![]() ![]() During this relatively short stage multiple domes grew on the volcano, reaching an altitude 2,100 m (7,000 ft). Swift Creek Stage: Swift Creek volcanism occurred between 16,000 to 12,800 years ago. The subduction zone creating the Cascade Range volcanoes. The debris avalanche was the most catastrophic event of the Cougar Stage, leaving a massive deposit behind. A debris avalanche is a mass of rock, soil and snow that runs down the side of a volcano to the valley floor, traveling several kilometers from the source, and leaving a horseshoe-shaped crater. ![]() The explosions varied to form lava flows and domes, large ash ejections, pyroclastic flows, a debris avalanche and lahars. Helens, taking place from 28,000 to 18,000 years ago. This stage was followed by a dormant interval from 35,000 to 28,000 years ago.Ĭougar Stage: The Cougar Stage was one of the most explosive periods for Mt. Evidence can be found in rocks as far as eastern Washington, many of which were altered by hydrothermal (hot water) activity, indicating explosive eruptions. Volcanologists have separated the eruption history of this volcano into four main stages, each followed by a dormant, nonexplosive period.Īpe Canyon Stage: This stage spans from 275,000 to 35,000 years ago and had two major lava dome eruption events. Helens has displayed both relatively quiet outpourings of lava and violent explosive eruptions of volcanic ash and rock fragments, known as tephra. The 275,000 year old geologic history of Mount St. Can you guess what may disturb the balance in the chamber and set off a volcanic eruption? Earthquakes! Just as a sudden and violent shake of a soda can will cause the liquid to escape quickly when opening, volcanoes will react to this quick change in motion and pressure by erupting onto Earth’s surface. These chambers behave similarity to a soda can, staying dormant most of the time unless a sudden disruption occurs. The less dense material rises, melting and absorbing surrounding rock as it bubbles upward to form magma chambers. The ocean crust partially melts at depth and also releases less dense materials (water and gases). Oceanic crust is more dense than continental crust, so as the Juan de Fuca plate collides with the North American Plate, it is forced downward, deeper within the Earth where temperatures are higher. Here, the Juan de Fuca (oceanic) plate dives beneath the North American (continental) Plate. Helens resides, is a perfect example of a fundamental concept in geology known as a subduction zone, a place where oceanic crust and continental crust collide. Inside the Monument, the environment is left to respond naturally to the disturbance.The Cascade Range, where Mount St. In 1982 the President and Congress created the 110,000-acre National Volcanic Monument for research, recreation, and education. Helens and the surrounding landscape were dramatically changed within moments.Ī vast, gray landscape lay where once the forested slopes of Mount St. The eruption lasted 9 hours, but Mount St. Searing flows of pumice poured from the crater. Wet, cement-like slurries of rock and mud scoured all sides of the volcano. Nearly 150 square miles of forest was blown over or left dead and standing.Īt the same time a mushroom-shaped column of ash rose thousands of feet skyward and drifted downwind, turning day into night as dark, gray ash fell over eastern Washington and beyond. A tremendous lateral explosion ripped through the avalanche and developed into a turbulent, stone-filled wind that swept over ridges and toppled trees. The avalanche rapidly released pressurized gases within the volcano. In a few moments this slab of rock and ice slammed into Spirit Lake, crossed a ridge 1,300 feet high, and roared 14 miles down the Toutle River. Helens erupted, shaken by an earthquake measuring 5.1 on the Richter scale, the north face of this tall symmetrical mountain collapsed in a massive rock debris avalanche. ![]() At 8:32 Sunday morning, May 18, 1980, Mount St. ![]()
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