volcanic activity

convergent plate boundaries

Lots of volcanoes form along subduction plate boundaries. The edges of the Pacific Plate are a long subduction boundary. Lines of volcanoes can form at subduction zones on oceanic or continental crust. Japan is an example of a volcanic arc on oceanic crust. The Cascade Range and Andes Mountains are volcanic arcs on continental crust.

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where volcanoes are found

Volcanoes rise where magma forms underground. Volcanoes are found at convergent plate boundaries and at hotspots. Volcanic activity is found at divergent plate boundaries. The map in Figure 8.1 shows where volcanoes are located.

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volcanic hot spots

Some volcanoes form over active hot spots. Scientists count about 50 hot spots on the Earth. Hot spots may be in the middle of a tectonic plate. Hot spots lie directly above a column of hot rock called a mantle plume. Mantle plumes continuously bring magma up from the mantle towards the crust (Figure 8.3). As the tectonic plates move above a hot spot, they form a chain of volcanoes. The islands of Hawaii formed over a hot spot in the middle of the Pacific plate. The Hawaii hot spot has been active for tens of millions of years. The volcanoes of the Hawaiian Islands formed at this hot spot. Older volcanoes that formed at the hot spot have eroded below sea level. These are called the Emperor Seamounts. Loihi seamount is currently active beneath the water southeast of the Big Island of Hawaii. One day the volcano will rise above sea level and join the volcanoes of the island or create a new island (Figure 8.4). Hot spots may also be active at plate boundaries. This is especially common at mid-ocean ridges. Iceland is formed by a hot spot along the Mid-Atlantic Ridge. Hot spots are found within continents, but not as commonly as within oceans. The Yellowstone hot spot is a famous example of a continental hot spot.

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divergent plate boundaries

There is a lot of volcanic activity at divergent plate boundaries in the oceans. As the plates pull away from each other, they create deep fissures. Molten lava erupts through these cracks. The East Pacific Rise is a divergent plate boundary in the Pacific Ocean (Figure 8.2). The Mid-Atlantic Ridge is a divergent plate boundary in the Atlantic Ocean. Continents can also rift apart. When mantle gets close enough to the surface, volcanoes form. Eventually, a rift valley will create a new mid-ocean ridge.

instructional diagrams

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questions

Iceland consists of volcanoes that formed over a hot spot.

-->  a. true

b. false

Volcanoes are scattered randomly across Earths surface.

a. true

-->  b. false

Many volcanoes occur along the mid-Atlantic Ridge.

-->  a. true

b. false

Volcanoes occur only in oceanic crust.

a. true

-->  b. false

Chains of volcanoes form above hot spots because of plate tectonics.

-->  a. true

b. false

The Hawaiian hot spot is no longer active.

a. true

-->  b. false

Hot spots never occur under continental crust.

a. true

-->  b. false

The majority of mantle plumes are found under the ocean basins.

-->  a. true

b. false

All hot spots are in the middle of tectonic plates.

a. true

-->  b. false

The molten rock of a volcano comes from Earths core.

a. true

-->  b. false

column of hot rock that lies below a hot spot

a. volcano

b. magma

c. lava

d. hot spot

-->  e. mantle plume

f. plate boundary

g. fissure

crack in Earths crust where magma pushes up through the crust

a. volcano

b. magma

c. lava

d. hot spot

e. mantle plume

f. plate boundary

-->  g. fissure

feature that forms when magma flows onto Earths surface

-->  a. volcano

b. magma

c. lava

d. hot spot

e. mantle plume

f. plate boundary

g. fissure

location where most volcanoes occur

a. volcano

b. magma

c. lava

d. hot spot

e. mantle plume

-->  f. plate boundary

g. fissure

fixed spot where magma rises through the crust and creates volcanoes

a. volcano

b. magma

c. lava

-->  d. hot spot

e. mantle plume

f. plate boundary

g. fissure

name for melted rock under Earths surface

a. volcano

-->  b. magma

c. lava

d. hot spot

e. mantle plume

f. plate boundary

g. fissure

name for melted rock after it flows onto Earth surface

a. volcano

b. magma

-->  c. lava

d. hot spot

e. mantle plume

f. plate boundary

g. fissure

The only place where the mid-Atlantic ridge is above sea level is

-->  a. Iceland.

b. England.

c. Greenland.

d. none of the above

Volcanoes form at

a. hot spots.

b. divergent plate boundaries.

c. convergent plate boundaries.

-->  d. all of the above

Many volcanoes are located

a. within the Eurasian plate.

-->  b. at the edges of the Pacific plate.

c. within the North American plate.

d. at the edges of the Atlantic plate.

Volcanoes form in a subduction zone where a tectonic plate

-->  a. is pulled down into the mantle.

b. remains over a hot spot.

c. pulls away from another plate.

d. creates a rift valley.

Examples of volcanic arcs that formed at convergent plate boundaries include the

a. island nation of Japan.

b. Cascade Range in Washington State.

c. Andes Mountains of South America.

-->  d. all of the above

About how many hot spots have scientists identified on Earth?

a. 5

b. 15

-->  c. 50

d. 500

Which U.S. state formed over a hot spot in an oceanic plate?

a. Oregon

b. California

c. Florida

-->  d. Hawaii

diagram questions

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