TECTONIC PLATES
Mt St Helens is on the plate boundary between Juan de Fuca and the North American plates, the boundary is also a part of the Ring of Fire. The Juan de Fuca plate is an oceanic plate and the North American plate is a continental plate. The two plates became a destructive plate boundary – when a continental plate meets an oceanic plate. The oceanic plate (Juan de Fuca) descended underneath the continental plate (North American plate) as it is denser. When the oceanic plate descends further and further under the plate reaching past the lithosphere, it reaches the asthenosphere part of the upper mantle, the plate begins to melt because of the friction between the two plates. Once the plate has melted it turns into extremely hot, liquid rock, which is known as magma. The magma then rises through the gaps of the continental plate and when it reaches the surface it forms a volcano. This process goes on for hundreds of thousands of years, as more years go by more magma rises. In this case when the magma rose, gases couldn’t escape causing the magma to build more pressure, which creates a big volcanic explosion/eruption.