Download Earth Science Notes NAME: Chapter Nine-Volcanoes

Earth Science Notes
Chapter Nine-Volcanoes
NAME:
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SECTION 1-Volcanic Eruptions
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1.
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.
2. Where does the heat come from? Most heat is produced by weight and pressure of the
overlying rock material.
b.
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.
2. Gases escape easily from some types of magma and thus never build up great pressure to cause
an explosive eruption. Ex. Kilauea
3.
b. Factor Two: “Magma Composition”
1. Mafic magma contains less silica, is very fluid, and produces quiet, nonexplosive eruptions.
a.
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.
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.
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 lava2. 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 lava4. 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 blocks2. 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-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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.
2. Usually only a few hundred meters high.
3. Ex. Pari´cutin
c. Composite Volcano or Stratovolcanoes: See Fig. 5 pg 228
1.
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.
c. When this cycle of lava and tephra is repeated over and over in alternating layers, a
composite volcano is formed.
d.
e.
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. Batholiths2. Dikes3. Sills4. 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 necksb. 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. Cratersd. 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.
b.
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.
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.
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.
2. This magma cools and adds material to the mid-ocean ridge and creates new lithosphere along
the rift.
3.
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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.
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.
4.
----------------------------------------------------------------------------------------------------------------------------------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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