Volcanoes
Section 1 Notes: Volcanoes and Plate Tectonics
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Volcano: is a weak spot in the crust where molten material, or magma comes to the surface
Magma: is a molten mixture of rock-forming substances, gases, and water from the mantle
Lava: when magma reaches the surface
Ring of Fire: formed by the many volcanoes that rim the Pacific Ocean
Volcanic belts form along the boundaries of Earth’s plates
o At plate boundaries, huge pieces of the crust diverge (pull apart), or converge (push
together). As a result, the crust often fractures, allowing magma to reach the surface
Island arc: an volcano that is created in a string of island
Hot Spot Volcanoes
o Hot spot: is an area where material from deep within the mantle rises and then melts,
forming magma
o A volcano forms above a hot spot when magma erupts through the crust and reaches
the surface
o Some hot spots lie in the middle of plate far from any plate boundaries, but other hot
spots occur on or near plate boundaries
o A hot spot in the ocean floor can gradually form a series of volcanic mountains
Section 2 Notes: Properties of Magma
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Physical and Chemical Properties
o Element: is a substance that cannot be broken down into other substances
 Carbon, hydrogen, and oxygen are examples of elements
o Compound: is a substance made of two or more elements that have been chemically
combined
o Each substance has a particular set of physical and chemical properties. These
properties can be used to identify a substance or to predict how it will behave.
o Physical property: is any characteristic of a substance that can be observed or measured
without changing the composition of the substance
 Examples of physical properties include density, hardness, melting point, boiling
point, and whether a substance is magnetic
o Chemical Properties: is any property that produces a change in the composition of
matter
 Examples of chemical properties include a substance’s ability to burn and its
ability to combine, or react, with other substances.
o Viscosity: the physical property of liquids
o Because liquids differ in viscosity, some liquids flow more easily than others
o The greater the viscosity of a liquid, the slower it flows
 Example: honey is thick, sticky liquid with high viscosity; honey flows slowly
 The lower the viscosity, the more easily a liquid flows
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Viscosity of magma depends upon its silica content and temperature
Silica: is made up of particles of the elements oxygen and silicon
Silica is one of the most abundant materials in Earth’s crust
The amount of silica in magma helps to determine its viscosity
The less silica magma contains, the lower its viscosity
Different types of lava
 Pahoehoe: is fast-moving, hot lava that has low viscosity
 The surface of a lava flow formed from pahoehoe looks like a solid mass
of wrinkles, billows, and ropelike coils
 aa: lava that is cooler and slower-moving
 Aa has higher viscosity than pahoehoe
 When aa hardens, it forms a rough surface consisting of jaded lava
chunks
Section 3 Notes: Volcanic Eruptions
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A volcano is more than a large, cone-shaped mountain. Inside a volcano is a system of
passageways through which magma moves
Magma chamber: beneath a volcano, magma collects in a pocket
The magma moves upward through a pipe, a long tube in the ground that connects the magma
chamber to Earth’s surface
Vent: molten rock and gas leave the volcano through an opening
Lava flow: is the area covered by lava as it pours out of a vent
Crater: is a bowl-shaped area that may form at the top of a volcano around the central vent
When a volcano erupts, the force of the expanding gases pushes magma from the magma
chamber through the pipe until it flows or explodes out of the lava
Geologists classify volcanic eruptions as quiet or explosive
o Quiet eruptions: a volcano erupts quietly if its magma is low in silica; low silica magma
has low viscosity and flows easily; the gases in the magma bubble out gently; lave with
low viscosity oozes quietly from the vent and can flow for many kilometers
o Explosive eruptions; a volcano erupts explosively if its magma is high in silica; high silica
magma has high viscosity, making it thick and sticky; the high-viscosity magma does not
always flow out of the crater
Pyroclasic flow: occurs when an explosive eruption hurls out a mixture of hot gases, ash, cinders,
and bombs
Volcano hazards
o During a quiet eruption, lava flows from vents, setting fire to, and then burying,
everything in its path
o During an explosive eruption, a volcano can belch out hot clouds of deadly gases as well
as ash, cinders, and bombs
Geologists often use the terms active, dormant, or extinct to describe a volcano’s stage of
activity
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Dormant: or sleeping volcano is likely to awaken in the future and become active
Extinct: or dead volcano is unlikely to erupt again
Section 4 Notes: Volcanic Landforms
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Volcanic eruptions create landforms made of lava, ash, and other materials. These landforms
include shield volcanoes, cinder cone volcanoes, composite volcanoes, and lava plateaus.
Shield volcanoes: lava that flows gradually build a wide, gently sloping mountain
o Shield volcanoes rising from a hot spot on the ocean floor created the Hawaiian Islands
Cinder cone: these materials build up around the vent in a steep, cone-shaped hill or small
mountain
o Example: Paricutin in Mexico erupted in 1943 in a farmer’s cornfield
Composite volcanoes: are tall, cone shaped mountains in which layers of lava alternate with
layers of ash
o Examples: Mount Fuji in Japan, and Mount St. Helens in Washington State
Caldera: the huge hole left by the collapse of a volcanic mountain
How does a caldera form: enormous eruptions may empty the main vent and the magma
chamber beneath a volcano
Features formed by magma include volcanic necks, dikes, and sills, as well as bathliths and dome
Batholiths: is a mass of rock formed when a large body of magma cools inside the crust
Dome mountain forms when uplift pushes a batholiths or smaller body of hardened magma
toward the surface
Geothermal activity: magma a few kilometers beneath Earth’s surface heats underground water
Hot springs and geysers are types of geothermal activity that are often found in areas of present
or past volcanic activity
Hot springs forms when groundwater is hearted by a nearby body of magma or by hot rock deep
underground.
Geyser: is a fountain of water and steam that erupts from the ground