The theory of plate tectonics holds that as the expanding oceanic crust is thrust beneath the continental plate margins, it penetrates deep enough into the Earth to be partly remelted. Pockets of molten rock (magma) result. These become the feeding chambers for volcanoes
About 600,000 years ago a stratovolcano, Mount Tehama, gradually built up here through countless eruptions. Before Lassen Peak was created, Mount Tehama had collapsed. Since its caldera was breached, no lake developed as occurred at Crater Lake in Oregon
Lassen Peak began as a volcanic vent on Mount Tehamas's northern flank. Considered the world's largest plug dome volcano, it rises 2,000 feet to an elevation of 10,457 feet.
In May 1914 Lassen Peak burst into eruption, beginning a 7 year cycle of sporadic volcanic outbursts. The climax of the episode took place in 1915, when the peak blew an enormous mushroom cloud some 7 miles into the stratosphere. The reawakening of this volcano, which began as a vent on a larger extinct volcano known as Tehama, profoundly altered the surrounding landscape. The area was made a national park in 1916 because of its significance as an active volcanic landscape.