A supervolcano is defined by its Volcanic Explosivity Index (VEI) which is measured from a magnitude of 0 to 8. The magnitude of 8 represents the largest eruptions in history and supervolcanoes tend to erupt at this magnitude.
Currently, there are three active supervolcanoes in the United States. The three supervolcanoes are located at Yellowstone, the Long ValleyCaldera, and the Valles Caldera.
The Long Valley Caldera is a depression in California and one of the Earth's largest calderas. It was formed as a result of a supereruption that occurred about 760,000 years ago.
The Long Valley Caldera is of special interest to scientists in California because it is active and contains a reservoir of more than 240 cubic miles of magma located underneath the valley.
Scientists believe that the Long Valley Caldera could potentially erupt again. When it erupted 760,000 years ago, its blast was 2000 times larger than that of Mount St. Helens. The blast created a caldera that was 20 by 10 miles and the land area sunk more than one mile into the space that originally held the erupted magma.
A supervolcano eruption could cause catastrophic destruction and loss of life.
According to the United States Geological Survey, a supervolcanic explosion could also result in toxic gas, falling ash, and changes to the global climate.
The chances of any of the three supervolcanoes in the U.S. erupting any time soon are rather small and most likely, there won't be any major eruptions for the next few thousand years. However, it is best to be prepared.