The high temperatures (900°C) and extremely high pressures that occur in the mantle layer of the Earth are enough to melt rock. The high pressure changes the rock into a viscous semisolid called magma. It’s a bit like silly putty – still able to move but very thick in consistency – not the runny texture of golden syrup that is often imagined.

This semisolid magma continues to move upwards through the crust, experiences less pressure and so becomes more fluid. The result is the lava we see erupting from active volcanoes.

Getting through the crust

The superheated molten rock in the mantle doesn’t normally make it through the many kilometres of crust that forms the ground that we walk on. Only in certain areas where the crust is fractured or broken (called fissures) – like at the edge of a tectonic plate boundary – can the molten mantle start to creep through.

The rock in the mantle is less dense than the crust that contains it so it will rise through any gaps. The molten magma is also hotter than the surrounding crust so it will begin to melt some of the solid rocks that surround it.

Crusts and tectonic plates

The edges of tectonic plates form an ideal location for volcanoes to form. The crust here is already ‘broken’, and as a plate is subducted or forced under another, it melts in the hot mantle region to form more molten rock. This becomes a ready supply of new molten magma, which can result in more volcanoes. (Learn more about this in the article ‘Plate tectonics, volcanoes and earthquakes’.)

The explosion

Once the molten mantle rock forces its way through the crust, it eventually erupts through the volcano as lava. This lava cools and forms rocks that scientists study to try and tell them more about what is happening to cause volcanic eruptions and how the mantle is stored under a volcanic area. (Explore the different types of explosions that occur in the article ‘Types of volcanoes’.)

What triggers eruptions?

Scientists are currently working on finding out what triggers eruptions and what causes the magma to be released.

Current theories suggest that an upsurge of magma is related to the presence of gases and water in the magma deep in the mantle that increase the pressure under hot spots and tectonic plate boundaries. Other scientists think that ‘earth tides’ may be important – these are twice daily deformations of the Earth’s surface caused by the moon, allowing the crust to weaken in places and allowing magma to rise.

One key to solving these questions is finding out how the mantle makes its way through to the surface. New research that is being done right here in New Zealand is looking at this question. For example, Professor Richard Price and his team are interested in how magma chambers are constructed and what cooled lava can tell us about processes that happen deep in the Earth’s crust. (Read about Richard’s work in the article ‘Exploring magma formation’.)

However, scientists are still searching for reliable ways to understand how volcanoes work and what causes them to erupt. A good understanding lies at the heart of predicting future eruptions and perhaps saving people’s lives. There are still important questions to be answered.

Nature of Science

Science doesn’t stand still. Even things that we thought we knew well are constantly being revised and revisited by experts around the world.

    Published 9 April 2010