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Earth Science Notes
Chapter Nine-Volcanoes
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SECTION 1-Volcanic Eruptions
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1. A volcano is a mountain that forms when molten rock, called magma, is forced to the Earth’s surface; lava and
pyroclastic material built up on the earth’s surface around a vent.
2. Rocks in the mantle melt to form magma, which is forced upward toward earth’s surface. When the magma
flows through vents, its called lava and forms volcanoes.
a. Why do rocks melt?
1. Magma results from the extreme heat of the earth's interior. Most magma forms 50 to 100
miles (80 to 160 kilometers) beneath the surface.
2. Where does the heat come from? Most heat is produced by weight and pressure of the
overlying rock material.
b. Most magma forms at plate boundaries, where one lithospheric plate, usually of oceanic crust, is
subducted beneath another plate.
c. As the magma rises, it melts gaps in the surrounding rock and forms a large chamber as close as 2
miles (3 kilometers) to the surface. This magma chamber is the reservoir from which volcanic
materials erupt.
2. Types of Eruptions--Two major factors determine what type of eruption may be experienced by a volcano:
a. Factor One: “the presence of trapped gases”
1. The presence of trapped gases such as carbon dioxide and water vapor can have a great
impact on the eruption of a volcano.
2. Gases escape easily from some types of magma and thus never build up great pressure to cause
an explosive eruption. Ex. Kilauea
3. Gases that get trapped under high pressure eventually escape, causing explosive eruptions such
as Mt. St. Helens (1980).
b. Factor Two: “Magma Composition”
1. Mafic magma contains less silica, is very fluid, and produces quiet, nonexplosive eruptions.
a. This type of lava pours from vents and runs down the sides of the volcano.
b. Because the magma is fluid, the trapped gases can escape easily in a nonexplosive
manner. Ex. Kilauea
2. Felsic magma produces explosive, violent eruptions such as those at Mt. St. Helens.
a. Usually forms in subduction zones.
b. This magma is very thick and contains a lot of silica. Because it is thick, it gets
trapped in vents, causing great pressure to build up beneath it.
c. When an explosive eruption occurs, the gases expand rapidly, often carrying pieces of
lava (pyroclastic material) in the explosion.
3. What Erupts from a Volcano?
a. Depending on how explosive a volcanic eruption is, magma erupts as either lava or pyroclastic
material. Pyroclastic material consists of the rock fragments created by explosive volcanic eruptions.
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b. Lava-Several Types: Like magma, lava ranges in consistency from thick to thin.
1. Blocky lava-is so thick in consistency that it barely creeps along the ground.
2. Pahoehoe lava-flows slowly, like wax dripping from a candle, forming a glassy surface with
round wrinkles. [Looks like layers of rope stuck together]
3. Aa lava-a slow-moving type of lava that hardens to form rough chunks; cooler than pahoehoe.
4. Pillow lava-forms when lava erupts underwater. It forms rounded lumps that are the size and
shape of a pillow.
c. Pyroclastic Material-Several Types:
1. Volcanic blocks-are the largest pieces of pyroclastic material. They consist of solid rock
blasted out of the volcano.
2. Volcanic bombs-are large blobs of magma that harden in the air.
3. Lapilli-means “little stones” in Italian, are pebble-like bits of magma that become solid before
they hit the ground.
4. Volcanic Ash-forms when the gases in stiff magma expand rapidly and walls of the gas
bubbles explode into tiny glasslike slivers.
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SECTION 2-Volcanoes’ Effects on Earth
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1. Volcanic cones are classified into three categories according to composition and form.
a Shield Volcano: See Fig. 5 page 228
1. Basaltic magma, thus quiet eruptions occur. Tend to see lava flow from vents and form layers.
These layers buildup to form a broad volcano with gentle slopes. Example--Hawaiian Islands
b. Cinder Cone Volcano: See Fig. 5 page 228
1. Formed by granitic magma, thus explosive. Here lava is ejected from the volcano where it can
form a steep-sided, loosely consolidated cinder cone volcano.
2. Usually only a few hundred meters high.
3. Ex. Pari´cutin
c. Composite Volcano or Stratovolcanoes: See Fig. 5 pg 228
1. Eruptions of these volcanoes vary between periods of quiet & violent!
2. The result:
a. Explosive period produces gas and ash, forming a layer of tephra-lava that cools and
hardens into different sizes of volcanic material.
b. During quiet period, erupting lava flows over the top of the tephra layer.
c. When this cycle of lava and tephra is repeated over and over in alternating layers, a
composite volcano is formed.
d. Usually form by two plate converging and forming a subduction zone.
e. Often develop into very high volcanic mountains.
f. Ex. Mt. Fuji in Japan, Mt. Rainier, Mt. Hood, Mt. Shasta, and Mt. St. Helens in the
United States.
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2. Features of Volcanoes--Intrusive features:
a. Batholiths, dikes, sills, and laccoliths form when magma solidifies underground.
1. Batholiths-Largest intrusive igneous rock bodies. (deep rock bodies) Can be hundreds of miles
wide and long.
2. Dikes-formed when magma cuts up through layers of rock (vertical structure).
3. Sills-a rock body that is formed when magma cuts across layers of rocks (horizontal structure).
4. Laccoliths-a rock body formed when magma bulges up to make a dome structure. Many
laccoliths are located below the Black Hills.
3. Other Features of Volcanoes:
a. Volcanic necks-the core of a volcano’s vent that remains after the outer layers of lava and
tephra have been eroded away from an extinct volcano.
b. Caldera-the large opening formed at the top of a volcano when the crater collapses into the
vent following an eruption. Crater Lake in Oregon is a caldera that is now a lake.
c. Craters-a bowl-shaped area that forms around a volcano’s central opening.
d. In the largest type of volcanic eruption, lava simply pours from long fissures in the Earth’s crust to
form lava plateaus.
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SECTION 3-What Causes Volcanoes?
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1. The key to understanding why volcanoes erupt is to understand how magma forms.
a. Volcanoes result from magma formed in the mantle.
b. When pressure is reduced, some of the solid rock of the already hot mantle melts to form magma.
c. Because it is less dense than the surrounding rock, magma rises to the Earth’s surface. It either
erupts as lava or solidifies in the crust.
2. Where Volcanoes Form:
a. Many volcanoes are located along subduction zones, where one plate moves under another plate.
1. The subducted plate dives deep into the asthenosphere where it melts.
2. A major zone of active volcanoes is caused by subducting plates encircling the Pacific Ocean.
This zone, called the Pacific Ring Of Fire, results from plates subducting along the Pacific
coasts of North America, South America, Asia, and islands of western Pacific Ocean.
b. When two oceanic plates collide the result is the formation of an island arc.
1. One of the two plates is forced under where it melts. Some of the magma breaks through the
surface behind the trench. The result is the formation of an island arc.
2. Examples--Aleutian Islands and Japan
c. Mid-Ocean Ridges provide areas where large amounts of magma can reach the surface of the earth.
1. As plates pull apart, magma wells upward along the rift zone.
2. This magma cools and adds material to the mid-ocean ridge and creates new lithosphere along
the rift.
3. The magma can also form underwater volcanoes.
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4. Most occur deep within earth’s oceans, however, one exception is found on Iceland. Iceland is a
part of the Mid-Atlantic Ridge that is above sea level. Iceland is sitting on two plates moving in
different directions. The middle of Iceland is cut by fissures, cracks through which lava flows.
d. Hot Spots are areas where volcanoes develop not along a plate boundary, but instead develop in the
middle of a plate.
1. Hot spots tend remain fairly stationary, but the plates above a hot spot continue to drift
slowly.
2. The result--The volcano that develops over the hot spot eventually is carried away from the
hot spot, thus cutting off the supply of magma that is needed to keep it active.
3. A new volcano forms where new crust has moved over the hot spot.
4. The Hawaiian Islands are an example of a chain of volcanic islands formed over a hot spot.
----------------------------------------------------------------------------------------------------------------------------------QUESTIONS TO CONSIDER:
1. Explain how the composition of magma determines the type of volcanic eruption that will occur.
2. Classify the main types of lava and volcanic debris.
3. If a volcano contained magma with small proportions of water and silica, would you predict a nonexplosive
eruption or an explosive one? Why?
4. List and describe the three main types of volcanoes.
5. Why do cinder cone volcanoes have narrower bases and steeper sides than shield volcanoes?
6. Briefly describe the difference between a caldera and a crater.
7. Describe the formation and movement of magma.
8. What two factors may cause solid rock to become magma?
9. Explain the relationship between volcanoes and plate tectonics.
10. If the Pacific Plate stopped moving, what might happen to the island of Hawaii?
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