Dynamic Pulsating Eruptions

Soufriere Hill exploding -part III

Photo - Barry Voight

What was needed to help understand the dynamics was a mathematical model of the magma’s rise to the Earth’s surface, the formation of the gas bubbles, the growth of crystals and the huge change in physical properties of the magma. Fortunately I started a very fruitful collaboration with Oleg Melnik from the Institute of Theoretical Mechanics at Moscow University who was already interested in volcanic flows and who was able to help me develop sophisticated and elegant mathematical models. Imagine a fluid with some gas dissolved at very high pressure, around 1000 atmospheres or more, coming up a very long tube, perhaps 5 km long. The tube has a diameter of about 30 metres and the fluid is a mixture of crystals and molten silicate rock at a temperature of about 850°C. At the top of the tube the pressure drops and all the gas dissolved in the fluid escapes and the material rapidly turns into a solid with a melting temperature of around 1100°C. During its journey up the tube the viscosity increases a billion times and the fluid turns into a strong stiff solid, creating a large blockage a few hundred metres below ground. Here earthquakes can occur as the material breaks up under the pressure of the escaping gases. Below the solidified and degassed magma pressure builds up and can sometimes lead to explosive failure of the magma. This model predicts many of the volcanic features we see on real volcanoes, including the pressurisation and eruptive cycles.