A magma reservoir 2.5 times larger than previously thought. If it erupted fully, it would blanket North America in ash and trigger a volcanic winter.
Ed Austin/Herb Jones, Public domain, via Wikimedia Commons
The Subterranean System
Beneath the geysers and hot springs of Yellowstone National Park lies a colossal, two-tiered volcanic plumbing system. The upper magma chamber, a sprawling mass of hot rock, begins three to nine miles below the surface. Research from the University of Utah, updated through seismic imaging, revealed this chamber to be approximately 55 miles long and 18 to 30 miles wide. It is not a cavern of liquid rock, but a sponge-like body of crystal mush with pockets of melt. Only about 5 to 15 percent of the upper chamber is actually molten. This shallower body primarily contains rhyolitic magma, a silica-rich and highly viscous material responsible for Yellowstone's explosive past.
Deeper still, between 12 and 28 miles down, lies a second, even larger reservoir. Discovered in 2015, this basaltic magma body is 4.4 times larger than the chamber above it. It contains an estimated 11,200 cubic miles of hot and partly molten rock. This lower chamber is the primary heat source, feeding basaltic magma upward into the rhyolitic system. The combined volume of this entire magmatic system is one of the largest known on the planet.
Imaging the Unseen
Scientists map these underground structures without ever drilling into them. The primary method is seismic tomography, which is a medical CT scan. The Yellowstone Volcano Observatory uses a network of seismometers to record earthquakes that naturally occur in the park, as well as seismic waves from distant quakes around the globe.
These seismic waves travel at different speeds depending on the material they pass through. They slow down when moving through hot or partially molten rock. By analyzing the arrival times of these waves at different stations, geophysicists can create a three-dimensional map of the subsurface. Another technique, called magnetotellurics, uses variations in the Earth's electromagnetic field to detect electrically conductive bodies like magma. These methods have allowed researchers to determine the size, shape, and even the melt percentage of the chambers far below.
The ground provides clues. Using GPS and satellite radar (InSAR), scientists monitor the surface of Yellowstone for upward and downward movement, known as uplift and subsidence. The caldera floor can rise or fall by several centimeters per year as magma and hydrothermal fluids shift below. Between 2004 and 2006, the ground uplifted by as much as 7 centimeters per year. More recently, from 2015 onwards, the caldera has been subsiding at a rate of 2 to 3 centimeters per year.
💡Fun Facts
The Yellowstone hotspot is stationary, while the North American tectonic plate moves southwest over it at about 2.2 cm per year, creating a 17-million-year-old track of calderas across the Snake River Plain.
The last supereruption, the Lava Creek event, occurred about 630,000 years ago and ejected approximately 1,000 cubic kilometers of rock and ash.
Since the last major caldera-forming eruption, there have been about 80 smaller, mostly non-explosive eruptions of rhyolite and basalt lava flows, with the most recent occurring 70,000 years ago.
The ground in the center of the caldera has risen by nearly 3 feet since measurements began in 1923, though it has experienced periods of both uplift and subsidence.
The park is open 24 hours a day, 365 days a year, though road access is seasonal and weather-dependent. Most visitor centers have seasonal hours.
Admission
Park entrance fee required.
Accessibility
The magma chamber is underground. The park itself has varied terrain with many accessible viewpoints, visitor centers, and boardwalks through geothermal areas.
