There are about 1,500 active volcanoes on Earth and that's just above ground. There are countless more under the oceans. Luckily, only about 20 to 30 of them erupt each year.
What Is a Volcano?
A volcano is basically a chimney. Molten rock from beneath the Earth's mantle erupts through volcanoes to the surface. Molten rock (which means melted rock) is called lava. Hot lava isn't the only thing volcanoes let out. They also release deadly gasses and cover the land for miles in ash.
Three basic types of volcanoes are recognized by geologists.
Composite volcanoes (or strato volcanoes) are formed from layers of lava and rock fragments from different eruptions. The word composite means “made up of different things.” This is the reason these volcanoes are called composite.
Composite volcanoes have gentle slopes at the bottom and a steeper slope at the top. They are smaller than Shield volcanoes but larger than Cinder Cone volcanoes. Composite volcanoes often have snow-capped peaks and look like tall mountains. They can be as tall as 8,000 feet.
Between eruptions, composite volcanoes are so quiet that many people forget that they are volcanoes at all. When they do erupt, it is usually a huge explosion. This is because the magma is thick and sticky (or viscous). When very viscous magma rises to the surface through the volcano, it clogs up the volcano's craterpipe. Gas in the craterpipe gets locked in and pressure builds up. This is kind of like blowing too much air into a balloon. At some point it will pop! A volcanic pop is more like a KA-BOOOOM!
Here is an example of a composite volcano before and after an eruption. You can see how the entire top of Mount Saint Hellens was blown off!
Shield volcanoes are huge. Layers of runny lava flows build them up. Since the lava is runny it doesn’t build up high, but spreads out instead. Lava spills out of a main vent or sometimes a group of vents. This forms a broad-shaped, gently sloping cone. Shield volcanoes may be produced by hot spots that lie far away from the edges of tectonic plates. Tectonic plates are the layers of the earth under the crust.
Shields also are found along the mid-oceanic ridge where sea-floor spreading is in progress. Sea-floor spreading is the process of two plates moving apart. This creates a kind of tearing in the earth that lava leaks out of.
The eruptions of shield volcanoes are usually not very explosive. Lava fountains form cinder cones and spatter cones at the vent.
Cinder Cone Volcanoes
A cinder cone is a steep hill formed above a volcanic vent. Cinder cones are the most common volcanoes in the world. They are formed by Strombolian eruptions. Strombolian eruptions are simply low-level volcanic eruptions, named after the Italian volcano Stromboli. The cones usually grow up in groups and they commonly occur near shield volcanoes.
Cinder cones are built from lava fragments called cinders. The lava fragments are shot from a volcanic vent. They pile up around the vent when they land and form a mound. Cinder cones grow fast and don’t typically get much bigger than 700 feet high.
Hot Facts About Volcanoes
Volcanic Rock Floats
Volcanic rock is also known as pumice. It is full of bubbles and holes. It is the only rock that floats in water.
Maleo Birds Need Volcanoes to Survive
The Maleo Bird keeps its eggs warm by laying them in a volcano's hot sand. The chicks have to dig themselves out after they hatch underground.
Volcanoes Are Great for Sunsets
When a volcano spews ash into the sky, the tiny particles reflect the sun's rays, creating gorgeous sunsets of pink and orange hues.
Volcanoes Can Inspire Art
The Scream is a famous painting by Edvard Munch. He painted his first version in 1893, ten years after the massive eruption of Indonesia's Krakatoa. Krakatoa's explosion was so huge that it colored the sunsets in Norway, half a world away where Edvard lived and painted. Some people believe Krakatoa's fiery sunsets made an impression on Edvard and he used those memories when he painted The Scream.
Volcanoes in Space!
Earth is not the only planet with volcanoes. Our solar system has many planets and moons with both active and extinct volcanoes. The most active volcanic object in our solar system is on Jupiter's moon Io. The largest volcanic eruption ever recorded occurred on Io in 2001.