18.3 Volcanoes

When magma reaches Earth’s surface it is called lava.

• • • •

Objectives

Describe

the major parts of a volcano .

Compare

and

contrast

composite volcanoes.

shield, cinder-cone, and

Contrast

the volcanism that occurs at plate boundaries.

Explain

the relationship between volcanism and hot spots.

Vocabulary

– vent – crater – caldera – shield volcano – cinder-cone volcano – composite volcano – tephra – pyroclastic flow – hot spot

• A

crater

is a bowl-shaped depression at the top of a volcano that is connected to the magma chamber by a vent.

Tephra

are rock fragments thrown into the air during a volcanic eruption.

• A

vent

is where lava erupts through an opening in the crust.

Calderas

•

Calderas

are large depressions that can form when the summit or the side of a volcano collapses into the magma chamber.

Volcanoes

Three main types: Shield Composite Cinder Cone

Shield

• Very fluid lava- has Iron (Fe) and Magnesium (Mg).

• Lava flows great distances • Mild eruptions • Gentle sloping • Some of the largest volcanoes in the world

Mauna Loa - Hawaii

Shield Volcano

Composite

• Made up of alternating layers of ash, cinders and lava • Lava rich in silica (Si) • Lava thick • Eruptions can be explosive • Steep slopes

Mt Fuji Japan

Composite

Cinder cone

• Smallest volcano type • Most abundant type • Lots of gas trapped in magma • Very explosive eruptions • Active for only a short time • Has bowl shaped crater in the center

Cinder Cones

Crater Lake

Lava Flow

Where do volcanoes occur?

– 80% found at convergent boundaries.

– 15% found at divergent boundaries. – 5% found far from plate boundaries.

Where do volcanoes occur?

Mediterranean Belt.

Pacific Ring of Fire.

Ring of Fire

Hot Spots

–

Hot spots

are unusually hot regions of Earth’s mantle where high-temperature plumes of mantle material rise toward the surface. – A plume does not move laterally, which results in a trail of progressively older volcanoes that formed as a plate moved over a hot spot.

Hot Spot Plumes

Pyroclastic Flows

Sakura-jima – A

pyroclastic flow

is a cloud of volcanic gas, dust, and other tephra traveling at speeds of nearly 200 km/h. The center of a pyroclastic flow can exceed 700 °C. Mt. Saint Hellens Soufriere Hills

Pinatubo Ash Deposit

Pyroclastic deposits

• Over 100 pyroclastic fall deposits in Oshima on the Izu Islands

Section Assessment

1.

Match the following terms with their definitions.

A.

an opening in the crust from which lava flows

B.

depression caused by a collapsed magma chamber

C.

a bowl-shaped depression around an opening in the crust

D.

volcanic materials that are thrown into the air during a volcanic eruption

E.

a cloud of rapidly moving, extremely hot volcanic material

Section Assessment

2.

How can chains of volcanoes that form over a hot spot track plate movement?

The hot spot is in a fixed location. All of the volcanoes in the chain were over the hot spot when they formed. The volcanoes’ movement and the direction of the chain’s alignment indicates the movement of the plate.

Section Assessment

3.

Identify whether the following statements are true or false. than 500 m high.

boundaries as compared to convergent boundaries.

false

______ The Mediterranean Belt is also known as the “Ring of Fire”.

true

______ It can be 700ºC in the center of a pyroclastic flow.

If Yellowstone Erupted

Miscellaneous Info

• Underwater volcano is called a seamount they look a lot like composite volcanoes.

• Most volcanoes occur at plate boundaries convergent plate boundaries.

• Volcanoes can occur in the middle of tectonic plates- these are called hotspots . Ex. Hawaiian Islands