Question

Use plate tectonic theory to explain the origin of Mount St. Helens (and by extension, the Cascades and all Andean-type mountains). Your answer should be no less than 500 words. In your answer, include a discussion of:

• the plate tectonic process involved
• the origin of the magma and rocks
• the composition of the magma and rocks
• the type of eruptions that occur there
• other geologic features associated with Andean-type mountains.

Answer 1

Mount St. Helens was formed by the subduction of the Juan De Fuca oceanic plate under the North American plate. This is a convergent/subduction type setting. So this region is marked by the presence of deep focus earthquakes and explosive volcanic activities.

Plater tectonics is the process which explains the movement of the solid lithospheric plate above the asthenosphere. The plate movement can be divergent or convergent or transform. Here the type of plate tectonic activity is a convergent plate boundary.

Here the magma produced by the melting of the mantle wedge and the melting is a flux type melting i.e. addition of volatiles. The magma formed here Is andesitic type. The composition is more viscous than the basaltic type magma and less viscous than the rhyolitic type magma. As this andesitic magma has much viscosity it produces explosive type volcanism. Generally this forms stratovolcano type feature.