Andesite is a type of rock that is thought to form on the edge of tectonic plates in a subduction zone. The subduction zone is where one plate is being forced under another and the subducting plate then melts in the hot mantle. Professor Richard Price studies andesite to try and understand how magma forces its way through the Earth’s crust to form a volcanic eruption.

The traditional model of a volcano shows a pool of magma being stored beneath the volcano. When the conditions are just right, the lava forces its way through a tube in the volcano and forces its way out to form a volcanic eruption. The centre of the North Island has many andesite volcanoes (Mt Ruapehu, Mt Taranaki) that have exploded with a great deal of force and fire in the past.

Professor Richard Price and his team at the University of Waikato have discovered that the explanation of a pool of magma waiting to erupt may not always be accurate.

The new model

Richard and his team have suggested that, instead of a pool of stored magma, the magma may permeate slowly through the underlying crust and be stored in many small pockets. Imagine the crust as a sponge filled with holes, each hole slowly filling with lava bubbling up from the mantle underneath. These pockets of magma remain separate until conditions cause an eruption. At this point, the sponge is compressed, and the magma is forced out of the pockets and out through the top of the volcano.

How these pockets of magma form varies depending on temperaturepressure and the chemical composition of the mantle. Different minerals in the mantle melt at different temperatures, so the rising material may melt some areas of the crust more easily than others. As it moves, it cools slightly and the result is a ‘crystal mush’ – like ice cream that has been left to melt and then refreeze. Each mush in each pocket is different, as it is based on the unique conditions under which it formed. This is magma, in storage pockets, waiting for something to trigger an eruption.

The eruption is no less dramatic, but in this model, you don’t need to wait for the big chamber to refill before another eruption could occur.

How did the scientists figure it out?

The textbook model of volcanoes and magma chambers is based on volcanoes from the United States and Europe where layers of lava can be observed lying one on top of the other, and each layer is distinct. When scientists starting looking at lava flows in New Zealand, they found that the lava didn’t layer nicely. Instead, there were pockets of rock with different chemical compositions, even from the same eruption.

Richard specialises in the crystals that are found in these different pockets of rock. The size and shape of the crystals that form tell the scientists about the conditions that existed when the magma was forming. Large crystals suggest that the magma has been sitting in one place for a while. Small crystals suggest the magma has been moving about and remelting.

Richard uses specialist equipment to record the changes in crystal size in different lava deposits and find out the conditions in which the magma was formed. When he examined the lava from Mt Ruapehu, he found all sorts of crystals, in all shapes and sizes. This meant that they had been formed under different conditions, not in one big melting pot. This evidence did not fit the model of one magma pool.

It has taken many years of work and detailed chemical analysis of many, many rocks, but Richard and his team can now say that they have evidence for a new model for volcanoes. The next step is for other scientists to look at their work and see if they can corroborate the findings. This is an important part of the process of science that has to happen before the new ideas will be accepted in the science community.

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