New research indicates the next major eruption of the Yellowstone volcano will occur near the northeast corner of the Yellowstone Caldera/NPS file, Jim Peaco

Deep within the Yellowstone Caldera, the bowl-shaped rock cauldron at the heart of Yellowstone National Park, there’s a clue that any future eruption will occur near the northeastern edge of the caldera.

But that eruption is not expected any time soon.

“There's no possibility of current-day eruptions happening at Yellowstone [National Park],” said Ninfa Bennington, a U.S. Geological Survey geophysicist.

It’s been nearly 650,000 years since the caldera last notched a significant eruption. Through the past 2.1 million years there have been three major eruptions: the Huckleberry Ridge eruption 2.1 million years ago, the Mesa Falls eruption 1.3 million years ago, and the Lava Creek eruptions roughly 640,000 years ago.

New research doesn’t predict when another major eruption will occur, but it does indicate that the most likely spot for one would be along the northeastern edge of the caldera.

In a paper that appeared early this year in the journal Nature, Bennington and seven colleagues pointed out that heat carried by basaltic magma in the crust beneath the caldera has fueled a lobe, or pocket, of rhyolitic magma away from the center of the caldera to the northeast.

Additionally, the scientists discovered that mid-to-upper crustal layers of the caldera are not underlain by one large reservoir of magma, but rather several lobes of rhyolitic magma.

“They're all different sizes from one another, but they're much smaller in total volume than if you had magma distributed across the whole caldera as a single reservoir,” said Bennington during a phone call from her office at the Hawaiian Volcano Observatory.

To understand what’s going on beneath the caldera, and to understand what might generate a violent eruption, you must understand two basic types of magma. Basaltic magma is like that typically found flowing downhill relatively fast during eruptions of Hawaii’s Kīlauea or Mauna Loa volcanoes. It is low in viscosity. Rhyolitic magma, on the other hand, has higher viscosity and requires great force to erupt.

“What this means for volcanic eruptions,” explained Bennington, “is that if you have [rhyolitic magma] being stored within a volcano, it’s really resistant to flow. You have to generate a lot of pressure to get that material out of the volcano. And so, what happens is that a lot of these rhyolitic eruptions are really explosive in nature.” 

A pocket of rhyolitic magma, labeled C4, is near the northeastern corner of the Yellowstone Caldera/USGS

Because much of Yellowstone’s basement is composed of rhyolitic magma rich in silica and very viscous, the landscape “has this history of having had really explosive eruptions,” she said.

But present-day conditions portray what might best be described as a simmering cauldron, and one with disparate pockets of magma. The basaltic magma pockets in the western portion of the caldera are cooling and “don’t have a heat source to sustain volcanic activity,” said Bennington. While the rhyolitic magma lobes are hot enough, they just haven't reached the pressure-cooker-like concentration necessary for an eruption.

When it might reach that point remains unknown.

The research also concluded that the amount of magma in the caldera’s basement is far below previous estimates that had placed the amount at “one to four times greater than the eruptive volume of the largest past caldera-forming eruption.”

The segregated pods of magma Bennington and her colleagues identified have volumes in line with “small-volume post-caldera Yellowstone eruptions.”

While the lobe along the northeastern edge of the caldera indicates that any future eruption would be towards that corner of the caldera, Bennington said the current magma concentration in that lobe is “too small to be able to host a present-day eruption.”