On May 18, a second earthquake of magnitude 5.1 made a huge part of the north face of the volcano collapse. It was the largest known debris avalanche in recorded history. At 8:32 a.m. Pacific Daylight Time, the magma inside of St. Helens exploded. On the Volcanic Explosivity Index scale, the eruption was rated a five, which is the same rating of the famous Vesuvius eruption in 79 AD.