Bishop Tuff road cut north of June Lake, Long Valley Caldera, CA

The Long Valley Caldera was formed 760,000 years ago when 600 cubic kilometers (150 cubic miles) of magma (molten rock) was erupted over the period of six days. As the magma was ejected, the roof of the magma storage region collapsed forming a 17 by 32 km (9 by 18 mile), oval-shaped depression known as the Long Valley Caldera. The deposits from the large eruption are called the Bishop Tuff and they can be seen in many areas outside the caldera.