<P> Roughly 700 years of dormancy were broken in about 1480, when large amounts of pale gray dacite pumice and ash started to erupt, beginning the Kalama period . The eruption in 1480 was several times larger than the May 18, 1980, eruption . In 1482, another large eruption rivaling the 1980 eruption in volume is known to have occurred . Ash and pumice piled 6 miles (9.7 km) northeast of the volcano to a thickness of 3 feet (0.9 m); 50 miles (80 km) away, the ash was 2 inches (5 cm) deep . Large pyroclastic flows and mudflows subsequently rushed down St. Helens' west flanks and into the Kalama River drainage system . </P> <P> This 150 - year period next saw the eruption of less silica - rich lava in the form of andesitic ash that formed at least eight alternating light - and dark - colored layers . Blocky andesite lava then flowed from St. Helens' summit crater down the volcano's southeast flank . Later, pyroclastic flows raced down over the andesite lava and into the Kalama River valley . It ended with the emplacement of a dacite dome several hundred feet (~ 200 m) high at the volcano's summit, which filled and overtopped an explosion crater already at the summit . Large parts of the dome's sides broke away and mantled parts of the volcano's cone with talus . Lateral explosions excavated a notch in the southeast crater wall . St. Helens reached its greatest height and achieved its highly symmetrical form by the time the Kalama eruptive cycle ended, about 1647 . The volcano remained quiet for the next 150 years . </P> <P> The 57 - year eruptive period that started in 1800 was named after the Goat Rocks dome, and is the first time that both oral and written records exist . Like the Kalama period, the Goat Rocks period started with an explosion of dacite tephra, followed by an andesite lava flow, and culminated with the emplacement of a dacite dome . The 1800 eruption probably rivalled the 1980 eruption in size, although it did not result in massive destruction of the cone . The ash drifted northeast over central and eastern Washington, northern Idaho, and western Montana . There were at least a dozen reported small eruptions of ash from 1831 to 1857, including a fairly large one in 1842 . The vent was apparently at or near Goat Rocks on the northeast flank . Goat Rocks dome was the site of the bulge in the 1980 eruption, and it was obliterated in the major eruption event on May 18, 1980 that destroyed the entire north face and top 1,300 feet (400 m) of the mountain . </P> <P> On March 20, 1980, Mount St. Helens experienced a magnitude 4.2 earthquake; and, on March 27, steam venting started . By the end of April, the north side of the mountain had started to bulge . On May 18, a second earthquake, of magnitude 5.1, triggered a massive collapse of the north face of the mountain . It was the largest known debris avalanche in recorded history . The magma in St. Helens burst forth into a large - scale pyroclastic flow that flattened vegetation and buildings over 230 square miles (600 km). More than 1.5 million metric tons of sulfur dioxide was released into the atmosphere . On the Volcanic Explosivity Index scale, the eruption was rated a five, and categorized as a Plinian eruption . </P>

When was the last time mount st helens erupted before 1980