Last week, we got a reminder about volcano hazards from the Tongariro eruptions in 2012. One of the stream valleys draining from near the Te Maari crater at Tongariro flooded, creating lahar-like conditions in the valley. Debris (sand, clay and rock) left over from the 2012 eruptions was remobilised by heavy local rainfall and washed out of the valley. Minor damage was done to a walking track.
Even though it’s been three years since the mountain erupted, moderate amounts of ash, sand and gravel remain as loose debris in the valley; it has never quite been cleared out. Due to the high intensity rain last week, the valley flushed out some more of this debris. We call this process remobilisation and refer to the floods as secondary lahars.
So when this stuff gets moving the consistency can range from that of muddy dishwater to that of thick wet concrete. Depending on the ratio of water to debris, within the flow there will be grain sizes from clay to boulders. This fluid has the potential to pick up much larger boulders and carry them down stream. This type of flow can be quite damaging when carrying large boulders, so you want to make sure you and all your stuff are out of the way.
These secondary lahars are often referred to by volcanologists as the ‘sting in the tail of a volcanic eruption’. They can deposit volcanic ash and debris over large areas down slope and in river channels. Volcanologist Brad Scott points out we’ve seen several examples of these events in Tongariro National Park. Following the 1995/96 Ruapehu eruptions ash and debris was washed off the mountains for years and similar is now occurring at Tongariro following the 2012 eruption at Te Maari.
Depending on the consistency of the flow the scientists have several technical names for these (volcanic mud flow, debris flow or lahar). In all cases it is a mixture of water and volcanic debris that moves rapidly down off the volcano. For now, it looks like Tongariro is still capable of producing these secondary lahars when enough rainfall occurs. Unfortunately the effects of the 2012 Te Maari eruption are still with us, albeit in a minor way.