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.
The factories and the subduction zones that underpin them are essentially coextensive. It probably bears repeating that each of the zones is where one of the world's many heavy oceanic plates slowly collides with one of the many lighter and thicker continental plates and slides, buckling as it does so, underneath. The zones are very long, and very thin. If unravelled they would extend for about 19,000 miles. But they are rarely more than sixty miles wide. The total area of the subducting world's assembly-lines amounts thus to about a million square miles – about the size of Greenland, or the American Confederate South, or Argentina.
And enclosed within the zones, and formed, allowed to grow, and then destroyed or mutated or otherwise dramatically affected by the processes going on inside them, are about 1,400 of the world's 1,500 historically active land volcanoes. Of all visible volcanoes, 94 per cent, in other words, stand within subduction zones. A mere handful of countries – Indonesia, Japan, America, Russia, Chile, the Philippines, New Guinea, New Zealand, Nicaragua chief among them, and in that order, play host to most of them: these nine countries are home to more than nine out of every ten volcanoes that are liable to erupt today or have done so in recent history.
The most readily recognizable subduction zones are those that enfold the Pacific Ocean. As a reasonably familiar example, consider that which runs along the western edge of South America, and which has created the chain we know as the Andes. * This is where the heavy basaltic Nazca Plate collides with the lighter granitic-and-sedimentary-rock South American Plate. (It does so simply because it is at the same time splitting itself away from its neighbour, the Pacific Plate, along what is called the East Pacific Rise – close to where the Isla de Pascua, Easter Island, is hoisted above the ocean surface.)
The subduction factory that results is a classic of its kind, creating dozens of volcanoes running from the Andean peaks of Ruiz and Galeras in the north, in Colombia, via Chacana, Cotopaxi and Sangay in Ecuador, Huaynaputina † in Peru, Lascar in Chile, Llaima and Villarica on the frontier between Argentina and Chile, and, at the southern tip of the continent, Monte Burney and Cerro Hudson, this last volcano erupting massively in 1991. All told there are sixty-seven volcanoes that have been manufactured by the processes of this one subduction zone – and since there are 4,000 miles separating northern Colombia from southern Chile, and since there is a sort of serrated regularity to the Andes, that means there is more or less one volcano piercing the sky every sixty miles.
Much the same number of volcanoes, with similar intervals between them, is to be found at the other subduction factories around the Pacific – in Alaska and the Aleutian Islands, in the Kamchatka Peninsula, in Japan and the Kurile Islands; and an even greater number is to be found in the most volcanic part of the world, the great subduction zone that stretches 3,000 miles from the northern tip of Sumatra to what is called the Bird's Head on the north-western tip (the West Irian side) of the island of New Guinea.
In this immense factory, there are at least eighty-seven volcanoes that make up much of the archipelago that politics has lately chosen to call Indonesia and the Philippines. Indonesia itself has and has had more volcanoes and more volcanic activity than any other political entity on the earth, in all recorded history. It is a country that is defined by its place at the heart of a subduction zone and is essentially made up of volcanoes and precious little else. On the island of Java alone today there are twenty-one volcanoes that remain fully active. Their eruptions are invariably spectacular and terribly dangerous. And because a very great many people live and work near the volcanoes (not least because the volcanic soil, thanks to the recycling mentioned earlier, is nutritious and ideally suited for farming), they are the cause of a dismaying number of deaths.
The earth fashioned three of the five greatest volcanoes of historic time in this one gigantic factory. The largest the world has ever known was made there: Mount Toba, which erupted 74,000 years ago in what is now northern Sumatra. It had a Volcanic Explosivity Index, or VEI, of 8 – the highest on a scale that is now universally used to classify all eruptions (save for those which merely ooze lava, without exploding). Toba's humongous
caption
The world and its pattern of tectonic plates. Where oceanic and continental plates meet, there is volcanic and seismic activity in great – and often terrible – abundance.
explosion – the curious adjective is now officially used to describe giant volcanoes, the equivalent of the cyclonic sea-state that is these days termed phenomenal left behind an immense lake, fifty miles long and fifteen wide, with the sheer caldera cliffs rising 800 feet straight out of the water. The eruption left layers of dust eighteen inches thick on the ocean floor 1,500 miles away, and must have placed a severe crimp on the development of such Ur-humans as were struggling for existence in those times: it must have lowered the ambient temperature by many degrees, and made even more harsh a climate that was already in the midst of changing into yet another Ice Age.
The eruption of Tambora in 1815, in this same subduction zone, is reckoned the second greatest in history, with an Explosivity Index of 7. (This index, which was first created at the Smithsonian Institution in Washington, is based on two features: the amount of material that is ejected in an explosion, and the height to which it is hurled through the atmosphere. These two factors are clearly observable in modern eruptions; they are also deducible from the records of the past. Even though there were no literate eyewitnesses to Toba, and very few to Tambora – which has to be the principal reason neither has lingered in the public consciousness, while Krakatoa clearly has – the total mass that was ejected from each eruption can be calculated with some precision from an examination of the local geological record, and the distribution of the ash falls far away on the seabed can suggest with fair accuracy the heights to which the columns rose into the sky.)
Third in the list is Taupo in New Zealand, which erupted in AD 180, hugely; and fourth is Novarupta in Alaska – better known as Katmai, on the landward end of the Aleutian chain – which did so in 1912. This last was the largest recent eruption on the North American mainland, but, because of its remoteness, it was little noticed except by what – in terms of calderas and domes and frozen lakes – it left behind.
And then, fifth in the list of all volcanoes known, with a VEI of 6.5, with more than six cubic miles of rock and ash and pumice and dust hurled dozens of miles into the lower stratosphere, with sounds heard 3,000 miles away, with tidal waves of enormous force and height, with shock waves that ran four times to the far side of the world and almost three times back, and with more people killed and more livelihoods ruined than by any other eruption in world history, comes Krakatoa.
Seven weeks after the eruption, when the dust had cleared, the Netherlands government ordered Dr Verbeek and his colleagues to investigate exactly what had happened. The team of four took off in the government hopper-barge the Kedirie, on 11 October, and spent the following two weeks examining every possible aspect of the now seemingly dead remnants of the mountain. The orgy of self-destruction was more than amply evident. It was, Verbeek wrote later, ‘the most interesting eruption witnessed by the human race until now’. But very little of the original mountain was left to see.
The southern quarter of the island remained but was sliced open, as if with a vertical carving-knife – so that the original peak of Rakata looked, from the south, almost the
same, but with everything to its north missing. The exposed northern face of Rakata was almost perfectly vertical, and in cross-section, when viewed from the north, quite perfectly triangular; it was pierced with vertical lines and radiating systems of lava-filled dikes and sills and plugs of newly formed rock, all covered with several feet of grey pumice dust, so that it looked from afar exactly as it was – a perfect cross-section, as one might see in a teaching-chart, of a one-time volcano that had been blasted in half and into oblivion.
But at least the summit of Rakata was, more or less, still there. The two northern peaks of Krakatoa, the summits known as Danan and Perboewatan, were, to the expedition's awestruck fascination, no longer anywhere to be seen. Nor was the little skerry of andesite known (because of its shape) as the Polish Hat: it had quite vanished, presumably vaporized in that one paroxysmal instant.
The very opposite had happened to the two small islands, Lang and Verlaten, which had once enfolded Krakatoa like a pair of parentheses. Rather than vanishing into thin air, these two now appeared to be very much larger than they had before. Their beaches, it turned out, had since the eruption become choked and swollen with enormous amounts of stranded pumice. Larger they may have been – but the essential difference about their appearance as maritime parentheses was that now they were enfolding and bracketing nothing – between them was just a huge expanse of empty, lifeless sea, with the immense broken fang of Rakata peak rising alone straight out of the ocean as a reminder of what had once been there.
The sea to the immediate north of the cliff was very deepnearly a thousand feet. Clearly an enormous new caldera had been created – almost the entire volcano had collapsed into an immense void below, and the cliff of Rakata, neatly bisected where it had been shorn off, was all that remained to the south of the collapse. Off to the north-east two entirely new little islands had risen from the waves, and were christened Steers and Calmeyer Islands; because they were composed of little more than stranded rafts of soft pumice they were eroded back to sea-level in very short order; on today's charts there are just warnings of ‘patches of discoloured water’, fifteen feet deep, suggesting where they used to be.
Back in 1885, when Verbeek wrote his official report, there were only the vaguest explanations of just why all of this might have happened. It was easy enough to describe what had happened – the science of descriptive vulcanology was in any case well advanced, and had been for many years. But when the vulcanologists of the day came to explain the reasons for the violent behaviour of their charges – as true for every volcano in the world as it was for Krakatoa – there was very little understanding of the processes of the world to offer them a basis for coming up with a theory.
After all, only a few decades before, many believed that basalt and flows of lava were simply precipitates from the sea. Until 1857 many geologists thought that volcanoes were caused by the bulging upwards of horizontal flows of lava – and not that they had been built vertically by the discharge of their own products. And at the time of Krakatoa's eruption Alfred Wegener – who first came up with the idea of continental drift, which was to lead to the theory of plate tectonics, and who might well have set the bewildered community of vulcanologists off in the right direction – was only three years old.
And so while in all the official reports and learned papers about the event there was plenty of description of the ruin and dismay that Krakatoa had caused, and though there was much speculation about why the volcano exploded with the violence it displayed, there was next to nothing by way of sensible wondering about the larger mechanisms that triggered it.
This was true for Verbeek in his report, for instance. He spent countless pages describing in detail clogged pipes, steam vents and collapses of central parts of the main volcano. What he concluded did display a remarkable prescience: he said that a good deal of the vanished volcano had foundered into the sea, and had not been blasted into the atmosphere. He suggested that the Plinian violence of the explosion was a result of sea-water mixing suddenly with the magma, and flashing over, turning into superheated steam, in a gigantic and uncontrollable explosion that is these days given the somewhat less than attractive name of a phreatomagmatic eruption. But he never tried to step back and wonder why Krakatoa was where it was, and why it did what it did in the first place.
The same was true also for John Judd, president of the Geological Society of London and author in 1881 of a then classic work, Volcanoes. He too wrote eloquently of the way in which hot magma and sea-water mixed, and of how pumice was created by a lowering of the magma's melting point by the addition of water – but, once again, he missed the central point. He never even tried to grapple with the central issue: why Krakatoa?
The last popular book on the subject * was written in 1964. Even then, still lacking any solid theory that might account for the world's inner processes, the author could only really describe what a volcano was (‘a hole in the ground through which hot gas, molten material and fragmentary products rise to the surface’), say where volcanoes were to be found and name the kinds of material that came from them. And when he arrived at the specific case of Krakatoa, which was described over many pages and with a quite magical literary skill, the author turned desperate. The book begins to speak in terms of ‘The Demon’ going in to ‘press the attack’, his ‘searching fingers boring into the defences’, and the pent-up energies of time' and ‘primeval forces’ readying themselves to do battle. One can hardly blame him. Neither he nor anyone else had an inkling of what really caused Krakatoa. And that was hardly his fault: he was simply a very few years too early.
But once plate tectonic theory was in place, all that changed. Nowadays there is a ready explanation for what happened and why. Essentially the same explanation accounts for the eruption of Toba at the north-western end of the subduction zone, and for Tambora's at its eastern end, and for those of all the other volcanoes in between.
Krakatoa erupted because of what happens when two plates collide – specifically, because of what happens when the northbound Australian Oceanic Plate collides, as it has been doing for many millions of years past and as it continues to do today, with that part of the Asian Plate that, for the sake of simplicity, we will call by the name it enjoys today, Sumatra.
The oceanic plate is cold and made of the heavier, darker, less acidic suite of rocks that underlie all oceans, so as it hits it begins to sink below the warmer and lighter rocks of which Sumatra and all other continents are made. As it sinks it takes with it, downwards, the small, wedge-shaped sliver of continental rock that it either scratched or smeared off the Sumatran edge; it also takes along with it some of the sands and clays that had accumulated on the Sumatran coast, the water that was trapped chemically in these, and a fair amount of atmospheric air and sea-water besides. This entire geological cocktail – an amalgam of cold and heavy basalt from the plate; granite-like rocks from the Sumatran crust; sands, clays, limestones and vast quantities of air and water – then plummets. And, as it does so, everything suddenly changes.
Water is the crucial ingredient in this process. Not only does it lubricate the motion of the plates and help the subduction continue, but, even in very tiny amounts, its presence lowers the temperature at which the rocks of the mantle will begin to melt. And since the water also lowers the density of the wedge-shaped mélange that is being swept and smeared downwards too, the molten rock that is being created below it finds that the rocks above it have suddenly become (thanks to the water) less dense, less rigid, less strong. They have become, in other words, a perfect exit route for the partly melted rock below, enabling it to rush upwards, to melt even further because of the decompression mentioned earlier. Then, with the dissolved carbon dioxide and water vapour suddenly turning back into gas and frothing out of solution, the whole mass rushes up and out as a torrent of phenomenal explosivity into the unsuspecting open air: as a gigantic and classical subduction-zone volcano.
That is why Krakatoa exploded. As to why it exploded
so
A cross-section showing the basic elements of an oceanic plate – upwelling new material from the centre, spreading outwards and then slid-ing beneath the lighter continental plate it then encounters. Volcanoes and earthquakes are an inevitable feature of this last process of subduction.
powerfully and so very noisily – this entirely different debate continues apace today.
There are some clues. The geography of the islands indicates, for example, that there was once an ancient super-Krakatoa, and that at some unspecified time in the past it exploded and collapsed into itself, leaving behind a caldera; the parenthesis-islands of Lang and Verlaten were clearly the caldera walls, the cliffs at the edge of the old volcano. The volcano that succeeded this then had three distinct peaks – Rakata, Danan and Perboewatan. Each was an exit passage for a gigantic magma chamber that clearly existed deep below the region.
So there can be little doubt from this simple evidence alone that the 1883 Krakatoa existed above a large chamber of magma, and that with the three exit-pipes above it weakening its roof, it had a propensity, in times of violent stress, to collapse. The question that has occupied the minds of many specialists in recent years is this: was it the fact that sea-water managed to get into this chamber at the moment of collapse that was the primary cause of the deafening explosion and the tidal waves? Or was it simply one contributory factor, with some other process also at work to make things even more dramatic?
A series of experiments performed in pressure vessels in a laboratory in Australia has suggested that other factors were indeed at work – but that they were complex and subtle. They suggest that a pulse of fresh basalt from deeper in the earth may have been unexpectedly injected into the base of the magma chamber; that this new pulse heated the existing magma above it, causing a violent convection current and the sudden frothing of even more gas – and the sudden breach of the chamber roof. This idea of magma mixing has lately taken hold: processes going on even more deeply within the subduction zone may perhaps have contributed to the might of the Krakatoa event.