According to Official Guinness Records,
The landslide on Mt St Helens on 18 May 1980, was the largest ever observed by a survivor, when about 2800million m3 96,000million ft3 of rock slipped off the mountain immediately prior to the volcanic eruption. This is equivalent to a block of earth nearly one and a half kilometers high, wide, and long (0.8 miles high, wide and long).
For a complete list of 1980 records, please visit 1980 Guinness Records in United States.