Lassen Volcanic

Declared in 1916, shortly after the last major eruption of Mount Lassen between 1914 and 1917 — the only documented volcanic eruption in mainland California in the 20th century before Mount St Helens — it is the only place in the world where all four principal volcano types recognised by volcanology occur together (shield, composite, lava dome and cinder cone), making it a world-reference field laboratory for the study of volcanic geology.