Mt. Shasta and Lassen Peak are both volcanoes located in Northern California. They were formed by the subduction of the Gorda Plate beneath the North American Plate. As the oceanic crust of the Gorda Plate descended beneath the continental crust of North America, it melted and rose to the surface, forming volcanoes. Mt. Shasta is a stratovolcano, which is a steep-sided volcano that is built up of layers of lava and ash. Lassen Peak is a plug dome volcano, which is a volcano that is formed by the eruption of viscous lava that does not flow easily.