Chapter 12 Section 1. What is a Volcano? Opening in the Earth (called a vent) that erupts gases, ash, and lava. –Volcanic mountains result from the build-up.

Chapter 12

Section 1

What is a Volcano?

Opening in the Earth (called a vent) that erupts gases, ash, and lava.– Volcanic mountains result from the build-up of

these materials.• The steep walled depression that forms around a

vent is called a crater.

– There are more than 600 currently active volcanoes.

Which is the most active volcano?

Kilauea in Hawaii.– Although Kilauea has been erupting for

centuries, the current series of eruptions began in 1983.

How do volcanoes form?

Deep inside the Earth, heat and pressure cause rock to melt, forming magma.

Because liquid rock (magma) is less dense than solid rock, it slowly rises, reaching the surface after many thousands of years.

Where do volcanoes occur?

Convergent boundariesDivergent boundariesHot spots

Where do volcanoes occur?

Divergent Plate Boundaries– Magma flows onto the floor of the ocean at rifts

(long, deep cracks in the ocean floor).– Over time the cooled magma (igneous rock)

can build up and form islands.– Example:

• Iceland – Mid-Atlantic Rift

• Surtsey - 1963

Where do volcanoes occur?

Convergent Boundaries– As the subducting plate is heated by increasing

temperature and pressure, it partially melts, forming magma that moves to the surface and usually causes violent eruptions.

– Examples:• Mount St. Helen’s – Cascade Mountains

Where do volcanoes occur?

Hotspots– Anomalies in the middle of plates where there

are large amounts of heat.– Hot spots are stationary whereas the plates

above them continue to move.– Examples:

• Hawaii

• Yellowstone