In this topic you will learn about different volcanic features.

Volcanoes occur in belts, or long lines. One circles the Pacific Ocean. Another runs along the Mediterranean Sea, through Iran, Indonesia, and on to the Pacific. Over 80 percent of all land volcanoes are found in these two belts. There are even more volcanoes at the ocean floor. Most volcanoes occur along plate boundaries.

The Hawaiian Islands are a chain of volcanoes, but they are in the middle of a plate. They are not at the edge. Geologists believe the plate is moving over a hot spot. A very hot part of the mantle where magma can melt through a plate moving above it, is called a hot spot.

Magma rises to the surface through cracks in the rocks above it. In time it reaches the surface and erupts through a central opening, or vent. Magma that reaches Earth's surface and flows out of a vent is called lava.

After many eruptions, the material cools and hardens around the vent. It can pile up into a big hill or even a mountain. This is called a volcano. A cuplike hollow that forms at the top of a volcano around the vent is called a crater.

What an eruption is like depends on how much gas is in the magma and how thick the magma is. Some magma is thick and has lots of gas in it. Gases explode out of the magma. Lava blasts outward. It hardens. Hot rocks, ranging in size from tiny droplets to huge boulders, fall to the ground, building up a steep-sided cone called a cinder-cone volcano.

If lots of gases have already escaped from the magma, it may just flow out of the vent. The lava spreads and hardens into a wide, gently sloped cone that forms a shield volcano.

Sometimes an eruption "takes turns." An eruption may explode forming rocks of different sizes. Then the eruption may switch to a quiet period when lava flows over the rocks. When this switching repeats over and over, it forms a composite volcano.

A geyser is an effect of volcanism. The main vent of a geyser is filled with water. The water is heated and turned to steam. Pressure builds up and a jet of hot water and steam erupts from the geyser.

The heat from geysers can be used. Geothermal energy is heat from below Earth's surface. In California, steam from wells dug into hot rock material is used to run power plants that produce electricity. Heated groundwater also concentrates valuable minerals such as copper, tin, gold, and silver. Over thousands of years, these minerals built up into deposits that are mined.