle
Volcanic Belts: Form along the Earth’s plate boundaries
o The boundaries Converge or Diverge
 Divergent Boundary: Plates move apart
 Ex: Sea Floor Spreading
o Rift Valley
 Lava pours out and volcano formed
 Convergent Boundary: plates collide
 Subduction at trenches
o The Crust breaks and magma can reach the surface
 The denser plate is subducting and melts to form magma
 The magma rises and breaks through the crust to form volcanoes
Places with many Volcanoes
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Ring of Fire: major volcanic belt
o Formed by many volcanoes that rim the Pacific Ocean
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Island Arc: string of islands created by volcanoes
o Follow the curve of the Trench
o Examples
 Japan
 New Zealand
 Aleutian Islands
 Caribbean Islands
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Hot Spots: An area where deep inside the mantle rises and melts
o Volcanoes form above a hotspot when magma erupts through the crust and reaches
the surface.
o Found…
 Near plate boundaries
 In the middle of plates
 Under the continents
 Ex: old faithful in Yellowstone national park
Properties of Magma
General Properties
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Element: substance that cannot be broken down into another substance
Compound: substance made of 2 or more elements
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Physical Property: any characteristic of a substance that can be observed or measured
without changing the composition of the substance
o Examples: Hardness and melting point
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Chemical Property: any property that produces a change in the chemical composition of the
matter.
o Examples: Ability to burn
Magma’s Viscosity
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Viscosity: the resistance of a liquid to flow
o Depends on the silica content
 More silica: Higher viscosity
 When lava cools it form rock rhyolite which is like granite
 Less Silica: Lower viscosity
 Flows fast, lava cools to form Basalt
o Depends on temperature
 Hot magma flows quickly
 Temperature difference produces 2 kinds of lava
 Pahoehoe- fast, hot lava
o Low viscosity
o Looks like: wrinkles or coils of rope
 Aa: slow, “cooler” lava
o High viscosity
o Looks: rough: jagged chunks
Volcanic Eruptions
Pele’s Hair: type of rock formed by hardened lava
sprays
Inside a volcano
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Magma collects in the magma chamber
Magma then rises upward to the vent
Parts of a Volcano
o Pipe: a long tube in the ground that connects
the magma chamber to the surface of the
Earth.
o Lava Flow: an area covered by lava as it flows out of a side vent
o Crater: a bowl shaped area that may form at the top of the volcano around the central
vent
Erupting Volcanoes
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How it erupts: Gases that are dissolved in the magma expand as the magma goes up the pipe
pushing it upwards until it reaches the top
Two kinds of volcanic Eruptions
o Quiet Eruptions
 Low viscosity magma
 Bubbles out of vents
 Can produce both kinds of lava
 Ex: Kilauea
o Explosive Eruptions
 High Viscosity magma
After Eruption:
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Lava breaks into fragments and cools quickly
Lava hardens into different sizes
o Small:
Volcanic Ash
 Fine rocky
o Medium: Cinders
 Pebble
o Large:
Bombs
 Larger (baseball)
Pyroclastic Flow: occurs during an eruption when hot gases, ash, cinders and bombs are
expelled
Types of Rock Created
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Obsidian: rock from lava that has cooled rapidly
o Glossy Texture
Pumice: forms when gas bubbles get trapped in fast cooling lava
Volcanic hazards
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Burning: quiet eruption can cover a large area
Clouds of deadly gas
Cinders
Bombs
Ash: can burn entire towns
o Roofs collapse
o Jet planes stall
o Avalanche, landslides and mud flows
Stages of Volcanic activity
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Active Volcano: a volcano that is erupting or shows signs of erupting soon
Dormant Volcano: a sleeping volcano
o It can awaken in the future
Extinct Volcano: a volcano that is unlikely to erupt again
Monitoring Volcanoes
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Different instruments are used to measure…
o The temperature of ground H2O
o Earthquakes around the volcano
o Gases escaping
Volcanic Landforms
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Shield Volcano
o Quiet Eruptions
o Gentle, sloping mountain
o Ex: Hawaiian islands are shield volcanoes that arise from hot springs
Cinder Cone Volcano
o Cinder, Ash & Bombs erupt
o Explosively forms cone shaped hill
Composite Volcano
o Alternate quiet and explosive eruptions
o Tall cone shaped mountains
Lava Plateau
o Lava flows out of cracks
o Thin, runny lava
o Ex: Columbian Plateau in Washington, Oregon, & Idaho
Calderas
o Not the same as crater
o This is a huge hole left by the collapse of a volcanic mountain
Land near a volcano
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Soil is Very Fertile
o Contains potassium, phosphorus, and other substances
Landforms from magma
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Sometimes magma breaks through cracks in the upper crust but does not reach the surface
Five different kinds
o Volcanic Necks: magma hardens in the pipe
o Dikes: magma hardens across rock layers
o Sills: magma hardens between horizontal layers of rock
o Batholith: large amount of magma cools in the crust
o Domed Mountain: magma forces the layers of rock to bend upward
Geothermal Activity
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Magma that is only a few kilometers beneath the surface heats the ground water
Occurs in an area of present or past volcanic activity
o Hot springs
o Geysers