When a volcano erupts hot gases and liquid rock come up to the surface from inside the earth. The material sometimes slides down mountains as lava or is hurled into the air as ash or small rocks. Volcanoes often destroy the land around them completely. Gases from eruptions can keep sunlight from reaching the earth and darken the sky for years. Volcanoes also build up new islands and mountains and make the soil a good place for crops to grow.
The earth's crust is made up of sections or plates. They always move and at places where they get together they collide or slide on top of each other.
90% of all volcanoes are located around the edges of the Pacific Ocean. The Pacific coasts of Asia, North America and South America form a big circle, which is called the Ring of Fire.
Sometimes volcanoes are found in places where plates move away from each other. In the Atlantic Ocean, for example, a long range of underwater mountains forms the border between two plates. Iceland is a gigantic volcanic island that has come up to the surface from this mountain range.
Other volcanoes form in the centre of plates, where magma comes up from deep inside the earth and breaks through the crust. Such areas, like the Hawaiian Islands, are called hot spots.
Hot liquid rock, called magma, makes up the inner part of the earth. When it rises to the surface it cools down and sometimes the flow of magma stops in the inner part. In other places, magma unites with gas and creates chambers below the surface of the earth. When the gas pressure gets higher, magma becomes lighter and it spews out of openings, cracks or vents at the surface.
When a volcano erupts three types of material come to the surface: lava, small pieces of rock or ash, and gas.
Magma that spreads out of a volcano slowly is called lava. It can reach temperatures of up to 1,200° C and glows red to white when it flows. In most cases, lava mixes with steam and gas. All types of lava contain silicon and oxygen. As lava moves down a volcano’s slope it cools down and becomes harder, creating very rough blocks or rock.
Small pieces of rock erupt from a volcano when magma is trapped inside and cannot get out. Gas pressure in the magma chambers gets high and suddenly material is blown out and thrown high into the sky.
Often, the pieces of rock are so tiny that they form ash or dust. Strong winds can blow volcanic dust hundreds or thousands of kilometers away from the place of eruption.
The two most common types of volcanoes are stratovolcanoes and shield volcanoes.
Stratovolcanoes are formed like a cone. The base is rather flat and they become steeper towards the peak. A crater usually forms at the top of the volcano. Such volcanoes develop when lava and ash come up from the inner part of the earth and build up layers. First, lava spreads over the surface of the volcano, cools down and becomes hard. Then pressure builds up underneath the layers of lava and ash and other materials are blown out. One of the most famous stratovolcanoes is Mount Fuji in Japan.
Shield volcanoes are dome shaped mountains which are flatter than stratovolcanoes and built of lava flows. They may start on the sea floor and become so large that islands can rise far above sea level. The Hawaiian mountains were formed this way. The islands, which rise about 10,000 meters above the ocean floor, are the world’s most massive volcanoes.
Volcanoes also create holes, called craters and calderas. Calderas are larger than craters and are formed when parts of the volcano collapse. Sometimes these holes are filled with water and form a lake.
Some volcanoes are active. They erupt very often, like Italy's Mount Etna, which erupts every few years. Inactive volcanoes have not erupted for a long time, but scientists warn that they may become active some day. Extinct volcanoes, like Mount Kenya in Africa, will probably never erupt any more.
Volcano experts do their best to warn the population when a volcano may erupt. Sometimes smaller earthquakes or clouds of gas from vents tell observers that a volcano could erupt soon. Most of the time, however, there are no signs at all.
Although volcanoes may cause a lot of damage and big eruptions can kill many people, there are also advantages of volcanic activity.
Volcanic lava and ash spreads over the slopes of mountains and creates good farmland. Volcanic rock is used to build roads, make special tools and ornaments.
The heat that is formed underneath a volcano is called geothermal energy. Hot water that lies below the surface is used to heat homes and greenhouses. Countries with many volcanoes like Japan, Iceland or Italy use this kind of energy to produce electric power.