Volcanoes A place where molten rock, hot gases, and solid rock erupt through an opening in the crust. It is also the mountain built up by these materials.

How Volcanoes Form Beneath the volcano is a source of magma. Magma rises to the surface through cracks in the earth. In time the magma reaches the surface. The magma erupts through the central opening known as the vent. When the magma comes out of the vent it is called lava.

Erupted material cools and hardens around the vent in a mound. After many eruptions it can pile up into a big hill or mountain called a volcano.

Parts of a Volcano Vent – central opening to the volcano Crater – cuplike hollow around the vent. Caldera – when a volcano collapses and forms a very wide crater Lava – magma that has reached the surface

Where Volcanoes Form Occur along the plate boundaries in belts or long lines. 80% of all Earth’s volcanoes are found within: Pacific Ring of Fire Mediterranean Sea through Iran

Pacific Ring of Fire

Rift Volcanoes Where plates are moving apart volcanoes form along the gaps at the edges of spreading plates. Most are located deep underwater along mid-ocean ridges.

Along Colliding Plates One plate plunges beneath another. The downward-moving plates melts. Magma forms. It moves upward through the rocks and forms volcanoes.

The Hawaiian Islands A chain of volcanoes, many not active In the middle of a plate Believed to be moving over a hot spot Kauai is the oldest island. It formed when it was located over a hot spot. As the plate moved, Kauai moved away from the hot spot and was no longer active.

Hot Spot A very hot part of the mantle As a plate moves over the hot spot, magma melts up through the crust and forms volcanoes. Other examples: The Azores and the Galapagos

What Causes Eruptions Magma is less dense than the rock around it. The magma rises up toward Earth’s surface. Near the surface there is less rock overhead pressing down on the magma. If the pressure decreases enough, it is like removing the cap of a shaken bottle of soda. Gases that were dissolved in the magma came boiling out. The gases shoot lava or partly hardened chunks of lava out the vent.

Types of Eruptions Depends on how much gas is in the magma and how thick the magma is.

Types of Volcanoes

Cinder-cone Volcano Has both explosions and flowing eruptions

Cinder-Cone Eruptions Magma that is thick and has a lot of gas in it. Lumps of magma may get stuck as the magma rises to the surface. In time the magma bursts free. Gases explode our of the magma. Lava blasts outward and hardens. What falls to the ground is a rain of hot rocks in sizes from tiny droplets to huge boulders. Builds into a steep-sided cone.

Shield Volcano flowing eruptions http://www. youtube. com/watch

Shield Volcano Eruptions Lava flows out of the vent due to a lot of gases already having escaped from the magma. The lava spreads out and hardens into a wide, flat mound.

Composite Volcano Explosive eruptions http://www. youtube. com/watch

Composite Volcano Eruptions The eruptions “take turns” An eruption may explode and the next time may flow out of the volcano. The switching of eruptions repeats over and over. Have beautifully symmetrical shapes The shape of one side of the cone matches the opposite side.

Active Volcanoes Erupting now or have erupted recently Many located in the Pacific Ring of Fire

Japan Made up of volcanic island including active volcanoes These islands were built up from the ocean floor along a convergent boundary. One plate is plunging under another. Molten rock pushed through the surface and forma string, or arc, of volcanoes. The volcanoes grew in size over time from undersea volcanoes to islands far above sea level.

Dormant Volcanoes When active volcanoes stop erupting Has not been active for a long time but has erupted in recorded history.

Extinct Volcanoes Has not erupted in recorded history.