Mount St Helens

Mount St Helens is a stratovolcano, or as they are also called a composite volcano. They occur at subduction zones. It is in the Cascade mountain range in Washington State, U.S.A, and is on the boundary of the North American Plate and the Juan de Fuca plate. When the oceanic plate (Juan de Fuca) subducts, dewatering occurs. Water is released from the subducting plate and rises. This water causes the melting point of the mantle rock to decrease. This causes melting and the partially melted rock rises because it is less dense the mantle rock.
The magma rises through the crust, adding silica rich rock, and pools into a magma chamber beneath or within the volcano. The pressure is lower here and the gases dissolved in the magma come out of solution and suddenly explode up through the volcano similar to the CO2 escaping from a shaken coke bottle.

Mount St Helens has gone through many eruptive stages. The current “stage” it is in now is called the Spirit Lake Stage and all stages before this are known as ancestral stages.
The first eruptive stage on record is the “Ape Canyon Stage” from 40,000 to 35,000 years ago. The Ape Canyon eruptive period ended around 35,000 years ago and was followed by 17,000 years of relative quiet.
The second eruptive period, the Cougar Stage, started 20,000 years ago and lasted for 2,000 years. Another 5,000 years of dormancy followed, but then came the Swift Creek eruptive period. Swift Creek ended 8,000 years ago.
A dormancy of about 4,000 years was broken around 2500 BC with the start of the Smith Creek eruptive period, around 1900 BC there was the largest known eruption from St. Helens ever recorded, judging by its effect on the environment. This eruptive period lasted until about 1600 BC and left 18 inches deep deposits of material 50 miles away in what is now Mt. Rainier National Park. Deposits have been found as far northeast as Banff National Park in Alberta, and as far southeast as eastern Oregon.   It is believed there may...