6 Volcanoes and Volcanic Hazards Free Online FlashCards

How do magma and lava differ?

Magma is molten rock below ground; when it erupts onto the surface, it is called lava.

What three processes can cause rock to melt?

Rock melts when pressure decreases as hot mantle rises, when added water lowers its melting temperature, or when surrounding rock is heated.

Where do volcanoes commonly form?

Volcanoes occur most often at divergent boundaries, where plates separate, and convergent boundaries, where a sinking plate releases water that helps melt mantle rock. Some also form above hot spots, such as Hawaii.

What conditions favor an effusive eruption?

Runny magma lets gas escape more easily, favoring effusive eruptions that produce lava flows or fountains.

How can viscous, gas-rich magma produce an explosive eruption?

Viscous, gas-rich magma can trap gas and build pressure. When released explosively, it fragments magma into tephra, ranging from fine ash to large blocks.

How do phreatic and phreatomagmatic eruptions differ?

In a phreatic eruption, heated water flashes to steam and blasts apart existing rock; new magma need not reach the surface. In a phreatomagmatic eruption, magma interacts directly with external water.

How does a shield volcano form?

Shield volcanoes are broad, gently sloping structures built mainly by repeated flows of fluid basaltic lava.

What builds a stratovolcano?

A stratovolcano is a steep cone built from alternating lava flows and deposits of ash and other fragments. Its eruptions can range from quiet lava emissions to violent explosions.

What materials build a cinder cone?

Cinder cones are usually small, steep-sided hills made mostly of loose fragments that fall around a vent.

How does a volcanic caldera form?

A caldera is a large depression formed when the ground collapses after a substantial amount of magma has been withdrawn from beneath a volcano.

What is a pyroclastic density current?

Pyroclastic density currents are fast, ground-hugging mixtures of hot gas, ash, and rock that can devastate areas in their paths.

What is a lahar, and where can it travel?

Lahars are rapid flows of water mixed with volcanic sediment. They can travel down river valleys, even after an eruption, and threaten communities far from a volcano.