What Are Volcanoes? - Eruption, Types & Facts

Instructor: Maria Howard

Maria is a teacher and a learning specialist and has master's degrees in literature and education.

Explore volcanoes, from the fiery explosion of Mount St. Helens to the slow-flowing lava on Hawaii's 'Big Island.' Learn about how volcanoes form and how they are classified.

From Cornfield to Volcano

In 1943 in the small village of Paricutin, Mexico, a farmer named Dionisio Pulido noticed that a crack spewing hot steam had suddenly appeared in his cornfield. By the end of what must have been a difficult day for Señor Pulido, enough ash, rocks and other materials had been expelled from the crack to form a hill 40 meters high (131 feet), about the height of a 13-story building.

As more debris spewed from the crack, the bigger the hill grew. Nine years passed, and both Pulido's farm and the village of Paricutin were taken over by this growing volcano, which eventually measured one-quarter of a mile high and blew ash and smoke as far as 200 miles away.

This photo taken in 1943 of the volcano at Paricutin, Mexico, shows just how bad things got for Pulido and his cornfield that year.
Photo of volcano in Paricutin, Mexico

What are Volcanoes, and How Do They Form?

Not all volcanoes, cracks in the Earth's crust which allow melted rock to rise to the surface, appear as quickly or dramatically as the one that formed in Dionisio Pulido's cornfield. In fact, many volcanoes start as mountains which lay dormant, or inactive, until shifts in the Earth's surface allow melted or partially melted molten rock, called magma, to pour out. (It's important to note that molten rock is called magma when it's beneath the ground and lava when it is expelled to the surface.)

The crust of the Earth is broken into plates, much like if you cracked the shell of a hard-boiled egg. The plates, like the pieces of eggshell, can move apart, move against each other and even move under one another. These plate movements create opportunities for magma to break through to the surface and create havoc in the form of volcanoes.

Places on Earth where plates crash into one another are called convergent boundaries, and unsurprisingly these boundaries are also places where subduction, or the movement of one plate under another, often occurs. Many of these plates come together deep under the ocean, where we cannot see how they are dramatically altering the Earth.

Burning Ring of Fire

The Ring of Fire, which is less of a ring and more of a horseshoe that encompasses the Pacific Ocean, is a collection of shifting plates that create many volcanoes and other plate-related activities, such as earthquakes and seafloor spreading, in places including Japan, Indonesia and New Zealand and down the west coast of the United States and South America.

The Ring of Fire spans over 40,000 kilometers (25,000 miles) and contains 452 volcanoes.
Map of the Ring of Fire

Many of the world's active volcanoes are found in the Ring of Fire, especially on the ring's eastern edge in places that include Indonesia, the Philippines, and New Zealand. However, sometimes, as is the case with the Hawaiian islands, lava can break through in the middle of a plate.

Many islands, such as the islands that make up Hawaii, form when undersea volcanoes grow tall enough to appear above the ocean's surface. The continually erupting volcanoes Mauna Loa and Kilauea contribute to the growth of the 'Big Island' of Hawaii, and future resort developers might be eager to know there is even a steadily growing volcano called Loihi about 914 meters (3,000 feet) away from rising above the surface of the Pacific Ocean.

Types of Volcanoes

There are currently over 1,500 active volcanoes on Earth, which means they have erupted before and can erupt at any time. But don't worry; only about 50 volcanoes erupt each year. Each eruption adds another layer of lava onto a volcano, and the type of lava flowing out of it gives the volcano its shape.

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