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.
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