The Indonesian word, lahar, is the technical term used to describe volcanic mud flows. This post explains the difference between two types of lahars (hyperconcentrated flows and debris flows), using videos that I recorded at Volcán de Colima as examples.
I’d been meaning to post these for a while. This month, the Accretionary Wedge (a collection of geoblog posts linked by a common theme) gave me an excuse. The current theme is ‘Geological Events That You’ve Directly Experienced‘.
The ingredients for a lahar
The summer of 2005 was the most active period at Volcán de Colima in nearly 100 years. Hundreds of explosions blasted from the crater; the largest produced pyroclastic density currents that dumped millions of cubic metres of smashed-up volcanic rocks upon the upper slopes and within the ravines (barrancas) on the flanks. Uncemented by clays, the deposits are a loose, rubbly mixture of boulders, cobbles, gravels and sandy and dusty ash.
Fast-forward to summer 2007, and the middle of the rainy season. Sweaty, humid nights dawn into blazing sunlit mornings, but clouds soon form. By 12.00hrs the summit of the volcano is lost and thunder begins to rumble. Mid-afternoon, every day, the tropical rain hammers down. And I mean hammers; rainfall of 100 mm in 3 hours is not uncommon. By contrast, London gets 750 mm in an entire year.
The first video shows what this kind of rain looks like. I had gone with Flo, a volunteer at the Universidad de Colima, to maintain a radar monitoring station on the south flank of the volcano, about 3 km from the crater. (Click here for a Google Earth file that shows the monitoring station and the debris-covered flanks of the volcano). We had barely started our work when the rain arrived, so we had to sit it out in order to finish what we needed to do.
The three ingredients for a lahar, steep slopes, loose debris, and heavy rain, were in place.
Hearing a gradually increasing roar from the barranca to our west, we went over for a look. A lahar was flowing in the bottom of the channel, crashing boulders and splashing mud and roaring with the white noise of rocky collisions and hammering rain. The second video shows it in action.
The technical term for this type of lahar is a hyperconcentrated flow. It contains mainly water, but with 20-60 vol% sediment. The finer material is mixed turbulently into the muddy water, but the bigger rocks are moved by bouncing or rolling along the floor. Deposits from the flow contain mainly silt/sand and gravel and can be stratified, although they often have no obvious structure and the bigger rocks are isolated, supported by their finer surroundings.
About 30 seconds into the video, a big raft of boulders comes charging down the valley (Flo gets very excited). You can see clearly how they jostle each other as they rattle past us. These ‘inter-grain collisions’ are important in allowing fragments of solid materials to flow.
Once we’d seen enough, and had begun climbing back up to the radar station, we felt the ground begin to really shake. Something much bigger was coming, so we ran toward the barranca, and arrived just in time to see a wall of boulders about 2-3 m high sweep round the corner like an oncoming train. Retreating to a slightly higher perch, I recorded the third video.
This time, the flow has a different regime, known as a debris flow. Debris flows are mainly sediment (>60 vol%), and the water mixes with the finer material to form a wet-concrete-like slurry that lubricates the flow of the boulders. This one was moving at over 10 metres per second (36 km per hour).
Inter-grain collisions keep the heavy boulders moving, and a process called kinetic-sieving, where the smaller rocks fall down gaps between the larger ones, can concentrate the largest boulders at the top and at the front of the flow. You can see this at the very start of the video. Also, because the slurry is so heavy with sediment, boulders can become buoyant. At about 20 seconds into the video, a ~1 m wide rock floats by on the surface of the flow.
Debris flows can form from hyperconcentrated flows that bulk-up by eroding sediment from the channel walls, especially if a large part of the wall collapses. I think that was what happened here. Hyperconcentrated flows can turn into debris flows if they are diluted by water from tributary streams, or if they deposit a load of their sediment when they reach flatter ground. The latter was the most likely fate of this debris flow.
Learning from experience
Having the direct experience of lahars at Colima reinforced three things in my mind. These are things that should have been obvious, but are more sharply defined having seen them first hand.
- Lahars happen all the time. From the tectonically inactive shores of the UK, lahars sound exotic and exciting, but in Colima they are just weather. High on the volcano’s flanks, small lahars occur just about every time that it rains. Generally, they deposit most of their material harmlessly within 5 km of the crater, where the gradient of the channel gets less steep.
- Lahars are powerful and dangerous. The momentum of the fast-moving boulder mixture is incredible, and the potential for destruction is clear. Bigger events can travel further from the volcano, reaching inhabited areas. Lahars from Volcán de Colima prompted evacuations in 2007, destroyed a house in 2000, and resulted in 20 fatalities in a town ~20 km from the crater in 1955. Elsewhere, Semeru, Mayon and Pinutubo have all produced fatal, rainfall-triggered lahars. This video from Semeru makes the Colima lahars above look tame.
- A lahar is a wet pyroclastic density current. The inter-grain collisions and other processes in a lahar are broadly the same as those that allow pyroclastic density currents (PDCs; aka pyroclastic flows) to travel so far from their source volcanoes. The difference is that PDCs are lubricated by hot gas, not water. This video from Mt Unzen, Japan is a good example. If you could see through the billowing cloud of 200°C dusty ash, the blocks and larger rocks rattling along the river bed would look very similar to those in the videos above.
I filmed these lahars while I was working as a postdoc at the Centro de Intercambio e Investigación en Volcanologia (CIIV) at the University of Colima. The CIIV has a volunteer scheme that provides opportunities for people to gain experience working on an active volcano. Volunteers need only basic computer-literacy, an ability to spend days out hiking, plenty of enthusiasm and at least a couple of months to spare. For more information, visit http://www.ucol.mx/ciiv.