Mount st. helens HELP NEEDED!!!?

Question

what kind of boundary caused wt. st. helens.

like... what formed it?

i know it was caused by and earthquake, but whatelse caused it is what im asking.

Answer 1

Convergent

Mount St. Helens is an active stratovolcano.

Mount St. Helens is associated with the subduction of the Juan de Fuca plate under the North American plate.  As the Juan de Fuca plate was subducted, the leading edge began to get very hot due to earth's internal heat.  The heat caused the plate "melt" and form magma.  This hot magma expanded and became less dense than the surrounding material.  Therefore, the magma began to rise.  It erupted onto the earth's surface to form the volcano we know as Mt. St. Helen's.

Answer 2

The 1980 eruption resulted from a large magma accumulation in a chamber on the north flank.  The magma is the result of the subduction process.  When enough pressure built up, the magma chamber exploded.

There is a subduction zone associated with Mount St. Helens and the other Cascade volcanoes. The subduction zone has been there a long time, 20 million years or so. It results from the subduction of the Juan de Fuca oceanic plate beneath the North American plate. At the present time, the Juan de Fuca plate is moving to the east-southeast at about 3 cm/year and the North American plate is moving to the southwest at 2.3 cm/yr. Because the oceanic plate is denser it is pushed beneath the continental part of the North American plate, which is made of rocks with lower densities.

First, as the Juan de Fuca plate is subducted under the North American Plate, parts of it and/or the N. American plate start to melt. This melted rock (=magma) rises toward the surface to feed the Cascade volcanoes.

Every once in a while enough magma accumulates under a volcano that it starts to expand and rise the last few km to the surface. This magma is very viscous. In the case of the big 1980 eruption, the magma accumulated high in the volcano and as it did it bulged the north flank very perceptibly outward.

Once the pressure becomes so great that the overlying rock of the volcano can no longer contain it the eruption starts. There is lots of gas dissolved within the magma and it is kept in solution by the overlying pressure (just like the fact that you don't see bubbles in a soda before you open it). However, as soon as the top of the volcano is blasted off the pressure is lowered. This allows the gas bubbles to expand and force even more magma out, which further lowers the pressure, which allows more bubbles to expand and on and on and on. This rapid depressurization is the reason for the high explosivity of the eruption.