Chapter 4Lesson 2Volcanoes

Students will be able to explain why a volcano erupts and describe how volcanoes build land.

Kilauea Volcano, Hawaii

How can scientists predict whether a volcano will erupt?

Where are volcanoes found? Volcanoes form on land and on the ocean floor. A

volcano is an opening in Earth’s crust. They are only located in certain places on Earth’s surface. Most volcanoes are found where plates meet.

The Ring of Fire – surrounds the Pacific Ocean. It follows the boundaries of the plates that meet around the Pacific Ocean. Volcanoes are more likely to erupt at plate boundaries than anywhere else on Earth. An eruption is an outpouring of melted rock, ash, gases, or a combination of these.

Brain pop - https://www.brainpop.com/science/earthsystem/volcanoes/

Where are volcanoes found? Volcanoes do not erupt at all plate boundaries.

Scientists conclude that volcanoes tend to erupt where one plate is pushed under another plate. When rocks in the plate that is being pushed down reach the heat and pressure in the mantle, they melt. Magma forms and pools in a chamber underneath the crust. Sometimes a crack forms above the chamber or the pressure in the chamber grows too great and the magma rushes upward toward the surface.

All volcanoes have at least one vent, or opening. Once magma reaches Earth’s surface, it is called lava. Over time, a cup-shaped depression may form around a vent. This depression is called a crater. Most craters are found at the top of a volcanic mountain. Sometimes the magma chamber beneath a volcano is emptied. The volcano may then collapse inside itself. The hole that forms is called a caldera.

Galapagos Islands - caldera

How does magma form geological features? Dike – when magma hardens in vertical

or nearly vertical cracks

How does magma form geological features? Sill – forms when magma hardens

between horizontal layers of rock. Dikes and sills can be large or small.

Laccolith – When the magma pushed into a sill does not spread horizontally. Instead it pushes upward and forms a dome shape. When a laccolith forms, it may raise the rock layers above it.

Batholith – the largest and deepest of all underground magma formations – huge and irregularly shaped. It reaches deep into the crust.

When lava comes out of a vent, it is liquid. The lava cools to form a solid layer of rock as it hardens. Over thousands of years, layers of lava may increase the height of a volcano and form a volcanic mountain. The mountain is new land that the volcano has built.

Volcanoes Active volcano – currently erupting Dormant volcano – has not erupted for some time Extinct volcano – a volcano that does not erupt anymore Shield volcano – built by thinner fluid lava that spreads

over a large area. These mountains have a broad base and gently sloping sides.

Cinder-cone volcanoes – built by thick lava that is thrown high into the air and falls as chunks or cinders. These mountains form as a cone shape with a narrow base and steep sides.

Composite volcanoes – built by layers of ash and cinders sandwiched between layers of hardened lava. The shape on one side of a cone formed by a composite volcano usually looks the same as the shape on the opposite side.

Shield volcano

Cinder-cone volcano

Composite volcano

Review How might a cone-shaped mountain form? It can form from rock fragments (a cinder-cone

volcano) or from alternating layers of rock fragments and lava(a composite volcano).

Explain an extinct, dormant, and active volcano.

Extinct – no longer erupting, dormant – is not erupting at this time, active – erupting at the present or recently

How do volcanoes build islands? The Hawaiian Islands are an island chain, or a line of

volcanic mountains. Scientists know that the islands rest on a slowly moving tectonic plate. As it moves, the plate passes over a stationary pool of magma called a hot spot. Over years the lava erupting formed a mountain. Eventually the mountain grew taller than the ocean’s surface and became a volcanic island. As the plate moved, that island moved away from the hot spot and a new island began to form. In areas where an ocean-floor plate is pushed under another ocean-floor plate, an island arc forms. As it is pushed down, it melts. Magma forms, rises upward, and erupts through the ocean floor. This forms a series of volcanic islands.

Review What information would you need to figure

out if islands formed from a hot spot?

You would need to know whether all the islands formed form volcanoes, if any of the islands had an active volcano, and the ages of the islands to determine if the extinct volcanoes where moving away from the active volcano, which is where the hot spot would be.

Review - games http://discoverykids.com/gamesvolcano-

explorer/

http://www.neok12.com/Volcanoes.htm

http://www.scholastic.com/play/prevolcano.htm