When Mount Pinatubo suddenly erupted on 9 June 1991, the power of volcanoes past and present again hit the headlines

当菲律宾的皮纳图博火山于1991年6月9日突然喷发时，火山的力量再次成为头条新闻。

A部分

Volcanoes are the ultimate earth-moving machinery. A violent eruption can blow the top few kilometres off a mountain, scatter fine ash practically all over the globe and hurl rock fragments into the stratosphere to darken the skies a continent away.

But the classic eruption – cone-shaped mountain, big bang, mushroom cloud and surges of molten lava – is only a tiny part of a global story. Vulcanism, the name given to volcanic processes, really has shaped the world. Eruptions have rifted continents, raised mountain chains, constructed islands and shaped the topography of the earth. The entire ocean floor has a basement of volcanic basalt.

Volcanoes have not only made the continents, they are also thought to have made the world’s first stable atmosphere and provided all the water for the oceans, rivers and ice-caps. There are now about 600 active volcanoes. Every year they add two or three cubic kilometres of rock to the continents. Imagine a similar number of volcanoes smoking away for the last 3,500 million years. That is enough rock to explain the continental crust.

What comes out of volcanic craters is mostly gas. More than 90% of this gas is water vapour from the deep earth: enough to explain, over 3,500 million years, the water in the oceans. The rest of the gas is nitrogen, carbon dioxide, sulphur dioxide, methane, ammonia and hydrogen. The quantity of these gases, again multiplied over 3,500 million years, is enough to explain the mass of the world’s atmosphere. We are alive because volcanoes provided the soil, air and water we need.

A部分

火山是终极的地球移动机器。猛烈的喷发可以将山顶的几公里吹掉，将细灰岩散布至全球，并将岩石碎片抛向平流层，使遥远的大陆上的天空变暗。

但典型的喷发——锥形山、大爆炸、蘑菇云和熔岩涌动——只是全球故事的一小部分。火山作用，即火山过程，真正塑造了世界。喷发造成了大陆断裂、山脉抬升、岛屿构建和地球地形的形成。整个海底都有火山玄武岩构成的底层。

火山不仅创造了大陆，还被认为创造了世界上第一个稳定的大气层，并提供了我们所需的土壤、空气和水。现在大约有600座活火山。每年它们为大陆增加两到三立方千米的岩石。想象一下，在过去的35亿年里，同样数量的火山持续喷放。足够解释大陆地壳的岩石组成。

火山口喷发出的物质主要是气体。超过90%的气体是来自地球深处的水蒸汽：足以解释35亿年来海洋中的水。其余的气体是氮气、二氧化碳、二氧化硫、甲烷、氨和氢。这些气体的数量再乘以35亿年，足以解释世界大气层的质量。我们之所以能够生存，是因为火山提供了我们所需的土壤、空气和水。

B部分

Geologists consider the earth as having a molten core, surrounded by a semi-molten mantle and a brittle, outer skin. It helps to think of a soft-boiled egg with a runny yolk, a firm but squishy white and a hard shell. If the shell is even slightly cracked during boiling, the white material bubbles out and sets like a tiny mountain chain over the crack – like an archipelago of volcanic islands such as the Hawaiian Islands. But the earth is so much bigger and the mantle below is so much hotter.

Even, though the mantle rocks are kept solid by overlying pressure, they can still slowly ‘flow’ like thick treacle. The flow, thought to be in the form of convection currents, is powerful enough to fracture the ‘eggshell’ of the crust into plates, and keep them bumping and grinding against each other, or even overlapping, at the rate of a few centimetres a year. These fracture zones, where the collisions occur, are where earthquakes happen. And, very often, volcanoes.

B部分

地质学家将地球视为一个有着熔融核心的半熔融地幔和脆弱的外壳。可以将其类比为煮熟的鸡蛋，软糊状的蛋黄、坚实但易弯曲的蛋白和坚硬的蛋壳。如果在煮沸过程中蛋壳稍微裂开，蛋白质就会流出并在裂缝上形成像夏威夷群岛这样的小山脉，但地球更大，下方的地幔更炽热。

尽管地幔岩石由上覆压力使其保持固态，但它们仍然可以像稠密的糖浆一样缓慢地“流动”。这种流动被认为是对流电流的形式，足以将地壳的“蛋壳”断裂成板块，并保持它们以每年几厘米的速度碰撞、摩擦或甚至重叠。这些碰撞发生的裂缝区域就是地震发生的地方，而且很常见地也是火山发生的地方。

C部分

These zones are lines of weakness, or hot spots. Every eruption is different, but put at its simplest, where there are weaknesses, rocks deep in the mantle, heated to 1,350℃, will start to expand and rise. As they do so, the pressure drops, and they expand and become liquid and rise more swiftly.

Sometimes it is slow: vast bubbles of magma – molten rock from the mantle-inch towards the surface, cooling slowly, to show through as granite extrusions (as on Skye, or the Great Whin Sill, the lava dyke squeezed out like toothpaste that carries part of Hadrian’s Wall in northern England). Sometimes – as in Northern Ireland, Wales and the Karoo in South Africa – the magma rose faster, and then flowed out horizontally on to the surface in vast thick sheets. In the Deccan plateau in western India, there are more than two million cubic kilometres of lava, some of it 2,400 metres thick, formed over 500,000 years of slurping eruption.

Sometimes the magma moves very swiftly indeed. It does not have time to cool as it surges upwards. The gases trapped inside the boiling rock expand suddenly, the lava glows with heat, it begins to froth, and it explodes with tremendous force. Then the slightly cooler lava following it begins to flow over the lip of the crater. It happens on Mars, it happened on the moon, it even happens on some of the moons of Jupiter and Uranus. By studying the evidence, vulcanologists can read the force of the great blasts of the past. Is the pumice light and full of holes? The explosion was tremendous. Are the rocks heavy, with huge crystalline basalt shapes, like the Giant’s Causeway in Northern Ireland? It was a slow, gentle eruption.

The biggest eruptions are deep on the mid-ocean floor, where new lava is forcing the continents apart and widening the Atlantic by perhaps five centimetres a year. Look at maps of volcanoes, earthquakes and island chains like the Philippines and Japan, and you can see the rough outlines of what are called tectonic plates – the plates which make up the earth’s crust and mantle. The most dramatic of these is the Pacific ‘ring of fire’ where there have been the most violent explosions – Mount Pinatubo near Manila, Mount St Helen’s in the Rockies and El Chichón in Mexico about a decade ago, not to mention world-shaking blasts like Krakatoa in the Sunda Straits in 1883.

C部分

这些地震带是弱点或热点。每次喷发都是不同的，但简单来说，在存在热点的地方，地幔深处的岩石被加热至1,350℃时开始膨胀并上升。随着上升，压力降低，岩石膨胀并变成液体，更迅速地上升。

有时它是缓慢的：来自地幔的巨大岩浆泡——在冷却过程中慢慢展现为花岗岩的喷发（如苏格兰天空岛或英格兰北部的大威恩辟士尔岩脉，像挤出的牙膏一样携带着哈德良长城的一部分）。有时候——如北爱尔兰、威尔士和南非卡鲁地区——岩浆上升得更快，然后以巨大而厚实的岩石层水平流出地表。在印度西部的迪干高原，有超过两百万立方千米的岩浆，其中一些厚达2400米，形成于50万年间的喷发。

有时候岩浆运动非常迅速。它没有时间在向上涌动时冷却。被困在沸腾岩浆中的气体突然膨胀，岩浆因热而发光，开始起泡，并以巨大的力量爆炸。然后稍微冷却一些的岩浆开始从火山口流出。这种情况发生在火星上，在月球上也发生过，甚至在木星和天王星的一些卫星上也发生过。通过研究证据，火山学家可以推测过去巨大爆发的威力。毛石轻且充满孔隙吗？爆炸是巨大的。岩石重，具有巨大的结晶玄武岩形状，如北爱尔兰的巨人之路？这是一次缓慢、温和的喷发。

最大的喷发发生在深处的中央海洋脊，新的岩浆正在将大陆分离并使大西洋扩大，每年或许增加五厘米。观察火山、地震以及菲律宾和日本等岛链的地图，可以看到所谓的构造板块的大致轮廓——这些板块构成了地壳和地幔。其中最引人注目的是太平洋“火环”，那里发生了最剧烈的爆发——比如马尼拉附近的皮纳图博火山、洛基山脉的圣海伦斯山和墨西哥的埃尔奇冈火山，还有像1883年苏门答腊海峡的喀拉喀托火山那样震撼世界的爆炸。

D部分

But volcanoes are not very predictable. That is because geological time is not like human time. During quiet periods, volcanoes cap themselves with their own lava by forming a powerful cone from the molten rocks slopping over the rim of the crater; later the lava cools slowly into a huge, hard, stable plug which blocks any further eruption until the pressure below becomes irresistible. In the case of Mount Pinatubo, this took 600 years.

Then, sometimes, with only a small warning, the mountain blows its top. It did this at Mont Pelée in Martinique at 7.49 a.m. on 8 May, 1902. Of a town of 28,000, only two people survived. In 1815, a sudden blast removed the top 1,280 metres of Mount Tambora in Indonesia. The eruption was so fierce that dust thrown into the stratosphere darkened the skies, cancelling the following summer in Europe and North America. Thousands starved as the harvests failed, after snow in June and frosts in August. Volcanoes are potentially world news, especially the quiet ones.

D部分

但火山并不容易预测。这是因为地质时间与人类时间不同步。在平静期间，火山通过形成由熔融岩石从火山口边缘溢出形成的强大锥体，自己封顶，然后熔岩缓慢冷却成一个巨大坚固的塞子，阻止了进一步的喷发，直到下方的压力变得不可抗拒。在皮纳图博火山的情况下，这需要600年。

然后，有时候只有很小的预警，火山就会爆发。1902年5月8日上午7点49分，马提尼克的蒙佩尔火山就是这样爆发的。在一个有2.8万人口的城市中，只有两个人幸存下来。1815年，突如其来的爆发将印度尼西亚的丹保罗火山顶部1280米吹掉。喷发非常猛烈，向平流层喷出的灰尘使欧洲和北美的夏季消失。数千人因收成失败而挨饿，6月下雪，8月有霜冻。火山潜在地威胁着整个世界，尤其是那些看似平静的火山。