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

火山作用是我们生活在一个充满活力的星球上最戏剧性的表现。火山是地球系统的关键部分，也是许多化学物质从地球内部循环到地表的主要方式。火山喷发可能非常猛烈，是造成灾难的原因之一。了解是什么控制着火山喷发的规模、火山活动的速度，以及火山喷发的规模和频率如何随时间变化，这是非常重要的。关于全球火山活动的最全面的信息来源是大洋钻探方案从深海收集的岩芯。每次大喷发时，火山灰都会被风吹到海底，形成一个独特的层，从火山延伸到数百到数千公里。自1968年海洋钻探计划开始以来，可以在世界各地收集的数百个岩芯中研究这些地层。该项目将首次创建一个火山灰层数据库。数据库中将包括火山灰层的厚度、喷发年龄和火山位置等信息。然而，使用这些信息并不简单。直觉上，距离火山1000公里的核心有一层厚厚的火山灰层，很可能是由一次非常大的喷发造成的，但有多大呢？此外，核心中的火山灰层序列必须提供一些关于这些非常大的喷发频率的信息，但这能被量化吗？该项目的目的不仅仅是从火山喷发中说出火山喷发很大，或者每百万年左右从一个火山区爆发几次非常大的火山喷发。为了提取更准确的信息，需要开发和应用描述火山喷发、火山灰在大气中的运输以及细火山灰沉积到海底的过程的模型。火山喷发的大小是由喷发的火山灰质量来衡量的。火山灰的质量越大，海底某处的沉积物就可能越厚。然而，重要的不仅仅是规模。火山灰在风吹的方向会更厚，如果风向不正确，在一些地区甚至可能根本不会沉积。喷发的强度是指单位时间内喷发的质量/更强烈的喷发将火山灰推高到大气中，并导致火山灰扩散到更大的区域。厚度还取决于火山灰颗粒的大小范围。产生更细火山灰的喷发往往会使火山灰扩散得更远。这些和其他对灰烬厚度的控制是不确定的，也可以独立变化。因此，这些控制的许多不同组合可能导致相同的灰分厚度。有了计算机，现在就有可能找出可能导致相同灰厚度的所有工艺和参数的变化。因此，可以推断喷发的强度，但这将是一个范围，而不是一个确切的数字。同样的方法也可以用来限制火山活动的速度，结果也将是一系列的值。对于大洋钻探岩心，这有点复杂，因为构造板块的运动通常会将地点移向火山，但这种影响可以在具有这些喷发大小和频率范围的模型中考虑在内，我们可以使用统计技术来推断关于地球上数百万年来火山活动的一些基本问题。喷发的频率随规模的不同而不同？总体而言，我们预计，就像地震一样，较大的喷发比较小的喷发更不常见，但这是什么关系？小喷发的频率可以从历史记录中推断出来，但岩芯中的火山灰层是推断较罕见的大事件的更好来源。火山活动的速率是否随着时间的推移而变化，活火山的位置是否发生变化？我们如何解释这些变化？

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?