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Volcanoes exist because the surface of our Earth is in constant motion.
The Earth’s outer layer is divided into plates, like a jigsaw puzzle. These plates are generally about 50 miles thick and float like rafts on a softer layer of rock. As the plates float, they bump together or pull apart. When plates collide, one plate dives under the other in a process called subduction. When a plate sinks deep enough, the rock melts and makes new magma for volcanoes. When plates pull apart, magma comes to the surface. This is called a rift zone. The lava that erupts builds broad plains and volcanic mountains.
Other volcanoes occur in the middle of plates, over places in the mantle where the magma is superhot, as if someone has turned up the gas burner on a stove. As a plate moves over a hotspot, the magma melts through it.
One of America’s most active volcanoes is Mount St. Helens, in Washington State, and another is Kilauea, in Hawaii.
▲ There are seven major plates, which move like conveyor belts. The arrows show the direction each plate is moving.
Volcano Alert
Scientists at the Smithsonian Institution in Washington, D.C., keep statistics on volcanoes worldwide. They estimate 1,300 volcanoes have been active during the past 10,000 years. Most of these volcanoes lie along a narrow chain, called the Ring of Fire, which circles the Pacific Ocean.
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Ten Most Active Volcanoes on Earth |
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1. Eyjafjallajökull, Iceland |
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2. Mount Vesuvius, Italy |
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3. Sakurajima, Japan |
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4. Mount Merapi, Indonesia |
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5. Mount Nyiragongo, Democratic Republic of Congo |
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6. Ulawun, Papua New Guinea |
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7. Taal, Philippines |
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8. Mauna Loa, Hawaii |
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9. Galeras, Colombia |
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10. Santa Mariá, Guatemala |
At rift zones, plates move apart and new crust forms.
At subduction zones, plates collide and crust slides down and melts.
Mantle
This layer of molten rock lies 120 miles inside the Earth.
Oceanic Plate
This is a portion of the Earth’s crust that’s underneath an ocean.
Hotspot
This is where magma melts through the Earth’s crust.
Continental Plate
This is a portion of the Earth’s crust that’s part of a continent.
◀ Mount St. Helens in Washington State is a typical subduction-zone volcano. Volcanoes at subduction zones often erupt explosively. Thick, sticky magma plugs their vents and traps gases. Pressure builds inside the volcano and explodes the plug like a cap on a soda bottle.
