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

火山活动是我们生活在一个充满活力的星球上的最戏剧性的表现。火山是地球系统的重要组成部分，也是许多化学物质从地球内部循环到地表的主要途径。火山喷发可能非常猛烈，是灾难的起因。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，是一项重要的研究。关于全球火山活动的最全面的资料来源是海洋钻探方案从深海收集的岩心。每当火山大爆发时，火山灰就被风吹到海底，形成一层独特的岩层，从火山向外延伸数百至数千公里。自1968年海洋钻探计划开始以来，这些层可以在世界各地收集的数百个岩心中进行研究。这个项目将首次创建一个火山灰层数据库。数据库将包括火山灰层的厚度、喷发的年龄和火山的位置。然而，使用这些信息并不简单。从直觉上看，距离火山1000多公里的地核上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，地核中的火山灰层序列必须提供一些关于这些非常大的火山爆发频率的信息，但这能被量化吗？这个项目的目的不仅仅是从这次喷发中得出这次喷发非常大，或者从某个火山活动区域每隔100万年左右就会有几次非常大的喷发。为了获得更精确的信息，需要开发和应用描述喷发过程、大气中火山灰的运输以及细火山灰沉降到海底的模型。火山喷发的规模是用喷发出的火山灰量来衡量的。火山灰的质量越大，海底某一特定地点的沉积物可能就越厚。然而，重要的不仅仅是规模。在风吹的方向上，火山灰会更厚，如果风向不对，在某些地区甚至可能根本不会沉积。火山喷发的强度是指单位时间内喷发的质量/更强烈的火山喷发将火山灰推到大气中更高的位置，并导致火山灰扩散到更大的区域。厚度还取决于灰颗粒的大小范围。产生更细火山灰的火山喷发往往会使火山灰扩散得更远。这些和其他控制灰厚度的因素是不确定的，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。现在有了计算机，就有可能找出所有可能导致相同灰厚的工艺和参数的变化。因此，喷发的规模可以推断出来，但将是一个范围，而不是一个确切的数字。同样的方法也可以应用于限制火山活动的速率，结果同样会是一个数值范围。对于海洋钻探的岩芯来说，由于构造板块的运动，通常会使一些地点向火山方向移动，这就有点复杂了，但这种影响可以在模型中考虑进去。有了这些喷发规模和频率的数值范围，我们就可以利用统计技术推断出有关地球上数百万年来火山活动的一些基本问题。火山喷发的频率随大小而变化？总的来说，我们认为像地震这样的大喷发比小喷发更不常见，但这是什么关系呢？小型火山喷发的频率可以从历史记录中推断出来，但要推断出更罕见的大型火山喷发，地核中的火山灰层是更好的来源。火山活动的速率是否随时间而改变？活火山的位置是否发生了变化？我们如何解释这些变化呢？

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?