The Kelut is a volcano located on the main island of Indonesia, Java. It is a relatively small stratovolcano, its summit elevation being only 1,731 meters, but it packs a deadly punch due to the summit crater lake it contains. It has been the source of some of Indonesia's most deadly eruptions, one of the most memorable being the eruption of 1919, which killed more that 5,000 people. These deaths were mostly due to the pyroclastic flows and lahars resulting from the eruption.

Large eruptions

The eruption of 1586 also caused a huge number of deaths, some reports say more than 10,000. This eruption ejected more than 1 cubic kilometer of tephra, which gives an indication of its size. For comparison: the Pinatubo (Philippines) eruption of 1991 had an tephra ejection volume of about 10 cubic km, the Mt. St. Helens (USA) eruption of 1980 had a volume of about 1.5 cubic km, and the Krakatau1 (Indonesia) ejected more than 18 cubic km of tephra in its 1883 eruption.

As an aside, the Krakatau is generally the most well known big eruption in recent history (geologically speaking), but the 1815 eruption of the Tambora, which forms the Sanggar peninsula of Sumbawa Island in Indonesia, is the largest in both number of deaths and ejected tephra. The eruption blew 150 cubic km of tephra volume into the sky, and killed an estimated 92,000 people, 10,000 of which were direct fatalities caused by tephra fallout and pyroclastic flows, and the remaining 82,000 due to starvation and disease following the event. In contrast, the estimated Krakatau tally was no more that 36,500 dead, mostly due to the resulting tsunamis.

The most recent rumblings of the Kelut were in 1990, when it erupted explosively (with an Volcanic Explosivity Index or VEI of 4). An estimated 32 people died. This low number of deaths was mainly due to tens of thousands of people being evacuated beforehand. Pyroclastic flows reached up to 8 km away from the volcano and fist-sized tephra fell up to 55 km southwest of the volcano. Subsequent lahars buried agricultural land under a layer of mud and debris.

Curbing the crater lake

After the eruption of the Kelut in 1919 it was recognized that something had to be done to minimize the risks posed by the crater lake. Ejection of water during eruptions caused lahars that wreaked death and destruction around the mountain. In 1926 construction of drainage tunnels was started to lower the level of the lake. This first project resulted in a lowering of the lake level by more than 50 meters.

In 1951 another eruption deepened the crater by 70 meters, rendering the tunnels ineffective in reducing the water volume of the crater lake. After repair of the tunnels an estimated 50 million cubic meters of water remained in the lake, which resulted in an increased death toll of 200 when the volcano erupted again in 1966. A new deeper tunnel system was constructed so that the volume of water in the lake prior to the 1990 eruption was only about 1 million cubic meters.

On January 19, 2001, the Volcanological Survey of Indonesia raised Kelut's Alert Level from 1 to 2 (on a scale of 1 to 4). During the first months of 2001 the temperature of the lake increased from 38.5° Celsius to about 51° in the first week of February, after which it dropped back to around 48°. Simultaneously the acidity of the water increased, the pH dropping from 6.3 to 5.3. At the moment (July 26, 2001) the Alert Level remains at 2.

No more crater lake?

It seems that during 2008 a lava plug has developed at the bottom of the crater lake, boiling off most of the water. At the moment it has completely filled out the old crater lake and has actually covered up the intake of the water tunnel.

Swimming in the crater lake

I once had the rather strange (and at the time scary) experience of swimming in the crater lake of the Kelut. My family and I were visiting friends that lived near Malang, which is not far from the Kelut, so we decided to drive out to it, hike up its slope and down into the caldera and have a swim in the lake. It should have been rather fun, but I had just seen a teen horror movie which featured a rather murky lake, missing persons and a lake monster (that turned out to be some digging rig - it was a long time ago), so I was rather apprehensive of swimming in this rather opaque, blue-green water. The thick crusts of moss or algae that were loosened and floated up by touching the slippery bottom didn't help much, either. However, after some initial wariness I had quite a nice time of it. The water was nice and warm, not too hot and I remember it tasted strange.

One other thing I remember from that hike to the lake is that my father was telling me how his father had somehow been involved in building or maintaining the original tunnels that were built after the 1919 explosion. I'll ask him again sometime what it was exactly so I can update this in the future.

Sources: - a picture of the interior of the caldera - and another picture - information on VEI - info on large eruptions - more on large eruptions, including estimated death tolls

1 Some would insist on calling it Krakatoa

July 26, 2001

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