What causes a volcano to erupt?

• There are an estimated 1,510 active volcanoes in the world
• One in ten people live within the 'danger range' of volcanoes
• An eruption is caused by magma escaping from below the earth's crust
• Volcanoes found on world's plate boundaries called the 'Ring of Fire'
• The word volcano comes from Vulcan, the Roman god of fire

Volcano eruptions happen when magma erupts from beneath the earth's crust. When the volcano erupts the magma becomes lava and it shoots into the air or runs down the side of the volcano.

How and why volcanoes erupt can be fully understood by firstly looking at the earth's crust. Then we can consider how magma is formed and then why it erupts as a volcano. Keep reading.

The earth is made of three layers: the core, the mantle and the crust. The core comprises the inner core and outer. The mantle has lower and upper mantle areas and the crust has one main layer.

The earth's core is solid iron. Pressure in this region is very high. The mantle is made from solid rocks and minerals. But due to the heat the rocks have become softer and ductile (meaning slightly bendy). The earth' s mantle is not molten. The mantle is in the form of a rheid. This is a form of solid that moves or deforms under pressure. Small movement therefore occurs in the earth's mantle.

On top of the earth's mantle is the crust. The crust comprises of tectonic plates which rest on the upper mantle. Tectonic plates are moving very slowly in time with the movement within the upper mantle.

 Volcanoes are formed where there are gaps in the earth's crust. These gaps are called Convergent boundaries. Convergent boundaries, also known as plate boundaries, are the area of the earth's crust where tectonic plates meet. These regions are known as either subduction zones where the boundaries collide or divergent boundaries where plates move apart.

One tectonic plate moves under the other plate. The plate is pushed down into the upper mantle which causes a change in the temperature and pressure above the plate in the mantle. Magma may then form as a result of lower pressure and increased temperatures.

 Magma forms within the earth's upper mantle. This happens when pressure decreases within parts of the upper mantle usually within several kilometres of the crust at subduction zones where a tectonic plate have moved under another plate. Lower pressure above the submerged plate and below the plate on top allows the rocks to begin melting. The rocks only partially melt and magma is a mixture of between 25 per cent and 50 per cent molten rock and other solid rocks and gases that are also found in the upper mantle.

Magma is less dense than rocks. This means that magma is also lighter and so moves upwards. This happens in the same way that air is less denser than water. For example, when a ball is placed underwater in a swimming pool it rises up to the surface because air is lighter and less dense than water.

Before volcanoes erupt the magma moves within the upper mantle. Magma may cool and forms igneous rocks and crystals below the surface. It may also move into magma chambers which are large pools of magma below the earth's crust. When a volcano erupts the magma continues moving towards the surface of the earth’s crust until it escapes.

 Magma rises up towards the earth’s crust. When the pressure within the magma chamber is greater than the strength of the crust it begins to break through.

Magma can rise towards the earth’s surface for a combination of reasons. Inside the magma chamber there are a number of gases that are mixed in with the magma. Just like in a lemonade bottle, the bubbles of gas rise towards the surface of the magma chamber. The pressure from the gas that is dissolved in the magma pushes against the earth’s crust until it can escape.

Contributing to an eruption can also be more magma filling the chamber. More magma is formed from melting rocks and it is then added to the chamber. This causes the pressure inside the chamber to increase until there is no more room. At this point the magma is forced out through the surface.

Inside the magma chamber there is a volatile mix of reactions taking place. Magma is not identical and the magma from one eruption will be slightly different to another. Likewise, the composition of magma in one chamber will be different to that of magma in other chambers. This determines the type of volcanic eruption that will take place.

Thicker, sticky magma results in stronger eruptions whereas thinner magma causes less extreme explosions. The thickness of magma is determined by the temperature of the magma and how much water, silca and gas it contains.

Silca is a crystalline rock material and causes magma to be thicker. Hotter magma is also thicker. Thicker magma makes it harder for the bubbles of gas to move through the magma and be released from the chamber and so there is more pressure the eruption is stronger. A higher amount of gases within the magma also causes a more forceful eruption as there is more pressure being pushed against the earth’s crust.

More runny magma makes it easier for the pressure from the gas bubbles to be released and so the eruption is calmer.

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