Mount Ruapehu Crater Lake
Lahar: Just after 11am on Sunday 18 March 2007 the tephra dam at Mt Ruapehu's Crater Lake collapsed and sent a lahar down the Whangaehu River. It was described as a moderate lahar and the systems set in place to both detect the start of a lahar and ensure public and property safety through an emergency reponse plan worked flawlessly and the lahar passed down the Whangaeghu River, its natural path, to the sea with minimal property damage.
TV1 and TV3 both carried very good video footage of the lahar and the aftermath www.tvnz.co.nz and www.tv3.co.nz - use search feature and type lahar.
Mt Ruapehu's Crater Lake and the lahar
Situated within Tongariro National Park, the volcanoes of Ruapehu, Tongariro and Ngauruhoe and their surrounding lahar ring plains comprise major landscape features in the central North Island. The Whangaehu Valley is the major lahar path from the volcanoes and is almost certainly the most active lahar (volcanic mud flow) path in the world.
Mt Ruapehu is one of the world's most active volcanoes with a record of major eruptions about every 50 years. It erupted in 1945 and in 1953 part of an ice wall collapsed allowing a lahar (volcanic mudflow) to sweep down the Whangaehu River and destroy the Tangiwai Rail bridge minutes before the Express train arrived. 151 lives were lost when most of the carriages plunged into the swollen river.
From time to time activity in the magma chamber in the vent below the lake gives rise to increased temperatures or release of gas. This may show as a change in surface colour of the lake or as a 'burp' or phreatomagmatic eruption as gas reaches the surface.
The eruptions of 1995 and 1996 built up a deposit about seven metres thick of tephra (ash, scoria and rocks) on the crater rim at the former lake outlet. The lake refilled behind this barrier to form a dam. On 18 March 2007 the uncosolidated material collapsed giving rise to a moderate sized lahar. This was a natural event but the department and other agencies had undertaken steps to ensure public safety and minimise damage to property.