Geology of Lassen Volcanic National Park

Lassen Volcanic National Park, and more specifically Lassen Peak – the largest plug dome volcano in the world, is part of the “Ring of Fire”, the ring of volcanoes, some active, some dormant, some extinct – which circumnavigate the Pacific Ocean, marking the very edge of the Earth’s crust. The tremendous volcanic and seismic disturbances which are so prevalent in this “Ring of Fire” occur due to the gigantic slabs grinding past and overriding each other.

As one of these gigantic plates, the ever expanding oceanic crust continues to dive beneath the margins of the continental plate, it is thrust deep within the Earth, deep enough to be melted  resulting in pockets of magma, or molten rock. These pockets create the volcanoes feeding chambers.

Countless eruptions created the Ring of Fire almost 600,000 years ago, and the creation of Mount Tehama. Mount Tehama gradually collapsed but there was no crater lake development, because the caldera (or basin) was breached. Remnants of the caldera today form Mount Diller, Pilot Pinnacle, Brokeoff Mountain and Mount Conard. If you draw a line connecting these peaks you’ll get some idea of the scale of the base of Mount Tehama, in excess of an incredible 11 miles wide.

Lassen Peak probably began it’s life on the north side of Mount Tehama as a volcanic vent. These days it rises more than 2,000 feet to an elevation in excess of 10,457 feet, the largest plug dome volcano in the world. The lava present within the park came from many such vents.