G E O L O G Y : I N S I D E T H E E A R T H C H A P T E R 3 V O L C A N O E S
Chapter 3: Volcanoes
Big Idea: Forces Inside Earth
See pp. 80-111
Essential Question: What causes volcanoes and how do they affect Earth’s surface?
As tectonic plates move due to convection currents in Earth’s mantle , stress builds up along the lithosphere’s plate boundaries and weak areas in Earth’s crust called hot spots.
Less dense magma rises through the crust in these stressed places to form a volcano . Most volcanoes form due to the forces built up from subduction (at convergent boundaries) and sea-floor spreading (at divergent boundaries). The Earth’s surface is affected when lava cools to form new landforms like volcanic mountains, volcanic islands and lava plateaus . Volcanic debris from erupting volcanoes covers the surface and eventually becomes part of the rock record .
Section 1: Volcanoes and Plate Tectonics
Volcano: A vent (opening) in the Earth's surface through which magma erupts; it is also the landform that is constructed by the eruptive material.
Magma: Molten rock containing liquids, crystals, and dissolved gases that forms within the upper part of the
Earth's mantle and crust. When erupted onto the Earth's surface, it is called lava.
Lava: The term used for magma once it has erupted onto the Earth's surface
Ring of Fire: a major belt of volcanoes that rims the Pacific Ocean
Hot Spot: An area in the middle of a lithospheric plate where magma rises from the mantle and erupts at the
Earth's surface.
Island Arc: a string of islands formed by the volcanoes along a deep-ocean trench
 Most volcanoes are found along the boundaries of Earth’s plates and over hot spots.
Section 2: Properties of Magma
Element: a substance that cannot be broken down into other substances by ordinary chemical or physical means
Compound: a substance made of two or more different elements that have been chemically combined
Physical Property: any characteristic of a substance that can be observed or measured without changing the composition of the substance
Chemical Property: any property that produces a change in the composition of matter
Silica: the major ingredient of magma; made of the elements oxygen and silicon; silica content ranges from
50 to 70% and affects viscosity.
Visocosity: Measure of the fluidity of a substance. Lower viscosity magma flows quickly and cools to form basalt. Higher viscosity magma flows poorly and cools to form rhyolite (like granite).
Pahoehoe: relatively hotter, faster flowing lava that has a smooth, hummocky or ropey surface
Aa: relatively cooler, slower flowing lava that has a rough, rubbly surface composed of broken lava blocks
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 Each substance has a particular set of physical and chemical properties that can be used to identify a substance or to predict how it will behave.
 Viscosity is the resistance to flowing. Some liquids flow more easily than others (low viscosity).

The viscosity of magma depends upon the amount of silica and temperature. High silica increase viscosity. Low temperature increases viscosity.
Section 3: Volcanic Eruptions
Vent: the main opening in a volcano through which magma and gas escape
Side Vent: a conduit on the side of the volcano through which magma escapes
Crater: the bowl shaped area around the central vent
Lava: molten rock flowing over Earth’s surface
Lava flow: the area covered by lava as it flows out of a vent
Pipe: the long tube underground that connects the magma chamber to the surface
Magma: molten rock underneath
Earth’s surface
Magma Chamber: the pocket of magma beneath the surface
Pyroclastic Flow: a ground hugging avalanche of hot ash, pumice, rock fragments and hot, toxic gas that rushes down the side of a volcano as fast as 100 km / hr
. Temperatures can exceed 500° C.
Active or Live Volcano: A volcano that is currently erupting, or has erupted during recorded history.
Dormant or Sleeping Volcano: An active volcano that is in repose
(quiescence) but is expected to erupt in the future.
Extinct or Dead Volcano: A volcano that is not expected to erupt again.
 The forces in dissolved, expanding gases push magma from the magma chamber through the pipe and out of the vent during a volcanic eruption.
 Eruptions can be either quiet or explosive, depending on the physical properties of magma.
 Quiet eruptions result from low silica, low viscosity magma, and can result in aa and pahoehoe.
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 Explosive eruptions result from high silica, high viscosity magma, which can plug the vent and cause a build up of gases. Explosive eruptions feature bombs and pyroclastic flow, and can result in rocks like pumice and obsidian.
 Geologists monitor volcanoes for swelling, changes in tilt or elevation, seismic activity, ground water temperatures and escaping gases.
Section 4: Volcanic Landforms
Cinder Cone Volcano Composite or Stratovolcano Shield Volcano
A volcano that has long gentle slopes produced by repeated eruptions of runny lava flows. Galapagos Isl.
Caldera: the hole left by the collapse of a volcanic mountain
A steep-sided volcano formed by the explosive eruption of cinders that form around a vent.
Cinders are lava fragments about 1 cm.
Paricutin
Volcanic Neck: a landform that results from magma hardening in a volcano’s pipe
A steep-sided volcano built by alternating eruptions of lava flows and tephra (rocky debris) deposits.
Mt. Saint Helens & Mt. Shasta
Batholith:
A massive rock that forms when magma hardens inside the crust
Lava Plateau: a landform built from repeated lava eruptions
Sill: hardened rock formed when magma squeezes between horizontal layers of rock http://core.ecu.edu/geology/woods/NewIgRxQuesAnsw2008_files/image004.gif
when magma forces itself across vertical layers of rock
Geyser: a fountain of geothermallyheated water and steam that erupts from the ground
(see also hot springs)

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