Chapter 12Section 4 - Volcanoes

Movement along a fault causes a decrease in pressure – decompression

A decrease in pressure causes a decrease in melting point

The temperature remains the same

Magma from the Asthenosphere

• A buoyant force acts on magma that forms from rock surrounding it.

• Hot, nearly molten rock in Earth’s asthenosphere can change to aliquid by decompression melting.

Why Magma Rises• Rising magma

may reach Earth’s surface if pressure conditions allow and the rock has conduits (openings) through which it can flow.

Eruptive Products

• All solid materials expelled by a volcano are collectively called pyroclasts.

• Volcanoes release a broad variety of superheated gases, the most common of which is water vapor.

• In addition carbon dioxide and gases composed of sulfur compounds are expelled.

Solids

Gases

• Viscosity is a measure of the resistance of a fluid to flow. • High viscosity is thick like pancake

syrup.• Low viscosity is runny like water.

Liquids• Lavas can vary considerably in

composition, which in turn affects their physical properties.

Plate Boundary Setting• Most of Earth’s volcanoes lie in subduction

zones where continental and oceanic materials are being mixed and partially melted.

Hot Spots• Hot spots are volcanically active sites

that arise in places where large quantities of magma move to the surface in large, column-like plumes.

• A hot spot under an oceanic plate forms volcanic island chains, such as the Hawaiian Islands.

• Yellowstone National Park is an example of a hot spot under a continental plate.

• Moderate slopes, 20 degrees• Made of pyroclasts (cinder, bombs)• Rock type – basalt• Relatively small in size• Examples: Sunset Crater (AZ), Hawaii

Types of Volcanoes• Volcanoes are classified according to

their size, shape, and the materials that compose them.

Cinder Cone Volcanoes

• Low slope angles, 2 to 8 degrees• Long, fluid (basaltic) lava flows• Rock type – basalt• Can be very large• Example: Mauna Loa, Hawaii

Shield Volcanoes

• Also called Stratovolcano• Made of lava flows and pyroclastics

(mostly ash)• Moderate in size• Rock type – andesite• Examples: Mt. St. Helens, Fuji, Pinatubo

Composite Volcanoes

Section Check

Question 1Where do most volcanoes occur?

AnswerMost volcanoes occur at plate boundaries where huge pieces of the crust pull apart or push together. As a result, the crust often fractures, allowing magma to reach the surface.

Section Check

Question 2What type of volcano is formed by an explosive eruption followed by a quiet eruption?

A. cinder cone volcanoB. composite volcanoC. fissure eruptionD. shield volcano

Section Check

AnswerThe correct answer is B. Composite volcanoes erupt explosively releasing large quantities of gas and ash. They are followed by quieter eruptions that form a lava layer over the ash.

Section Check

Question 3How does a hot spot volcano form?

AnswerA volcano forms above a hot spot when magma erupts through the crust and reaches the surface. Hot spot volcanoes may lie in the middle of plates far from any plate boundaries or near or on plate boundaries.