What controls the explosivity of volcanic eruptions? A study of magma degassing at Taupo, New Zealand and Mono Craters, California

Volcanic eruptions are one of the most dangerous natural phenomena, which can generate deadly clouds of gas and ash and even affect global climate by injecting millions of tonnes of gases into the atmosphere. It is therefore important for geologists to understand how volcanoes work and one of the most fundamental questions is whether a volcanic eruption is going to be explosive or effusive. Explosive eruptions, such as the great eruption of Taupo, New Zealand ~2000 years ago, are extraordinarily violent events, producing plumes of ash and gas over 25 miles high. These plumes scatter ash over a huge area, destroying buildings and poisoning livestock, and can potentially affect millions of people. If the plumes become unstable and collapse the risks are even greater, as fast-moving pyroclastic flows (hot clouds of gas and ash) are generated, which destroy all in their path. On the other hand, effusive eruptions involve the gentle flow of lava, which seldom reaches more than a couple of kilometres from the vent, and risks are minimal. The key difference between these two types of eruption appears to be the fate of gases that are dissolved within the magma. These gases (mostly water) are present in the magma when it first collects deep beneath the volcano, and form bubbles when magma decompresses as it rises towards the surface. This reduces the magma density, causing the rapid acceleration up the volcanic vent that leads to explosive eruptions. If, however, the gases can escape from the magma (degassing), either into the rocks around the vent or through bubbles to the surface, it is generally thought that the driving force for explosive eruptions has gone, the bubbles collapse and thick magma oozes slowly to the surface. However, the story cannot be so simple and surprisingly little is known about how magma degassing controls the style of eruptions. New preliminary data from Taupo suggests that degassing does not always lead to effusive eruptions, but if not, why not? I will tackle this problem by measuring the concentration of water and other volcanic gases in ash from two contrasting eruptions: the incredibly explosive Taupo event and the 1340 AD Mono Craters eruption, a smaller-volume eruption that began explosively but quickly became effusive. Measuring the dissolved gas content in ash provides a snapshot of how the gas was escaping from the magma at one moment during the eruption. By analysing ash from different layers one can track how degassing patterns changed during the eruption and whether this influenced the eruption style. Previous researchers have measured gas in ash particles from Taupo and Mono, but some of their techniques were inaccurate, and nobody has tried to the particles of bubbly ash (pumice) that makes up the vast majority of the ash layers. Instead, they focussed on particles of bubble-free ash that were ripped from the vent walls and may be recycled from earlier phases of the eruption, and therefore not representative. This was because they were uncertain whether the water dissolved in pumice was the original magmatic water, rather than external water that had hydrated the particle surfaces. By combining different measurement techniques I will be able to separate magmatic from external water, and so provide the first data on degassing of pumice, which will provide important new insights into the mechanisms of explosive eruptions worldwide. The techniques either involve driving off the external water by baking the ash, or analysing only a tiny area of the ash, and avoiding any hydrated areas such as bubble walls. I will also measure, for the first time, the gradient in gas content around bubbles in pumice, which will indicate roughly how quickly the bubbles were forming. This will provide useful new constraints for scientists modelling explosive eruptions. My research will therefore provide an unprecedented level of information about exactly how degassing controls the explosivity of eruptions.