Young River Landslide Biggest In 16 Years Geologists have found that the Young River landslide in Mt Aspiring National Park is the biggest landslide in New Zealand since the top fell off Mount Cook in 1991. Using a tripod-mounted laser scanner, geologist Chris Massey and surveyor Neville Palmer, both of GNS Science, have measured the volume of the landslide at 13 million cubic meters. The Mount Cook rock avalanche of 14 December 1991 resulted in 16 million cubic meters of debris being deposited on the Tasman Glacier. The Young River landslide occurred on August 29 and was discovered by a helicopter pilot three weeks later. Seismic waves generated by the landslide were the equivalent of a magnitude 2.9 earthquake, and were recorded by seismic instruments in the central South Island.