All About Volcanoes

What Is a Volcano?

A volcano is an opening in the surface of the Earth where hot, melted rock and gases can escape from deep underground.

From the outside, many volcanoes look like mountains. But a volcano is special. Deep below it, the rock is so hot that it has melted into a thick, glowing liquid. Sometimes this liquid bursts out of the top. When that happens, we say the volcano erupts.

Scientists who study volcanoes are called volcanologists. They watch volcanoes carefully to learn how they work and to keep people safe.

In this book, you will find out how volcanoes form, why they erupt, what their different parts are called, and where some of the most famous volcanoes in the world are. Let's begin our journey deep into the Earth!

Inside the Earth

To understand volcanoes, we first need to look inside our planet. The Earth is a bit like a giant ball with layers, a little like an onion.

The thin, hard outer layer that we live on is called the crust. It is the rocky ground beneath our feet. Under the crust is a very thick layer called the mantle. The mantle is so hot that some of its rock can slowly flow, like very thick honey. Right in the centre is the core, the hottest part of all.

The Earth's crust is not one solid piece. It is broken into giant pieces called tectonic plates, a bit like a cracked eggshell. These plates fit together and slowly move over the soft, hot rock below. They move so slowly that we cannot feel it, only about as fast as your fingernails grow.

Most volcanoes form at the edges of these plates, where the heat from below can find a way to escape.

Magma and Lava

Deep underground, the heat melts solid rock into a thick liquid. This melted rock is called magma. Magma is incredibly hot, far hotter than boiling water, and it glows red, orange and yellow.

Magma is lighter than the solid rock around it, so it slowly rises upward, like a bubble rising in a fizzy drink. It collects in a large space underground called a magma chamber.

When the magma finally bursts out onto the surface of the Earth, it gets a new name. We call it lava. So magma and lava are the same melted rock — magma is the name we use when it is inside the Earth, and lava is the name we use when it is outside.

When lava flows out, it slowly cools down and turns hard. Over many years, layer upon layer of hardened lava builds up to form the cone shape of a volcano.

Why Do Volcanoes Erupt?

Why does the magma suddenly burst out? The answer is pressure.

Deep underground, magma is squeezed by the heavy rock all around it. The magma also contains gases, a bit like the bubbles trapped inside a fizzy drink. As more magma rises, the pressure builds up and up.

Have you ever shaken a bottle of fizzy drink and then opened the lid? Whoosh! The drink sprays out because all the pressure is suddenly let go. A volcano erupts in a similar way. When the pressure underground becomes too great, the magma and gas force their way up and burst out of the top.

Some eruptions are gentle, with lava that flows out slowly like a glowing river. Other eruptions are explosive, blasting ash, rock and gas high into the sky.

The Parts of a Volcano

Every volcano has the same main parts. Let's name them.

• The magma chamber is the large pool of magma deep underground.
• The vent is the main pipe that carries magma up from the chamber to the surface.
• The crater is the bowl-shaped opening at the top of the volcano.
• The cone is the mountain shape made from layers of cooled lava and ash.

During an eruption, lava, ash, gas and rocks rush up through the vent and out of the crater. A tall cloud of ash and smoke can rise high above the volcano. Sometimes lava also escapes through cracks called side vents on the slopes of the cone.

Different Types of Volcanoes

Not all volcanoes are the same. They come in different shapes, depending on how runny the lava is and how the volcano erupts.

A shield volcano is wide and gently sloped, like a warrior's shield lying on the ground. Its lava is runny, so it flows a long way before it cools. The huge volcanoes that make up the islands of Hawaii are shield volcanoes.

A stratovolcano, also called a composite volcano, is the tall, steep, cone-shaped kind that many people picture. It is built from layers of hardened lava and ash. Mount Fuji in Japan is a famous stratovolcano.

A cinder cone is a smaller, steep volcano made from bits of rock and ash that pile up around the vent.

Scientists also describe volcanoes by how busy they are. An active volcano can erupt. A dormant volcano is sleeping but might wake up one day. An extinct volcano is one that scientists believe will never erupt again.

Famous Volcanoes

Volcanoes are found all over the world. Here are a few famous ones.

Mount Vesuvius in Italy is one of the most famous of all. Long ago, in the year 79, it erupted and buried the Roman town of Pompeii under thick ash. The ash kept the town hidden for hundreds of years, and today people can visit and see how the Romans once lived.

Mauna Loa in Hawaii is one of the largest volcanoes on Earth. It is a gentle shield volcano with wide slopes.

Mount Etna on the island of Sicily, in Italy, is one of the most active volcanoes in the world. It erupts often, and scientists study it closely.

Many volcanoes sit around the edge of the Pacific Ocean in a giant ring. There are so many that this area is nicknamed the Ring of Fire.

Can Volcanoes Help Us?

Volcanoes can be dangerous, but they are not all bad. They give us some useful and surprising gifts.

When ash from a volcano settles on the land and breaks down, it makes the soil very rich. This is wonderful for farmers, because crops grow well in volcanic soil. That is one reason people choose to live near volcanoes, even though they can erupt.

The heat inside the Earth near volcanoes can also be used to make electricity. This is called geothermal energy, and countries like Iceland use it to power and heat their homes.

Volcanoes have even helped to build our world. Many islands, including Hawaii and Iceland, were made by volcanoes erupting under the sea and slowly building up land over millions of years.

Staying Safe

Because some volcanoes can be dangerous, scientists keep a careful watch on them. They use special machines to measure tiny shakes in the ground and to spot when magma is moving below.

If a volcano shows signs that it might erupt, scientists can warn the people who live nearby. This gives families time to move to safety. Thanks to this careful watching, many lives are saved.

What We Learned

We have travelled deep inside the Earth and back again! Let's remember what we found.

A volcano is an opening where melted rock escapes from underground. That melted rock is called magma when it is inside the Earth and lava when it pours out. Volcanoes erupt when pressure from gas and magma builds up and bursts free. They come in different shapes, like wide shield volcanoes and tall stratovolcanoes. Volcanoes can be dangerous, but they also make rich soil, give us energy, and even build new islands.

The Earth is a powerful and amazing place, always changing beneath our feet.

Hungry for more true stories about our world? Dig into the past with Dinosaurs A to Z, or dive below the waves in Explorers of the Deep Sea.