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Volcanoes: What they are and how they shape our planet

You probably know something about volcanoes. Maybe you’ve seen a volcano. Maybe you’ve even visited one. But did you know that volcanoes come in many sizes and shapes, and that they have big impacts on our world? Or that once in a while, a new one even pops up unexpectedly.

What causes volcanic eruptions?

What causes volcanic eruptions?

Volcanoes form where melted rock (called magma) from deep underground erupts onto Earth’s surface. Once the magma reaches the surface, it’s called lava. Lava then cools and hardens into volcanic rock. Along with lava, volcanoes can erupt steam, ash, and gases.

To understand where magma comes from in the first place, it helps to understand Earth’s layers. Picture a hardboiled egg. Earth’s core is the middle, like the yolk. Above that, like the egg white, is the mantle. Earth’s crust is the very thin, brittle layer on the outside, like the eggshell.

Most magma comes from the upper mantle, just below the crust. Because of pressure, density, or other factors, magma from the mantle can rise into the crust. Magma also forms within the crust under conditions of high heat, extreme pressure, or both.

As it rises, magma can pool in big chambers underground called magma chambers. As pressure builds up in a magma chamber, the magma chamber overflows. Magma rises to the surface through vents and fissures. We know those vents and fissures as volcanoes.

Types of volcanoes

Location, location, location

Like so many other features of our dynamic planet, the location of many volcanoes has a lot to do with plate tectonics. That is the interaction of the huge moving plates that make up Earth’s crust.


Where two plates pull apart, magma can well up from the mantle into the space between. That creates volcanoes, such as the ones in Iceland.

Other volcanoes form where one plate dives beneath another as they collide. The plate begins to melt as it sinks deep into the hot crust. Heat and water from the sinking plate rise, causing material above it to melt. That melting creates magma, which rises. That magma can erupt onto the surface as a volcano.

Not all volcanoes form at plate boundaries. Some famous volcanos have formed in the middle of plates. The islands of Hawaii are volcanoes formed by a plume of magma rising from the mantle in the middle of a tectonic plate. As the plate moves, the stationary plume below it has created a chain of volcanoes. The newest Hawaiian Island, the Lö’ihi Seamount, is forming right now south of the big island of Hawaii. It is still 3,000 feet below the ocean surface. But someday it will be the next island in the chain.

In fact, here’s a fun fact: most volcanoes are underwater. Because of this, volcanic eruptions sometimes cause tsunamis or big ocean waves.

Types of volcanoes

There are several types of volcanoes including cinder cones, stratovolcanoes, shield volcanoes, and fissures.

Cinder cones

Cinder Cones

Cinder cones are relatively small, steep volcanoes. They are made from a jumble of lava fragments. This lava typically contains a lot of gas and tends to spatter and explode as it erupts. That breaks it into small chunks called cinders. In some regions, small cinder cones dot the landscape. One day in 1943, in Mexico, a brand-new cinder cone called Paricutin popped up in a farmer’s field.



Stratovolcanoes are the big, beautiful cone-shaped volcanoes many people think of when they think “volcano.” Stratovolcanoes are made from many layers of thick lava, ash, and cinders. When they erupt, watch out! There might be a lot of ash, steam, and gas. Sometimes a stratovolcano explodes violently, destroying its own top and leaving a crater in its place. That happened at Mount St. Helens, Washington, in 1980.

Shield volcanoes

Shield Volcanoes

Shield volcanoes are built from layer after layer of runny lava, like pancake batter. They are not steep at all, but can cover a lot of area. Although they don’t usually erupt violently, that runny lava can run a long way! Kilauea in Hawaii is a very active shield volcano. Hot, moving lava from this volcano has buried roads and houses.



Fissures are long cracks from which lava erupts. Fissure eruptions can be spectacular, as curtains of glowing red lava burst from the ground. These eruptions can appear suddenly and then subside within days.

How volcanoes have shaped our planet

Lava, lava and more lava! Besides just lots of rock, volcanoes have had many other effects on our planet.

Underwater volcanic activity is believed to be where Earth’s earliest life may have started. When volcanic soil is eroded, it makes rich soil for farming.

Volcanoes can also affect climate, weather, and even airplanes. In 1883 an Indonesian volcano called Krakatoa erupted violently. The eruption sent gases and ash sky high into the atmosphere, partially blocking the sun. That made the next summer a chilly one all across the globe. In 2010, eruptions in Iceland sent a cloud of volcanic ash into the sky. Airlines had to cancel flights, because the ash can clog jet engines.

Volcanologists are always learning more about volcanoes. They set up instruments that help detect when magma is on the move inside a volcano. They measure invisible gas emissions that might increase just before an eruption. These scientists hope to predict more precisely when active volcanoes might erupt, so people who live nearby can get safely out of the way.

Written by Laura McCamy

Edited by Beth Geiger, MS Geology

Illustrated by Renee Barthelemy