Reconstructing eruptive processes from volatile distribution in volcanic glass

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There are two broad categories of volcanic eruption: explosive and effusive. Explosive eruptions produce tall clouds of ash and pumice, which may fall over a wide area, and hot, fast-moving pyroclastic flows, which pose an acute hazard to local communities. Effusive eruptions produce slow-moving flows and domes of lava and are usually much less hazardous. Many volcanoes, such as Mt St Helens (USA), Merapi (Indonesia) and Soufrière Hills (Montserrat, a UK territory in the West Indies) may erupt either effusively or explosively, and may switch between these styles during a single eruption. This is particularly hazardous because these switches may happen with no warning, making effective hazard management very difficult. Consequently, understanding the causes of transitions in eruption style is one of the most important goals in volcanology.This study will develop a new tool that will help volcanologists to understand why volcanoes switch between eruption styles. The tool can be used at so-called "silicic" volcanoes that have a particular type of magma that is very sticky (viscous). Because the magma is so viscous, these volcanoes are capable of producing the most violent types of eruption. One such volcano is Novarupta in Alaska - its eruption in 1912 was the most powerful of the 20th century and the fourth most powerful in the last 1000 years. We will focus our study on this eruption because it switched eruption style several times, and because it has exceptionally well-preserved deposits of the magma that it erupted.Volcanic eruptions are driven by buoyancy forces that arise when water, which is initially dissolved in the magma while it is stored underground, comes out of solution and forms bubbles of steam -much like the formation of bubbles in champagne, which cause it to spray out when the cork is popped. Our new tool works by measuring how much water is left in the magma when it cools, and how it is distributed around the bubbles that are 'frozen' in when the magma cools. A PhD study conducted by members of this research team has shown that it is possible to reconstruct the changes in pressure and temperature that a sample of magma experienced during eruption by measuring the way that the water is distributed around the bubbles. We do this using a spectroscopic technique that can make maps of the water distribution that are accurate to a few thousandths of a millimetre. By understanding the differences in pressure and temperature history of magma samples that were erupted explosively and effusively, we can determine what physical differences caused the change in eruption style.We will use artificial pumice samples, which are produced in the laboratory and therefore have very well constrained 'eruption' conditions, to test and perfect our tool. We will then apply it to samples from the eruption of Novarupta in 1912. There are a few specific questions that we want to answer, the most important of which are: Are the switches in eruption style caused by changes to the way the gas comes out of the magma? If so, what causes the change, and where in the volcano's plumbing system does this happen?Finding the answers to these questions won't just help us to understand the Novarupta eruption, but to understand why eruptions that switch between effusive and explosive are so common at silicic volcanoes. This will have impact well beyond our study. It will help volcanologists to work out whether a new eruption is likely to switch in style; it may even allow us to work out what signs to look for that a change in style is imminent. Ultimately, this will help to protect at-risk communities from one of the most serious natural hazards.