Lahars – Floods of Volcanic Mud

jeremy phillips

In this installment, I will introduce some of the hazards and research questions concerning the strange-sounding Lahar, or volcanic mudflow, if your Indonesian is a little rusty. Firstly some things about me: I am an academic Volcanologist with interests in developing models of volcanic processes that can be used to assess their hazard in practical situations, so I have broad interests in uncertainty in hazard modeling. Secondly, this is my first blog post ever (there’s probably some term for this), so here goes…

Lahars are a mixture of volcanic ash, other rocks and soils, and water, which have a consistency similar to wet concrete. They are formed in two main ways: when a volcano erupts hot ash onto its snow covered or glacicated flanks and surroundings, or when intense rainfall remobilizes volcanic ash deposits from previous eruptions or ongoing activity. The resulting mixture of ash and water tends to flow in existing valleys, that are typically steep-sided higher on the volcano and feed into flatter river systems. Like other hazardous mass flows, they are erosive on steep slopes (this is where the ‘other rocks and soils’ come from) and form deposits on shallow slopes, but in lahars the addition of rock and soils by erosion is extreme – the volume of the Lahar can increase by up to a factor of ten due to this effect. The flow transports a high concentration of solids, providing the ability for large boulders to be transported in the flow, meaning that building damage can be considerable, when the flow overtops river channels in habited regions. They can also be extremely fast moving – have a look at for three amazing clips of recent lahars in Japan.

Despite their initiation in localized regions, Lahars can be very large and destructive. A key driver in the development of volcanic hazards communication videos in the 1980s was the Lahar resulting from the eruption of Nevada Del Ruiz in Colombia in 1985, which destroyed the town of Armero 74 km away with the estimated loss of 23,500 lives. Lahar activity is common in countries with high snow-covered volcanoes such as Japan, those that include the Andes, countries with active volcanoes and seasonal rainfall including Indonesia and the central Americas, or simply where there has been a large recent explosive eruption, such as areas of the Western US affected by the 1980 Mt St Helens eruption. Lahars are closely related to mudflows and debris flows, and these hazards are globally widely-distributed.