It’s not hard to see why researchers are worried about a Yellowstone eruption. If the supervolcano were to erupt, the consequences would be devastating. The ashfall alone could render vast areas uninhabitable, while the injection of sulfur dioxide into the stratosphere would trigger widespread temperature drops, crop collapse, and famine.

No doubt, Yellowstone erupting would be a global catastrophe.

The timeline is also pretty concerning. Yellowstone had massive eruptions 2.1 million years ago, 1.3 million years ago, and 640,000 years ago, as well as several smaller ones. An eruption in the near future would fit the timeline.

All of that is enough to make scientists nervous about a potential eruption, but we have some good news. According to a new study using a method called magnetotellurics, there’s not enough molten material pooled in one place to trigger a massive eruption anytime soon.

A hot hotspot

Yellowstone is what’s called a hotspot, where large areas of molten material stay in one place as the Earth’s tectonic plates move around. This type of magma is called “basaltic”, and it tends to produce pretty mild eruptions. Hawaii is also a basaltic hotspot, for instance. However, hotspots can also melt the material around them, changing the composition of the lava and producing a type of molten material called rhyolite.

Whereas basalt magma is more liquid, rhyolite is more viscous. It doesn’t flow readily, and when it erupts, it can cause explosive eruptions. This is the type of magma that’s troubling researchers.

The problem is that telling the two lavas apart is difficult, particularly when they’re several kilometers underground.

The most widely used method to survey the Yellowstone subsurface is the seismic method. With this approach, researchers measure how seismic waves travel through the Earth’s crust to infer the composition and structure of the subsurface. Variations in wave speed reveal important details about the presence and distribution of molten rock. For instance, seismic waves slow down when passing through partially molten regions.

This method has provided invaluable insights into the vast magma reservoirs beneath Yellowstone, but it has its limitations.

Seismic data alone cannot fully capture the complexity of Yellowstone’s subsurface. So increasingly, scientists are combining it with other techniques like magnetotelluric imaging, which is highly sensitive to the presence of interconnected melt.

A magnetotelluric view

Magnetotellurics (MT) is a passive geophysical method that uses natural time variations of the Earth’s magnetic and electric fields to measure the electrical properties of the sub-surface. This type of data is particularly sensitive to the presence of melt. For an area like Yellowstone, it’s a great fit.

With this approach, the team of researchers led by Ninfa Bennington from the USGS mapped the molten material under Yellowstone up to a depth of around 50 km.

The study revealed seven distinct low-resistivity anomalies beneath Yellowstone. Each of these anomalies corresponds to regions of partial melt or hydrothermal activity. The vast majority of this is made of basaltic melts, although there are some rhyolitic areas as well as some transition zones.

Crucially, the melt fractions in these rhyolitic reservoirs were well below the threshold considered “eruptive” (around 40%). Instead, these melts are locked in a crystalline “mush,” incapable of generating a major eruption under current conditions.

Don’t count Yellowstone out just yet

A Yellowstone eruption is unlikely in the near future, but things could be heating up.

The largest rhyolitic storage zone lies under the northeast part of Yellowstone Caldera, with a volume comparable to the smallest past caldera-forming eruptions (approximately 400 cubic kilometers). However, the melt fraction remains too low for this magma to erupt.

Interestingly, basaltic melts from the lower crust appear to migrate toward this northeast region. This can provide heat that fuels the overlying rhyolitic reservoirs. In time, this extra heat input could lead to Yellowstone rhyolitic eruptions.

The study also suggests a potential shift in the locus of future volcanic activity. While past rhyolitic eruptions were distributed across the caldera, the northeast region now appears to be the primary site of magma accumulation. This is consistent with broader patterns observed in the region.

In summary, there’s a lot of potentially dangerous molten material near the current caldera, but it’s spread too sparsely to trigger a major eruption. This is pretty much what researchers were expecting in the Yellowstone system.

This finding has significant implications for hazard assessment. Although a major Yellowstone eruption is not imminent, the northeast caldera’s active magma system warrants close monitoring. The interaction between basaltic and rhyolitic magmas could, under certain conditions, lead to localized eruptions or increased hydrothermal activity.

The study was published in Nature.