The
Next I continued up the hill towards the villages of Rangkah and Kinah Rejo as I'd been told that these places had been stricken by lahars as well. I wasn't prepared for the absolute and complete devastation I saw here. These villages were almost completely obliterated by lahars that moved through along with the latest eruptions of Merapi last October. 33 people were killed and the local economy, which was based on cattle, was completely upended as the debris flows followed poisonous gas clouds through the area. This area as well has turned into a tourist destination, and the people of the area have partnered (1) with the Department of Tourism for the Jogjakarta region to collect an entry fee of 5000 rupiah (65 cents or so) per vehicle. The folks on duty told me that there are 35 attendants that work rotating shifts of 10 days or so (after 10 days the whole shift is replaced). This seemed like a creative way to provide temporary employment, but see the note below.
What is a lahar, exactly?
A lahar (sometimes also referred to as a "debris flow" or "mudflow") is a rapidly flowing mixture of rock debris and water from a volcano. The word "lahar", which has entered the technical lexicon of geography and geology, is from Javanese and loosely translates to "mudflow", but this is misleading because they don't contain much silt, sand, or clay, which is what makes mud muddy. Lahars are interesting from a geographic perspective because they arise from a combination of volcanic and climatic process. They occur on both active and extinct volcanoes and can be devastating due to the extreme force of the flow; lahars have the consistency of wet concrete and flow as fast as rivers. One lahar killed 23,000 people in 1985 after a relatively minor eruption of the Nevado del Ruiz volcano in Colombia. Lahars can also displace thousands of people, which increases their vulnerability to hunger and disease. Lahars are different from other mass movements (like landslides and rock falls) because of the materials involved, the way the material moves down slope, and the triggering mechanisms. They are very mobile and can cover up to 20 times the area of an avalanche with the same amount of material. There are several different types of lahars, but they can be loosely lumped into two categories: "hot" lahars and "cool" lahars.
Hot Lahars (also called "primary lahars") are those associated with volcanic eruptions. Lahars require water, which can come either from convective rainfall associated with the air rising from the heat around the volcano or from melted glacial ice in the case of alpine volcanoes. In the tropics hot lahars often occur when an eruption coincides with the rainy season. Around Mt. Merapi hot lahars also happen when pryoclastic material and other debris mixes with one of the many rivers that drain the mountain. On average this happens once every two years or so in some channels. The lahar that destroyed Kinah Rejo was a hot lahar.Cool Lahars ("secondary lahars")don't coincide with an eruption. Rain erodes away accumulated lahar material, which is very unstable and unconsolidated to begin with. Around Merapi this most frequently occurs during the rainy season, which lasts from November to April. According to the government's geological agency, they are most likely to occur when there is more than 40 millimeters (around a foot and a half) of rain in a 2 hour period, but in reality this varies depending on slope, type of rainfall, elevation, and a number of other factors. The lahar that covered Magelang Road was a cool lahar.
In addition to the obvious impacts around Merapi, it's been suggested (see Walter et al reference below) that lahars can amplify earthquake damage. This is because the material is loose compared to solid bedrock, and so volcanic vibrations loosen the material and cause it too shift and settle. It's kind of like what happens when you shake a cereal box. Thus structures built on old lahar deposits are more likely to be damaged. Human actions can also exacerbate the damage done by lahars. Deforestation and conversion to farmland reduces the hydraulic roughness of volcano flanks and leads to more severe lahar events.
Mass Movements?
Lahars
are an example of what geomorphologists (2) call "mass movements". This technical term is pretty self explanatory. Another example would be the landslides I described in the Dieng Plateau post. Mass movements are typically the result of some sort of slope failure. A very simplistic way to think about this is to imagine the Earth as a dynamic system that is trying to level itself out. It does this by using rain, waves, wind, and seismic activity to move material from high places to low places. However, most of the time the forces that are trying to tear mountains and hills down are balanced by those trying to hold the mountain up. Slope failure happens when these forces are temporarily out of balance, and the resulting mass movement only stops when a new equilibrium is reached. There are all sorts of mass movements, but they are generally classified on the basis of the type of movement, the speed, and how much water is involved. I've included a diagram I took from the textbook I use to teach Introduction to Physical Geography at UH (3). I'll write more about the specifics of mass movements in future posts, but this is a good place to start.Acknowledgments:
Many thanks to my friends Wanto and Mike at Wisma Bahasa for giving me directions and recommendations for this post.
(1) According to the people working the gate, they get paid 14000 rupiah a day, just under two dollars. However, when I was there there were dozens of visitors. The remaining money goes to the department of tourism for the province of Yogyakarta. I'm not sure why this is or where that money goes.
(2) A geomorphologist is a geographer that studies landforms and their life cycles.
(3) Do any of these types of mass movements look familiar to you? What types of slope failures are most common in Hawai'i? Can you use the internet to find some newspaper stories about different
References:
Christopherson, Robert. Every couple of years or so. Geosystems: An Introduction to Physical Geography. New York: Pearson/Prentice Hall. A bunch of pages.
Kerle, Norman, and Clive Oppenheimer. 2002. Satellite Remote Sensing as a Tool in Lahar Disaster Management. Disasters 26:2, pp140-160
Lavigne, Franck. 1999. Lahar Hazard Micro-Zonation and Risk Assessment in Yogyakarta City, Indonesia. GeoJournal 49: pp173-183
Rodolfo, Kelvin. 1989. Origin and Early Evolution of Lahar Channel at Mabinit, Mayon Volcano, Philippines. Geological Society of America Bulletin 101, pp414-426.
Walter, TR, and 14 coauthors. 2008. The 26 May 2006 Magnitude 6.4 Yogyakarta Earthquake South of Mt. Merapi Volcano: Did Lahar Deposits Amplify Ground Shaking and Thus Lead to the Disaster? Geochemistry, Geophysics, Geosystems 9:5.
















