"The third explosive eruption of Mount St. Helens occurred Thursday evening, June 12, 1980. It was preceded by a buildup of harmonic tremor similar to that which preceded the May 25th eruption. The tremor began at low levels about midday and increased in amplitude throughout the afternoon. A marked increase was noted at 7:05 p.m., and reports of an eruption cloud rising to an altitude of at least 13,000 feet were received 5 minutes later. Tremor dropped off in strength immediately after the eruptive pulse and fluctuated at moderate amplitudes for more than 2 hours.