Development and analysis of a database of volcanic ash layers from ocean drilling cores as a record of global explosive volcanism

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Volcanism is the most dramatic manifestation that we live on a dynamic planet. Volcanoes are a key part of the Earth System and the major way in which many chemicals are recycled from the Earth's interior to the surface. Eruptions can be very violent and a cause of disasters. It is of fundamental interest to understand what controls the size of volcanic eruptions, the rates of volcanic activity and how size and frequency vary with time. The most comprehensive source of information on global volcanism is from the cores collected from the deep sea by the ocean drilling programme. Everytime there is a large eruption volcanic ash is blown by the wind and deposited on the sea floor to form a distinctive layer which can extend hundreds to thousands of kilometres from the volcano. These layers can be studied in the hundreds of cores collected around the World since the Ocean Drilling Programme began in 1968. This project will for the first time create a database of volcanic ash layers. Information that will be included in the database will be the thicckness of the ash layer, age of the eruption and location of the volcano. It is however not simple to use this information. Intuitively a thick ash layer in a core over 1000 kilometres from the volcano is likely caused by a very big eruption, but how big? Also a sequence of ash layers in a core must give some information about the frequency of these very big eruptions, but can this be quantified? The project aims to do more than just saying from the eruption that the eruption was very big or that the several very big eruptions every million years or so from an area of volcanism. To extract more exact information requires development and application of models that describe the processes of eruption, transport of volcanic ash in the atmosphere and settling of fine ash to deposit on the ocean floor. The size of a volcanic eruption is measured by the mass of ash erupted. The greater the mass of ash the thicker the deposit is likely to be at a given place on the sea floor. However it is not just the size that matters. The ash will be thicker in the direction that the wind blows and may not even deposit at all in some areas if the wind is not in the right direction. Intensity of an eruption is the mass erupted per unit time/ more intense eruptions push ash higher in the atmosphere and results in the ash being spread over a greater area. The thickness also depends on the range of sizes of the ash particles. Eruptions that produce finer ash will tend to have the ash spread much further. These and other controls on ash thickness are uncertain and also can vary independently. Thus many different combinations of these controls may result in the same ash thickness. With a computer it is nowadays possible to find out all variations of processes and parameters that can lead to the same ash thickness. Thus magnitude of the eruption can be deduced, but will be a range rather than an exact number. The same approach can be applied also to constrain rates of volcanism and again the result will be a range of values. With ocean drilling cores this is somewhat complicated by the movement of the tectonic plates which commonly moves sites towards the volcanoes, bu this effect can be taken into account in models With these ranges of values of eruption size and frequency we can use statistical techniques to deduce some fundamental questions about volcanism on earth over the many millions of years. What does the frequency of eruptions vary with size? On the whole we expect like earthquakes that the bigger eruptions are less common than smaller ones, but what is this relationship? The frequencies of small eruptions can be inferred from historical records, but the ash layers in the cores are much better source to deduce the rarer big events. Have the rates of volcanism changed with time and have the locations of actiove volcanoes changed? How can we explain these changes?