An excursion to Krakatau (Part 1)

Dr G Trevor Watts (UK)

Some researchers credit a major eruption of Krakatau (or Krakatoa, if you prefer) in the middle of the sixth century with the triggering the Dark Ages (Figs. 1 and 2). Whether or not this is true, the volcano was certainly responsible for about 36,000 deaths on the coasts of Java and Sumatra on 27 August 1883. The blasts probably didn’t kill many, if any, people, directly. The horseman of this particular apocalypse was the tsunami that was created by the explosions or landslides (like the explosion of Mt St Helens) or by a section of seabed suddenly rising.

Fig. 1. Representation of the primordial Krakatau volcano.

Fig. 2. Representation of the primordial Krakatau volcano.

Before the 1883 eruption, Krakatau was a small archipelago of three islands. Sertung and Panjang were remnants of an original volcano that destroyed itself in prehistoric times (Fig. 3). In the middle of the caldera that was created in that destruction, a new island began to form, with three volcanic peaks – Rakata, Danan and Perbuatan, the latter being the most active in 1883.

Fig. 3. A new island grew, with three volcanoes: (from left to right) Rakata, Danan and Perbuatan.

Its last eruptive phase began on 20 May of that year, with columns of ash and steam being thrust to a height of more than ten kilometres.

Fig. 4. Only two segments of the original volcano remained – the present-day islands of Sertung (upper) and Panjang (lower)

Local people and Dutch officials visited the island in the following weeks and months, and noted increasing activity, as well as great rafts of floating pumice. The central volcano, Danan, also joined in the activity and, on 26 August, a tremendous explosion heralded the beginning of the end of the island. Ash rose to three times the height of the original eruption, and the explosion was heard more than 150km away in Java. Explosions continued the following morning. Some of these were heard more than 4,000km away.

Around 20 cubic kilometres of pumice and ash were discharged. The ash cloud stretched into the stratosphere and spread around the world on the jet stream, creating wonderful sunsets and reducing world temperatures slightly. The vast eruption had emptied the underlying magma chamber very rapidly and the remains of the three-peaked island collapsed into the resulting chasm. Perbuatan and Danan had gone when the dust and pumice settled.

Only half of Rakata was left, split down the centre as it slid into the newly formed collapse caldera that measured six kilometres across.

Fig. 5. Rakata, flows and Anak Krakatau slope.

Then it began again. By 1927, continuing underwater eruptions had built up enough lava to breach the surface, and the island was reborn as Anak Krakatau.

Fig. 6. Perbuatan and Danan had disappeared completely. Only half of Rakata was left, with its near-vertical cliffs facing into the new caldera.

For 45 years, Krakatau remained thus: three islands, the remains of two separate, huge eruptions thousands of years apart (Fig. 6).

Fig. 7. The most active volcano, Perbuatan, exploded with extreme violence.

Erosion by the waves, however, saw the island disappear and reform periodically until it became a permanent feature in 1952. Since then, it has continued to grow with intermittent Strombolian eruptions of ash, cinder and lava – on its way to recreating the stratovolcano of old. Sometimes, the eruptions are on a greater scale, reaching Plinian proportions and causing occasional pyroclastic flows, with ash clouds reaching into the stratosphere.

Krakatau sits on a 3,000km-long line of subduction. Here, the Australasian tectonic plate is being squeezed below the Eurasian plate, forming the deep sea Sunda Trench and its continuation, the Java Trench. The sinking basaltic plate melts as it descends, forming magma. It also pushes up the rim of the overlying plate, cracking it. As this happens, many earthquakes are caused and lines of weakness are created, through which magma can rise. In turn, a string of volcanoes is created. Over the ages, many of these have coalesced to form the island arc that contains Sumatra, Java, Bali and all of the other islands to the east, as far as the Banda Sea.

In the autumn of 2007, I set off on an extended trip to Indonesia, organised by Volcano Discovery, a firm based in Germany, to visit a selection of volcanoes (Fig. 8). I travelled from west to east – Krakatau to Bali – with a local guide, Doni, who is a vulcanologist with the Indonesian volcano monitoring service. It was supposed to be a group trek, but no one else booked the trip, so it became personalised, but expensive. From Jakarta, we travelled to Anjer (or Anyer), on the west coast of Java.

Fig. 8. Krakatau, in the middle of the Sunda Strait, with the Sunda subduction zone to the south-west. This gives rise to the island arc of Indonesia.

This is a small port with a beachfront hotel (Fig. 9), the beach being an equal mix of rock, sand, shell and coral fragments, with local fishermen and women scouring the rock pools at low tide.

Immediately north of the hotel is a lighthouse: a very traditional edifice – white and slender, 11 storeys high (Fig. 10). It was completed in 1886 and replaced the one destroyed by the tsunami that followed the famous eruption of Krakatau of 1883.

Fig. 10. The ‘new’ 1886 lighthouse at Anjer.

Alongside the present lighthouse is the base of the old one, brick-built and crumbling, despite efforts to preserve the remains. However, it is right on the edge of the beach and every tide washes around it, slowly destroying it (Fig. 11).

Fig. 11. Base of the original lighthouse at Anjer.

Anjer is on the shores of the Sunda Strait and Krakatau is approximately in the middle. In the film, notoriously entitled ‘Krakatau East of Java’, the ship Batavia (the former name of Java) has Anjer as its home port. Quite how it managed to get in and out of this tiny harbour is beyond me. Visits to the volcano usually depart from Karangbolong, about 60km further south, but we negotiated with a local boat owner for a departure from the harbour next to the hotel, costing around 1.4 million rupiah – around £75.

We had a powerful six-metre speedboat with twin Yamaha 115 outboard motors, and three young crewmembers. Departing from the tiny harbour just before eight in the morning, we were seen off by three other locals who seemed to be amused, or perhaps bemused. Apparently, this was the first time the boat owner had taken tourists to the islands from this port (Fig. 12.).

The sea was a pale turquoise and very clear. But, once we were away from the shelter of the shore, it was choppy, with waves around one and a half meters high. There was a strong cross swell that sometimes coincided with the main run of the waves, causing the boat to surf in a headlong dash down the long, steep faces, like a ride at a water park. After the slide down the wave would come the massive crash into the oncoming wavein a great, drenching lurch. At other times, the waves and swell were at odds with each other and the sea was broken and confused. Despite the changing, churning sea, the strong fumes from the open petrol tank and the smoke from the three chain-smoking crew, I felt fine.

I focused alternately on the horizon and the tiny swallows that skimmed and darted a few inches above the sea’s broken surface. Doni, however, took to his bed and stared steadfastly at the ceiling. Off the starboard bow, the massive bulk of an island gradually rose above the sea mist, and another was faintly seen beyond it. This was cheering for the crew and we steered towards the nearer one. However, after a long detour, much cigarette-stoked discussion, checking of the sat nav and calls to the base by mobile phone, it was decided that, unfortunately, it wasn’t Krakatau! It was another volcano, Sebesi, with Sebuku in the distance and we were around 20 miles further north than we should have been.

This was a mixed blessing – the upside of which was heading straight for us. It was a local fishing boat, 25m of pure utility and strangely picturesque, with heaps of trawl nets and barrels (Fig. 13). A quick decision was made – fish would be good for dinner tonight, so the crew contacted the ship over the boat’s radio. At first, we were told they had had no luck and were fishless.

An increase in the price offered, however, seemed to do the trick, and we pulled up alongside them for a relatively calm five minutes. Even so, we very carefully exchanged a few rupiah for a bucket full of silver fish, and the two boats parted, rocking and rolling apart, with many cheery waves between crews. And so we continued in a generally southerly direction, looking for Krakatau. The downside of the detour didn’t come until later.

Eventually, another low island was spotted, faint on the horizon. We peered at it for half an hour as we drew closer. There was more animated discussion and consultation with Doni, and it was decided that this was Sertung, the westernmost of the islands that formed the rim of the Krakatau caldera. As we approached, we could just make out another, steeper peak beyond, faint wisps of fumarolic steam intermittently rising from it. We circled round the outside of the island group and entered the wide, sea-filled caldera, with the towering sheer cliff face of the island of Rakata on our left.

And there was our first close view of Anak Krakatau, the present, active ‘Child of Krakatau’ (Fig. 14).

Fig. 14. Sketch of Anak Krakatau now, showing the trail from the camp on the beach up to the seismological station atop the eroded parasite cone. The trail then splits; one route goes downwards to the three parasite craters that emit vapours most of the time. The main trail drops briefly, and then rises through patches of fumaroles, sulphur and cinders. Then it curls round the higher slopes to the main crater.

By this time, there was a low, black and ominous trail of smoke emanating from the peak. As we circled the volcano, we could see several large areas of fumaroles billowing white steam and fumes from its higher flanks. The volcano seemed to wear them like a shroud at times, white and draped over its shoulders. As we continued around the island, we saw the ten-metre high cliffs where flows of very jagged and broken a’a lava from the 1960s had ended in the sea, and were now being reshaped by the pounding waves.

Fig. 15. Present day Krakatau, with Anak Krakatau growing spasmodically.

Rounding the last of the lava flows, we came to the forested side of Anak Krakatau and a beach, towards which the boat backed delicately through low breakers and then into a welcome calm patch.

After that, it was an easy drop between the motors into barely half a metre of warm water. This was a relief, as I was desperately endeavouring not to slip and drop my camera, rucksack, boots or camping equipment.

Fig. 17. Panorama showing the fi nely layered ash cliff and lava flow at the end of the beach, Krakatau.

Our landing point (Fig. 18) was the official entrance to the Krakatau Natural Reserve (Cagar Alam Krakatau), where a permit, as well as our signatures in the visitors’ book, was needed to enter. Two wardens, heavily armed with submachine guns, greeted us. They are stationed here to protect the fishing area, mainly from dynamiters. It is OK for the locals to use traditional fishing methods though, and there was a group of their boats parked on the beach just north of the landing point.

Soon, a tent was set up for me, a hammock for the guide, and a tarpaulin spread on the ground for the boat’s crew. Lunch consisted of a club sandwich, cold cola and even colder chips with tomato sauce, as we waited for the baking heat of the midday sun to ease off. A small party of researchers from Nottingham and Oxford Universities, who were surveying the development of flora on the islands, were camping nearby. They told of hearing explosions two nights previously and seeing fire bursts from the peak. That was two days after the Sumatran earthquakes of early/mid September 2007.

Fig. 19. High water pumice on Krakatau’s beach.

As the others dozed in the shade, I wandered along the beach, across fine sand and a mountain of litter. Litter!! On Krakatau!! It seems that the currents wash discarded packets, ropes and flip-flops onto some beaches here from all over the region. It wasn’t possible to walk more than a few hundred metres, as a fallen tree completely blocked the narrow beach, and its low cliff was composed of loose, layered ash that was too unstable to climb. So I retraced my steps and followed the shoreline north.

This section is clear of man-made rubbish. It consists of beautiful, soft sand, marked by occasional exotic fish, knobbly fruits and huge nautilus shells. A clear line of white pumice pebbles shows the high-water level, in stark contrast with the dark sand and darker basalt pieces. I strolled past the collection of fishing boats, with their owners settled for the season, and presently resting under their canvas shelters. Not wishing to invade their privacy, I tried not to look at what they were up to.

The beach here is backed by a five-metre cliff of the finest ash I have ever come across (Fig. 20). Almost like talc, but grey and orange, it is unstable and unclimbable. It has been deposited over a thousand or so eruptive episodes, with fine layers alternating with pebbly lapilli and broken pyroclastic fragments. Some layers are horizontal and others flow over earlier undulations in the ground. The beach and cliff come to an abrupt end where a lava flow dating from 1996 drops into the sea.

Fig. 20. Ash layers in the beach cliff face, Krakatau.

This forms a sudden, six-metre wall of tumbled a’a rock that cuts through the dense forest. It is already being colonised by small shrubs and ferns and, incredibly, some trees survived the flow and are still growing up through the lava. I climbed up the near vertical face of the flow, over extremely sharp and loose boulders, to see its jumbled surface. It was too rough and jagged to wander far with just sandals on my feet, so after about 100m, I dropped down into the forest. This is mostly dense deciduous woodland, with a few clearings.

It was not easy to negotiate through the patches of thick undergrowth, so instead of going back to the tent through the trees, I found a spot along the cliff where it was possible to jump and run down the steep slope onto the beach.

Fig. 21. Krakatau beach with fishing boats.

This produced a thick cloud of rising dust, very dirty feet, and a powerful smell of burning wood that must have been fixed in the ash since it fell, perhaps decades ago (Fig. 22).

By the time I got back to the camp, it was time to set out for the crater by a track which leads directly through the trees at the back of the wardens’ shelter. There is a sign ‘Keep your rubbish’ – with the armed guards behind you, I felt that one should probably follow this order.

Rising steadily along the path, the trees eventually clear into open areas of sand, pebbles and ash that are regularly replenished from above, and which are colonised by grasses and shrubs. The track here is steeper and open to the sun, which burns down and makes noonday ascents unbearable. But now, as we climbed a steep, steady gradient, with a couple of winding turns, a breeze that had been light was becoming stronger. As we rounded the slope more to the south, it was positively cooling, and very welcome.

The main seismological monitoring station, which is fenced off and solar powered, is at around 150m above sea level and sits on top of a parasite cone. It gathers data from three sensors on the mountain and sends the information to the mainland monitor, south of Anyer. Later, I saw some read-outs at the mainland centre, including the records from the previous week, when there had been a series of earthquakes off the west coast of Sumatra, 100 miles or so to the north-west, and along the same tectonic line as Krakatau.

Downwards, to the left of Krakatau’s monitoring station, a faint trail peters out in the direction of the flanking fumaroles and a series of three other parasite craters that smoke from time to time. This is the volcano ‘breathing’ during periodic increases in fumarolic activity. These had been getting more frequent, according to Doni, who had climbed to the crater more than a 100 times before.

**Fig.** 22. Minor craters in a fumerole field, Krakatau.

After the seismological station, the trail actually drops gently for 100m, because of the station’s position, perched on an old parasite cone, the crater of which has been eroded almost to invisibility. I don’t like to do downhill when I’m putting effort into an ascent, but it wasn’t long before the trail rose again and the climb proper began.

The walk is steep and hot, but not too tiring. The trouble of bringing heavy boots and walking poles was justified on the climb, and on the descent, too. The slope of the active cone looks dauntingly steep and long. Much of it is very loose ash, lapilli and rocks of many colours – white, yellow, orange, red and black. Down to the left, there is the eroded crater rim of another parasite cone that erupted periodically from 1927 to 1948.

Further below, there are views of a 1997 lava flow that reached the sea, and the darker 2001 flow that partly rode over the earlier one but didn’t quite make it to the shore. Beyond the waterline and across the sea-filled caldera, the view of the long, low island of Panjang disappeared behind us, lost round the flank of Anak Krakatau as we climbed higher and higher, and now it was the sheer 1,000-foot cliffs of Rakata on our left that dominated our view.

Fig. 23. Fumeroles on the flank of Anak Krakatau.

A field of lava rubble and slopes of ash and lapilli surround the faint trail that we followed, with scattered, large lava bombs up to a couple of metres across (Fig. 24). Some of these look as though they are extremely fresh, with superb ‘bread-crust’ surfaces.

Fig. 24. Lava bombs on Krakatau’s slopes.

There are also several areas of hissing holes – some encrusted with white, yellow or orange crystal growths, and most emitting wisps of steam and periodic whiffs of sulphur. Above, sea eagles cruise the updraft, circling, on the lookout for small rodents, birds and lizards.

The breeze became cooler and stronger as we climbed further round the shoulder of the cone. Heaven must have a breeze such as this, we said – so welcome, cooling and drying.

Even at my slow pace, this steep, exposed section took only 40 minutes, and suddenly the black and red slackened off to a gently inclined rock field, much of it completely crusted in white crystalline deposits.

The whole climb took us only an hour and ten minutes, including many photo stops and a brief diversion down the side trail towards the parasite craters and another seismological monitor near them. It was almost disappointing that it was so easy – to climb a volcano as famous as Krakatau in little more than an hour – it didn’t seem hard enough somehow. In the next part of this article, I will describe my journey around the rim of the volcano and into its crater.