Yellowstone volcano

The Yellowstone volcano is a caldera in Wyoming and Idaho that spans 43 by 28 miles (69 by 45 kilometers), with a magma chamber beneath it measuring 50 miles (80 kilometers) long and 12 miles (20 kilometers) wide. The entire park sits above this system, which feeds the geysers, hot springs, and mud pots scattered throughout the landscape.

Three large eruptions took place over the past 2.1 million years, with the most recent one occurring 640,000 years ago and forming the caldera seen today. Since then, magma has accumulated beneath the crust without producing another major eruption.

Rangers guide visitors to overlooks where they can watch steam vents and hot springs powered by the volcanic system below. Geologists work on site and often explain to visitors how monitoring equipment functions and what it reveals about activity beneath the surface.

Hundreds of small earthquakes occur each year but most go unnoticed and simply reflect normal activity in the system. Visitors can walk safely on boardwalks near thermal areas as long as they stay on marked paths and away from thin crusts.

The underground reservoir holds roughly 1,000 cubic miles (4,000 cubic kilometers) in volume, but only between 5 and 15 percent of it is actually molten rock. The rest consists of solidified or partly solidified material that surrounds and insulates the magma.