cavingliz non-cave albums: Lahar from Gunung Merapi, Java

Saturday, January 15, 2011

Lahar from Gunung Merapi, Java

Early January I was in Yogyakarta, Java, Indonesia, for the Asian Trans-Disciplinary Karst Conference 2011. After the papers and presentations we had 2 full days of field trips (each day was 15 hours!). The only nonkarst visit was to see a lahar from the 2010 Gunung Merapi eruption.

Although I have been to several volcanoes in Indonesia, it was my first time to see a lahar, and to see the devastation it caused. Gunung Merapi which is north of Yogyakarta, is one of the most dangerous volcanoes. Indonesia has 1/3 of the world’s most active volcanoes, and has 129 volcanoes. Merapi is one out of six of the most dangerous, because of the frequency of eruptions, every 5-13 years, and the dense population of people who live on the slopes.

It is feared for its deadly pyroclastic flows - avalanches of hot rocks and gas that are generated when parts of new lava domes in the summit crater collapse and slide down the sides of the mountains.

In Oct 2010 Merapi woke up again. It started erupting on Oct 26. At that time the danger zone was 10 km radius, 5 days later Merapi was more active and the danger zone was widened to 15 km.

On 26th Merapi had explosive eruptions producing lava flows and heat clouds, and smoke rose 1.5 km vertically above the summit. The first fatalities occured on that day.

On 30th ash fell more than 30 km away including on the city of Yogyakarta. Black soot fell across a vast area. Hundreds of fleeing residents clogged the roads. Heat clouds flowed into rivers and also rose 3.5 km above the summit. A pyroclastic flow headed towards some rivers. Then an explosion from Merapi resulted in a 2 km high fire ball rising from the top of the mountain. It's hard to imagine this, though I saw photos later.

Ash fell onto Jogyka and the airport was closed on and off over several days. I wondered whether we would be able to get to the conference.The biggest eruption was on 5 Nov. The safety zone was widened to 20 km radius. 275 people died on that day. These eruptions were the biggest since the 1870s. One village 15 km away was covered by ash up to 30 cm.

More than 100,000 people had been evacuated from the area. The World Heritage Site of Borobudur was covered with ash and was closed for a few weeks.

Ash clouds are solids suspended in hot air. They can move 150 kph or 40m/s down slopes, and people cannot outrun this. They lead to ashfall deposits.

A pyroclastic flow is a fast-moving current of extremely hot gas (which can reach temperatures of about 1,000 °C) and rock, which travel away from a volcano at speeds generally as great as 700 km/h (450 mph). The flows normally hug the ground and travel downhill, or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope. They are a common and devastating result of certain explosive volcanic eruptions.

Lahars do a lot of damage. A lahar is a flow of mud or debris composed of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. When mixed with heavy rains this heavy mud flow can carry huge boulders and trees down with it. These rocks are like battering rams and destroy bridges and houses etc. The term lahar originates from the Javanese language, a shortened version of "berlahar".

As we were driving towards Merapi on 10 Jan we found that the main road had been buried by lahar due to heavy rains the night before. So we had to find an alternative route. This lahar made front page news in the national papers.

The heavy rain caused the cooling lava in the river to explode, which destroyed some dykes.

We parked at the lahar site and walked up alongside to an embankment on the Gendol River. We were 14 km from the summit. This river was filled with pyroclastic flow then the lahar occurred because of the heavy rain. Last night the main road to Semarang was covered to a depth of 4 m.

The wide river was just a small trickle, and it was bizarre to see the water running downstream yet the clouds from the river were blowing upstream. I'm not sure if the these clouds were steam from the heat, or gas. I think it was steam. The ground on the river bank was quite warmer and of course much hotter in the river bed.

Dozens of bulldozers were working to remove the debris. They were filling lorries with the ash, filtering out all the large material. This ash is used in construction sites in the city and elsewhere. Many trees had been killed alongside the river and several houses destroyed.

It was a scene of devastation. There was only a mild smell, as the gases had presumably all escaped over the last 2 months.

It was quite fascinating to see it, but of course very sad, especially considering the people who had been killed and those who had lost their homes and livelihoods. A few enterprising people were selling food and drink and even souvenirs along the river bank.

On the road we passed several Red Cross tankers that were delivering water. And there were many signs locating evacuation centres and shelters.

And at the time of writing, 15th the authorities are worried that lahar in the Opak river may threaten Prambanan, a World heritage site, Candi Prambanan. This site was damaged during the 2006 earthquake.