Krakatoa

by Simon Winchester

  Pumice is one of the better known by-products of vulcanism. The most widely used products are, of course, volcanically produced rocks, used for building stone. These range in type from the dark basalts and gabbros, such as were used for making the spires of Cologne Cathedral, to the paler andesitic rocks, as are to be found in the temple lanterns all around Kyoto. Volcanic soils, especially a group called andosols, are uncommonly rich in minerals, and, in the attractive phrasing of the Encyclopedia of Volcanoes, ‘have nourished many ancient civilizations’. And scores of the most ordinary, everyday items have incorporated volcanic products – particularly a highly absorbent mineral derived from weather ash called bentonite – in their manufacture: batteries, surfboards, refrigerators and air conditioners generally make use of ash invisibly, while the cheap building material known as breeze block makes no secret of its origins: crushed-up coke, cinders, furnace clinker and, in places where it is readily available, finely ground pumice and volcanic ash.

  Pumice resides also in many an old-fashioned bathroom, sitting beside the loofah and the scrubbing brush. It is pleasantly abrasive and, because of the high proportion of gas bubbles that are caught in it before it solidifies in flight, it is of such low density that it floats quite readily. Makers of distressed denim fabrics also like to use it in their giant washing machines, where it brushes softly against the cloth, whitening and ageing it in a way that finds favour with youngsters.

  But these more benign uses of pumice tend to shroud the awful truth about the immense tonnage of it that was released by Krakatoa. The headmistress of a mission school in Zanzibar, * off the east coast of Africa, wrote to the Royal Society in response to the appeal, to report that

  … about the third week in July 1884, the boys… were much amused by finding on the beach stones which would float, evidently pumice-stone. The lady who was with them… also noticed that there were a quantity of human skulls and bones ‘all along the beach at high water-mark’; these were quite clean and had no flesh remaining on them, and were found at intervals of a few yards, two or three lying close together.

  The closer to the volcano, the thicker the rafts of pumice, of course. On the west coast of the island of Kosrae, in what is now Pacific Micronesia, huge plates of pumice sixteen inches thick were hauled from the beach early in 1884: they were covered with barnacles, and many were accompanied by the roots of huge trees, with extra pumice lumps caught up in their roots, helping them stay afloat. These trees, torn up and floated 3,000 miles to the east, were presumably parts of Krakatoa's old forests – the same forests that had been noted and painted by Captain Cook's homebound expedition in 1780, and those that the Sundanese boat-builders had been cutting when they were forced to flee for their lives from the first eruptions of May.

  The crews of ships moving through fields of pumice – such as those ‘acres in extent' that were encountered by one vessel coming into the Sunda Strait from Australia in January – were struck by the peculiar sound of the bow slicing through the rock. There was no real noise, ‘just a soft sort of crushing sound‘. And all the passing ships did their best to avoid desecrating the terrible cargo the pumice rafts all too often carried. A crewman on the vessel Samoa, which was heading south-westwards, off into the Indian Ocean, wrote of the nightmarish unreality of such encounters:

  For two days after passing Anjer we passed through masses of dead bodies, hundreds and hundreds of them striking the ships on both sides – groups of 50 and 100 all packed together, most of them naked. We passed a great deal of wreckage, but of course we cannot tell if any vessels were lost. We also passed bedding chests and a number of

  Rafts of pumice, many laden with the remains of victims, drifted as far away as Zanzibar.

  white bodies, all dressed like sailors, with sheath knives on them. For ten days, we went through fields of pumice stone.

  What the seamen witnessed on the Samoa, and on the Bothwell Castle, and on the Loudon and the Berbice and the Charles Bal and the Kedirie and a score of other ships beside that scoured the Sunda Strait during those weeks in late August, September and October does not bear too much repeating, so awful is what they have to say. Most reports were much more dreadful than the following account, which was published in a letter to The Times, from a correspondent writing from Batavia in October:

  The British ship Bay of Naples had called at these islands and had reported that on the same day, when 120 miles from Java's First Point, during the volcanic disturbances, she encountered carcasses of animals including even those of tigers, and about 150 human corpses, of which 40 were those of Europeans, besides enormous trunks of trees borne along by the current.

  Yet it is what they did not see that remains, to take the longer view, of greater significance. For what they did not see was this: the half-mile-high pointed peak of Krakatoa. They did not see it smouldering menacingly in the aftermath of what it had done, as most volcanoes are prone to do.

  Usually – whether named Vesuvius, or St Helens, or Pinatubo, or Unzen or Etna – a volcano explodes, and in so doing causes manifold devastation and death. Then it simply stands there, raging and smoking ever less fiercely, presiding with a titanic smugness over the ruin it has so lately made. But, uniquely in much of volcanic history, Krakatoa did no such thing.

  For Krakatoa had gone. Six cubic miles of rock of her, most of the island's great bulk, had just vanished, either blown into the sky or collapsed into the sea, and with the most thunderous roar and the greatest loss of life ever recorded in certain history.

  For so long before that Krakatoa had been an island of no consequence. It had been little more than a genial, easily recognized sailors' companion spotted off the bow of an approaching ship, a sea-mark that would always help guide any navigator who was feeling his way up or down this most vital waterway between Java and Sumatra. It was that old ‘island with a pointed mountain’, and nothing more.

  Now in the middle of the summer of 1883 it had suddenly and without much warning gone totally berserk, sent the sea berserk as well and then had, essentially, vanished. Not vanished in name, perhaps; not vanished from memory either; and recently, and in much the same form, it has been reborn (as we shall see). But in August 1883 this inconsequential little island went mad and disappeared. The reasons just why this happened – as it did, and when it did – have occupied the minds of a great community of geologists around the world for all the long years since.

  4. The Explanations

  Why did Krakatoa happen? Why, indeed and more generally, do volcanoes do what they do? Why does the terra firma upon which we so confidently and innocently secure all our lives, sometimes and so capriciously tear itself open and cause such fearful destruction as it does so?

  To those caught up in such a moment of appalling terror, such as the thousands whose lives were wrecked in 1883, it all must seem a most monstrous injustice, a terrible cheek perpetrated by the earth and its presiding deities. Krakatoa is a stark reminder of the truth of Will Durant's famous aphorism ‘Civilization exists by geologic consent, subject to change without notice.’ Yet geology, which is an unemotional and rational science, allows us to step back from our shock and dismay at such events, to accept a longer view – and to be awed by something rather different: that despite her seemingly cruel caprices, this planet in fact enjoys by and large an extraordinarily fortunate situation.

  The simple, very obvious features of the earth – its location in space, its size, the processes that led to its creation, processes that include the very volcanic events that took all the lives west of Java – happen to have been suited perfectly, when taking the long view, to the sustenance and maintenance of organic life.

  To victims of a volcanic eruption like this, of course, the very reverse must seem true. But consider location, for instance. Planet earth is sited just close enough to the star around which it orbits to derive only benefits from the latter's infernal solar heat. It is neither so close as to risk the boiling of its oceans and the loss of its water into outer spa
ce by photo-dissociation in the upper atmosphere, nor so far away that all its present liquid water remains uselessly and inconsumably frozen.

  The size of the earth is spot on too. Thanks to its moderate size its gravitational pull is just right. It is strong enough to overcome in particular the escape velocities of the molecules both of water and carbon dioxide, which means that we have a sheltering canopy – a benevolently situated greenhouse, even though this is a word with more negative associations today – that first allowed life's building blocks to be assembled, and then ensured that the fragile living entities so made could be cosseted against the perilous radiations from outer space.

  And then there are the volcanoes – just the right number, of just the right size, for our own good. The deep heat reservoir inside the earth is not so hot, for instance, as to cause ceaseless and unbearable volcanic activity on the surface. The amount of heat and thermal decay within the earth happens to be just perfect for allowing convection currents to form and to turn over and over in the earth's mantle, and for the solid continents that lie above them to slide about according to the complicated and beautiful mechanisms of plate tectonics.

  Plate movement and convection and the volcanic activity that is their constant handmaid may not seem, to the victims of eruptions and tidal waves, to be in any way benign, or to be good for the planet as a whole. And yet, taking the long view once again, they most certainly are: the water, carbon dioxide, carbon and sulphur that are so central to the making and maintenance of organic life are all being constantly recycled by the world's volcanoes – which were also the probable origins of the earth's atmosphere in the very first place. It is not merely that volcanoes bring fertile volcanic soils or useful minerals to the surface; what is more crucial is their role in the process of bringing from the secret storehouses of the inner earth the elements that allow the outer earth, the biosphere and the lithosphere, to be so vibrantly alive.

  Almost all our neighbour planets are, so far as is known, volcanically lifeless. They are also, on all the available evidence, more or less biologically lifeless – and that is quite probably at least in part because they are so volcanically dead. Only Io, one of Jupiter's many moons, seems to sport a significant number of volcanoes: spectacular sulphur-rich fountains of magma have been seen spouting on its surface. But there is no suggestion of plates or of any movement of the solid crust, either on Io or on any planet or moon known to exist between Mars and Pluto. The vigorous business of plate movement apparently does not occur on planets that are hotter than our own; nor does it on those that are much more frozen and more deeply dead.

  But it is the movement of the plates, and the internal storms that rage below and cause them to slip beneath or alongside one another or tear themselves apart along their suture-lines, that is the driving force behind our earth's highly unusual degree of vulcanism. Plate movement, as well as shaping the planet's topography, also creates most of the very vulcanism that is central to its life. Plate tectonics, in other words, is the key to it all – and any examination of just why Krakatoa happened as it did, and how it did, must inevitably now refer to this newly minted catalogue of knowledge about the workings of the earth.

  It was of course not always so. In the distant past, whenever the earth behaved with terrible and unanticipated violence, mankind could do little more than wonder, horror-struck, at the sheer effrontery of it. In very early times this wonderment was answered, inevitably, mainly by religion and the making of myths. Volcanoes were hills occupied by temperamental gods: they could be appeased by frequent sacrifice. The appeasing flesh could be that of a young human (a small child thrown every twenty-five years into the crater of a particular Nicaraguan volcano, for instance, would guarantee its quietude) or an animal (Javanese today toss chickens into the crater of Mount Bromo – superstition plays an important role in East Indian attitudes towards their volcanoes still).

  The ancient Greeks and Romans then hammered some kind of order into their beliefs, as might be expected: the idea of the existence of Hades, the nature of such gods as Pluto and Vulcan, the character of Titanic monsters like the fearsome, wild-eyed and flaming-tongued Typhon were all connected with the wayward behaviour of an earth that all then knew had a terrible and dangerously hot interior. It was no coincidence that the gateway to Hades – believed by the Ancients to be in the earth's centre – was the Romans' most notorious local volcano, Mount Etna, with its gas-belching vents known as solfataras, and the phrase ‘sailing to Sicily' was for a while a euphemism for entering the fiery furnaces of the Devil's domain.

  The seers of the classical world were on rather shakier ground when it came to deciding just why, other than for divine reasons, there was just so much heat inside the earth. The Greeks – the philosophers Anaxagoras and Aristotle in particular – favoured the human analogy of trapped wind, with the friction of the escaping wind causing the generation of heat, a sort of volcanic vindaloo. The Romans, on the other hand, and among them most notably Lucius Seneca, favoured the notion that the heat came from the combustion of a vast inner-earth storehouse of sulphur – and in some Roman poetry of the time this idea extended to the burning of deeply buried reservoirs of alum, coal and tar.

  This idea, that volcanoes were the consequence of the steady burning of a finite store of earthly combustibles, exerted a grip on the scientific mind for centuries. Then, as chemistry developed as a science, so its innumerable secrets offered themselves as the favoured sources for all the necessary heat, and were widely accepted as doing so. During the seventeenth and eighteenth centuries a great many seers – Isaac Newton among them – believed that so-called exothermic chemical reactions were the answer. By 1807, when the Geological Society of London, the world's oldest such body, was founded, the oxidation of newly discovered alkaline metals, such as sodium and potassium, was thought to be an answer.

  Even as late as the 1920s there were two now notoriously blinkered scientists who clung to what might seem today quite fatuous chemical theories. One of them, Arthur Louis Day, proposed in 1925 that volcanic heat was due to a series of complex chemical reactions between gases, and he won support from the redoubtable and influential Sir Harold Jeffreys, * while at the same time dismissing vulcanism generally as a phenomenon that was merely ‘local and occasional, not perpetual and worldwide’.

  However, in tandem with all those chemists and physicists who for so long had such an influence on geophysical thinking, there were also other natural philosophers – René Descartes most notable among them – who started out on what would prove to be the right track. In the mid seventeenth century Descartes – better known for his cogito, ergo sum, and for his legacy of Cartesian coordinates – came up with a quite revolutionary idea: that the earth originated by way of gravitational attraction and gaseous condensation, that heat was an essential primordial component of this process, and that its slow decay resulted in the earth having three internal concentric parts: a highly dense and incandescent liquid core, a half-cooled plastic central region, and a cold, solid and comparatively light crust. Moreover, there was ample primordial heat left over from the creation process to power all known volcanoes for a very, very long time.

  The subsequent advent of the science of field geology, the furious debates that went on between Neptunists, who believed all rocks to have precipitated from a primeval ocean, and Plutonists, who saw countless of them as having their origins in melting and magma, belongs to another story, temptingly diverting though the various interlocking sagas may be. In essence, though, the mystery that occupied most minds for most of the late nineteenth and early twentieth centuries was simply why rocks melt – what combination of physics and chemistry, of depth, of heat and of the presence or absence of water in the mix of minerals would lead a rock to become plastic and mobile and molten, and then to emerge on the surface and cool and harden and solidify back into rock once again.

  The chemist and the chemistry that sought to answer questions about the make-up of the earth in earlier ti
mes had been overtaken by the physics and the physicists seeking to do much the same in recent years; and though the physics answered much of the detail involved, many of the fundamental questions remained doggedly, in essence, unresolved.

  Or at least they did until that memorable July day in 1965 when, as I have explained in an earlier chapter, the soft-spoken and self-effacing Canadian geologist J. Tuzo Wilson managed to combine both the chemistry and the physics of the earth into one, inaugurating the science of plate tectonics. In doing so he launched a brand-new and all-encompassing global theory that would offer the answer to almost everything volcanic that had ever been wondered at.

  Uniquely in the solar system this planet sports a crust that is, by virtue of this process, being constantly destroyed and regenerated – an ever mobile chemical factory where materials that exist in solid, liquid and gaseous states are being recycled endlessly. They are burst out from the middle of oceanic plates by a process, newly understood, that allows upwelling materials to melt without heat being added to them – to melt simply because the pressure on them is relieved by their being convected upwards and outwards towards the atmosphere. * There are volcanoes here in these mid-crustal ridges, big but not especially explosive volcanoes, mountains that ooze basalt, like those in Hawaii and Iceland, the Azores and the rift valleys of East Africa. † They are the stuff of research and fascination in their own right. But they are the distaff side of the volcano of this account, the Alpha to Krakatoa's Omega, the mid-plate reciprocal to all that goes on at the plates' edges, the other side of the story.

  For the materials that rise up in the middle, along with whatever they sweep before them on their way, are in due course swept down again at the peripheries of plates. They are swept down by the process that is most crucial of all, which, though an essential part of earthly regeneration, also leads directly to the making of highly explosive, dramatic and deadly arc volcanoes like Krakatoa. Colloquially the phenomenon that exists at these plate edges is known among geophysicists and vulcanologists as the subduction factory – and Krakatoa stands front and centre in one of the largest and most complicated of these extraordinary, world-shaping entities.