Case Study Detail Mount St Helens is on the plate boundary between the Juan de Fuca plate and North American plate. When it erupted it permanently changed the surrounding landscape. A destructive plate boundary. Prior to the eruption there were over 10,000 earthquakes. The final earthquake triggering a massive landslide (the largest landslide recorded. The volcanic plug blocked the exit to the volcano so magma started to push its way sideward and created a cryptodome to the North of the mountain. The landslide triggered the eruption creating a lateral eruption and released a pyroclastic flow travelling well over mph and over o c.
