What is a better starting point for our proposed list of new Decade Volcanoes than a volcano that operates in a way that almost no other volcano does? As the number ten of our list we hereby present a volcano that equally blends being an artesian well and an andesitic tephra producer of note. Without further adue, let us move on to Gunung Kelud.

Within a distance fifty kilometers out from the volcano lives almost two million people and the city of Malang (824 000 people) is inside that distance. As such it more than well has the potential of producing a 1 MDE (MDE = Million Death Expectancy) for a larger event. Gunung Kelud is already one of the deadliest volcanoes in history with a confirmed death toll that has surpassed 10 000 dead, with half of those during a single eruption. The modes of lethality include pyroclastic flows, lahars, volcanic ash and widespread scattering of volcanic debris. Due to the form of its eruptions and way of operation it is likely to also be able to produce pyroclastic base surges and caldera forming events.

Before we move on to the details of the volcano I would like to state that this volcano is relatively well studied and monitored, but the way the volcano works needs to be better understood since it is the archetype of its own class of volcanoes. What we learn at Kelud can be applied to several other artesian crater lake volcanoes, and this type of volcanoes are among the deadliest known to mankind.

Background

Kelud is an inconspicuously small volcano that is only 1 731 meters high. The edifice is craggy and unevenly formed; one could even go so far and say that this is certainly not one of the more beautiful volcanoes of the world. The edifice contains several lava domes and spatter cones that gives evidence that the volcano has not always erupted through the massive wide bore crater that usually contains a crater lake. The current crater formed during a Colossal explosive eruption that took place around 230 BC and subsequent Cataclysmic eruptions has further enlarged the crater. Especially the Paroxysmal VEI-5 eruption of 1586 deepened and widened the crater.

Gunung Kelud is well known for its highly explosive eruptions, in the last one hundred years the volcano has suffered no less than five separate Cataclysmic VEI-4 eruptions. The most deadly of the series was the 1919 eruption when the brim full crater lake was disgorged in seconds during the onset of eruption. This caused massive lahars that travelled through canyons and rivers a distance of 37 kilometers. The total death toll of this eruption exceeded 5 000 people, mostly through drowning or crushing as the lahars inundated them. This led to the Dutch (colonial times) ordering an active mitigation scheme. A series of nine narrow tunnels was drilled through the mountainside so that the water could run off, thusly greatly diminishing the volume of the crater lake.

As the 1951 VEI-4 eruption happened the level of the crater lake had been lowered 50 meters and the Lahars produced were minute compared to in 1919. This eruption lowered the crater floor 70 meters and destroyed the old tunnels. This led to a new large crater lake forming, and during the 1966 VEI-4 eruption 200 people died from the lahars. This led to the construction of new lower tunnels and as the 1990 VEI-4 eruption happened the crater lake only contained one million cubic meters of water. During the 2014 massive VEI-4 eruption the tunnel was damaged and it is also reported that the crater floor once again has been lowered. It is as of now no news that new tunnels will be constructed, of if the old ones will be restored.

To date the tunneling at Gunung Kelud represents the only effective active mitigation campaign of any volcano on the planet. We therefore believe that it is imperative that the active mitigation program will be continued and that the effects of active mitigation of crater lakes can be further studied using Kelud as a natural laboratory. Even though the 2014 VEI-4 eruption was very powerful only two people died as their roofs collapsed on top of them. The reason for the low death toll is to be found in the active mitigation as well as with the highly skilled work performed by the Indonesian authorities prior to, during, and after the eruption.

The upcoming eruption was tracked weeks in advance and evacuations were performed continuously as the authorities received new data from local scientists. At the same time the military prepared for the upcoming cleanup process and moved heavy equipment into the region. Within hours after the eruption roads was started to be cleared from ash and debris and within a month most of the 12 304 destroyed or damaged buildings had been repaired or resurrected. This is a feat most countries would be well advised to study. Never had a large natural catastrophe been handled better.

The two cycles of Kelud

What sets Kelud apart from its explosive andesitic arc brethren is that its eruptions are driven in a bi-modal fashion. It is generally too small to produce repeated eruptions on this scale. So what makes this inconspicuous volcano so powerful? Let us take a look and marvel at the answer!

First we have to take a look at the types of eruptions that occur at Kelud. The primary type is highly explosive and contains juvenile magma that enters from depth. This magma is high in volatiles such as water from the melting of the subducting plate and rich in volcanic gasses. This combination is what makes andesitic volcanoes so explosive compared to mantle derived basalts.

Kelud normally produces a series of explosive eruptions until the magma is depleted of volatiles. This leaves a “dreg” of stale but still hot magma in the magma reservoir and conduits. As that is pushed up it produces a gentle effusive dome building eruption that emplaces a volcanic dome. In the last one thousand years those domes has been emplaced inside the crater at the center, but there are several emplaced domes outside of the crater that has been left untouched. The latest cycle of new magma intrusion probably started in 1951 and ended with the extrusion of a lava dome in 2007. This dome was most likely extruded partially because new fresh magma had started to enter from the magma acreation zone at depth. We will soon come back to what happened in greater detail.

Most crater lakes in volcanoes in the world forms as rain or snow falls inside an enclosed crater. These lakes are normally fairly cold and rapidly turn acidic from sulphur leaching into the water over time. In Keluds case the water comes up from the ground. This is evidenced in that there is no connection between water entering the lake and the rain season. Directly after an eruption hot water comes up through the fractures left by the explosive eruption. This initial water is high in chloride and holds a ph of 6 and contains potassium and sodium. The chloride content points to a deep source and the water is driven upwards by heat convection. There is a shallower source of water that has a slightly more acidic component in the form of a magnesium calcium sulphide.

Both types of waters start to emerge directly after the eruption, but over time the deeper source is gummed up and most likely squeezed shut as new magma comes up from the depth. This changes the water and makes it slightly more acidic (ph 5). During the last stages prior to an eruption the water changes again as Boron is released and the acidity falls again. Here we find the answer to the explosivity of Kelud. Not only is the magma high in water as a volatile, we also have pure water introduced directly into the volcanic system at depth. The amount of water introduced into the magmatic system is probably in the order of 250 liters per second.

In 2007 the lava dome was extruded into a very small crater lake due to the tapping of the water through the tunnels. It is easy to believe that the hot extruded dome did away with the water due to cooking it off, but this is just a small part of the truth. Instead it plugged the natural passage ways of water up through the volcanic vent. Then the water pressure started to rise rapidly as the natural outlet was hindered. Basically the extruded lava dome made Kelud into a gargantuan pipe bomb. The eruption in 1919 probably had seen a dome extrusion into the crater lake a few years prior to the big eruption. But then the lake was sufficiently large to quench the extruding dome making the process go even more rapidly. And as the dome was blasted out it dumped out all of the water in a matter of seconds.

Kelud and noise

In an innovative study sonar was used to measure sounds emanating from the volcano. The sound is produced as water boils at depth, and as gases are ejected into the crater lake. The ambient water noise level was measured at 130dB. During the last month before the 1990 eruption the noise level increased to 140dB as more gases was pushed out as magma started to rise and fracturing increased. This actually makes it possible to predict eruptions at Kelud by measuring sound.

The Future of Kelud

There are two separate dangers of Kelud that have to be taken into account:

First of all we have to look at the crater lake itself. For every eruption the floor of the crater is lowered and the potential volume of the lake increases. If the tunnels had not been operating during the 2014 eruption the water content would have been quite a lot larger than during 1919. Sufficiently so that the lahars would have travelled in excess of fifty kilometers causing widespread destruction, killed people, and taken away the livelihood of many subsistence farmers.

Due to the active mitigation this did not happen. But, let us play with the idea that the Indonesian government decides at some stage that the cost of constantly rebuilding tunnels is to daunting and leave the volcano as it is. Or even worse, also diminishes the resources for the volcanologists guarding the volcano. In this scenario we would be waiting for an eruption that would kill between 10 000 and 100 000 people as large scale lahars barreled down valleys, rivers and canyons. I do though not think that the Indonesians will do such a thing. They know but to well what Kelud is capable of and they also know that the cost of the tunnels is low compared to the lives they save.

The second risk lies in the nature of Kelud itself and its eruptive history. The system is gaining strength as the system evolves and it is already capable of producing a large VEI-5 to a small VEI-6. This is not guesswork; this is what the volcano already has done. An eruption of this severity that has a short run up would kill everyone within a ten kilometer radius from pyroclastic flows, ash fall, lahars and volcanic debris beating people to death. It would also kill people up to 30 kilometers out, but leave a possibility for surviving. Expected death rate for such an eruption is expected to be between 10 000 and 100 000 casualties. This number will obviously be lower due to the Indonesian authorities being adept and fast at performing evacuations as the warning signs come in.

There is though the problem that there is evidence that the eruptions are ramping up in power over time. The frequent powerful eruptions that we are seeing are a sign that the magmatic reservoirs are evolving and growing, and sooner or later the eruptions will become too powerful to contain and a caldera event will most likely happen at Kelud. If, or perhaps more to the point, when it happens the volcano is almost perfectly constructed to produce the most deadly of the primary killing factors, the pyroclastic base surge. Other andesitic volcanoes around the world that has the same shape and wide bore crater has often produced such surges, and if that happens nothing will survive within fifty kilometers out from the volcano. The ash fall would kill even further out as several meters of ash fall over villages and cities. And tertiary effects such as famine and diseases spread from contaminated water will kill even more people.

 

Kelud and the proposed new Decade Volcano Program

Kelud has a known history of explosive and dangerous eruptions. It is situated in a densely populated part of Indonesia. It has several modes of lethality that has been evidenced historically. From a scientific standpoint it bears to study, both how it is driven and also due to the unique attempt at active mitigation. Due to this we have chosen Gunung Kelud as the number ten on our list.

CARL