My. saint Helens was a long slow process of Continental Drift; it begins with the subduction of the oceanic plate (in the case of Mt. Saint Helen Juan De Fuca Plate) beneath the continental plate (North American Plate). This occurs because the weight of the water on the oceanic plates increases the density of the oceanic crust to the point where it slides underneath the continental crust. The oceanic plate goes farther and farther beneath the continental plate and lithosphere and it reaches the asthenosphere portion of the upper mantle where the plates melt into magma. When the rock melts into magma, water is also in the processes in its gaseous state and increases the volume of the rock and hence decreases the density of the rock. The gaseous water enters the magma begins to rise through the lithosphere and toward the surface due to its lower density where the magma solidifies into volcanoes and igneous intrusions.