The largest volcanic mudflow was the Osceola Mudflow, which occurred some 5,600 years ago and began on top of Mount Rainier in the US Cascade Mountain range. The result of a volcanic eruption, it consisted of a debris flow of around 3 km³ (105,944 ft³) that hurtled down the mountain at around 90 kph (56 mph). By the time it had reached the Puget Sound, more than 100 km (62 miles)away, the mudflow still had a thickness of around 30 m (98 ft). After it had subsided it had created 460 km² (177 miles²) of new land on the coast.
Comments below may relate to previous holders of this record.