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Mount St. Helens is a large eruptive cone consisting of lava rock interlayered with ash, pumice, and other deposits. The mountain includes layers of basalt and andesite through which several domes of dacite lava have erupted.
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, with little warning, 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 km2). More than 1.5 million metric tons of sulphur dioxide were released into the atmosphere.[
lahar mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. Lahars are extremely destructive: they can flow tens of meters per second, be 140 metres deep, and destroy any structures in their path.
The collapse of the northern flank of St. Helens mixed with ice, snow, and water to create lahars (volcanic mudflows). The lahars flowed many miles down the Toutle and Cowlitz Rivers, destroying bridges and lumber camps. A total of 3,000,000 m3 of material was transported 17 miles (27 km) south into the Columbia River by the mudflows.
The St. Helens May 18 eruption released 24 megatons of thermal energyit ejected more than 2.79 km3 of material.
The removal of the north side of the mountain reduced St. Helens' height by about 400 m and left a crater 1.6 km to 3.2 km wide and 800 m deep, with its north end open in a huge breach. The eruption killed 57 people, nearly 7,000 big game animals12 million fish from a hatchery.It destroyed or extensively damaged over 200 homes, 185 miles (298 km) of highway and 15 miles (24 km) of railways.
A pyroclastic flow is a fast-moving current of hot gas and rock which reaches speeds moving away from a volcano of up to 700 km/h (450 mph). The gas can reach temperatures of about 1,000 °C. Pyroclastic flows normally hug the ground and travel downhill, or spread laterally under gravity. They are a common and devastating result of certain explosive volcanic eruptions.
