Volcano | How is a volcano formed?


Imagine that the Earth’s crust is a very thin skin on top of a large ball of molten rock. Like a puzzle, the Earth’s crust is made up of multiple parts called tectonic plates. These can be different sizes, but are mainly the size of continents and move very slowly (only 1 – 9 cm per year).

Where the plates crash into each other are the areas of the Earth where volcanoes, earthquakes and geysers are found.

If two plates move apart, a crack is formed through which magma (the hot molten rock from the centre of the Earth) can escape. This type of volcano largely occurs at the bottom of the sea, so is almost never seen. Only if the amount of magma is large enough will it reach the surface of the sea and form an island. A long time ago, Iceland was formed in this way.

In the areas where the tectonic plates move towards each other, we find large chains of mountains that have been formed by the plates pushing each other up.

When two plates crash into each other, one plate is pressed under the other. The frictional resistance between them melts the first plate. At the same time, magma rises.

Only a small number of the Earth’s volcanoes are formed in this way, but it is these volcanoes that are the most violent and have the most dangerous eruptions.

Over time, volcanoes are also formed in the middle of plates in hot spots. Hot spots are warm areas of the Earth’s crust where a warm upstream of magma breaks through. Hawaii was formed in this way.


The areas where the tectonic plates collide or move alongside each other are also the world’s most active areas for earthquakes. These occur when the plates are positioned closely together, generating friction, and then occasionally move with small or big jerks.

The sea to the west of Indonesia is an example of an active earthquake area. The latest major activity took place on 26 December 2004 when an earthquake measuring 9 on the Richter scale caused a tsunami that rampaged through the Indian Ocean coastlines.

Another area with frequent earthquakes is the San Andreas Fault in the American state of California. In San Francisco, houses are built to withstand earthquakes.


In the fissures between the tectonic plates, it is also possible to find geysers. A geyser is a special form of warm spring that periodically bursts out and sends a spray of hot water and steam into the air.

To create a geyser, there must be lots of ground water and the temperature of this must be high. And by ‘high’, we mean over 100°C, which can occur if the glowing lava is close to the Earth’s surface.

Iceland is the only place in Europe where you can find geysers.

Erupting volcanoes

The duration of a volcanic eruption is very variable. It could last a couple of days or several years. The volcano Kilauea on the Hawaiian Islands has been erupting since 1983 and is still doing so.

A volcano is built of stone. Not pebbles, gravel or rocks, but the material stone. A volcano builds itself up over time; with many eruptions, lava bursts out of the volcano’s crater or flows out and remains lying around the crater.

Lava is liquid rock from the inside the Earth. Once the lava has cooled and set to stone, the volcano will have increased slightly in height. Sometimes, ash is also sent out of the crater. This is powdered stone or gravel that settles round the crater.

The change between lava and ash can give a layered effect. If the lava is very hot, then it is very runny and forms a flat volcano. If it is cooler then it does not flow as far and the volcano has steeper sides.

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