Every so often, a seemingly high number of earthquakes strike the Yellowstone National Park region. Whenever this happens, with an inevitability rivaling Old Faithful itself, the Internet takes the opportunity to remind anyone with a clicking finger that Yellowstone sits above a massive reservoir of magma capable of globally destructive mayhem.
A recent example comes from this characteristically muted Daily Mail piece (titled “Is it About to Blow? Yellowstone Supervolcano is Hit by 878 Earthquakes in Just Over TWO WEEKS”) from 28 June 2017:
A swarm of nearly 900 earthquakes have hit Yellowstone National Park since 12 June, according to experts. The park sits on one of the world's most dangerous supervolcanoes and recent activity has raised fears the supervolcano is about to blow.
If it were to erupt, the Yellowstone volcano would be one thousand times as powerful as the 1980 Mount St Helens eruption, experts claim - although they say the risk is 'low'.
That there are frequent earthquakes in Yellowstone should come as no surprise. There are thousands of detectable earthquakes in the Yellowstone region in any given year. The entire park sits above a shallow pool of magma fed by an enigmatic hotspot deep within Earth’s mantle. The iconic hot springs and geysers — a result of the interaction between constantly flowing groundwater and the heat of magmatic activity — punctuate a massive volcanic wasteland that was once formed through the wholesale collapse of entire mountains into the depths of the Earth, as described by the USGS:
The Yellowstone region has produced three exceedingly large volcanic eruptions in the past 2.1 million years. In each of these cataclysmic events, enormous volumes of magma erupted at the surface and into the atmosphere as mixtures of red-hot pumice, volcanic ash [...], and gas that spread as pyroclastic [...] flows in all directions.
Rapid withdrawal of such large volumes of magma from the subsurface then caused the ground to collapse, swallowing overlying mountains and creating broad cauldron-shaped volcanic depressions called “calderas.”
Those three eruptions can accurately be described as cataclysmic. The 2.1 million year old Huckleberry Ridge eruption stands as one of the five largest single eruptions known to our planet, and it left behind a 60 mile wide caldera. A smaller, but still earth-altering eruption occurred 1.3 million years ago. The most recent super eruption, 640,000 years ago, created what we now refer to as the Yellowstone Caldera, a massive crater 35 miles wide and 50-miles long.
So-called super-eruptions like these can have global consequences, altering weather patterns, disrupting agriculture, and temporarily cooling the globe. As a precaution, the USGS and other scientific organizations have developed efforts to monitor predictive signs that such an event could be imminent.
The Yellowstone Volcano Observatory monitors in real-time seismic activity, land deformation (from GPS and satellite measurements), and thermal changes or chemical signals from the gases being released (from surface detectors).
While the observatory witnesses a plethora of tectonic and volcanic activity on a yearly basis — the earthquakes it detects are not evidence of an impending cataclysm, or even an impending eruption. Due to the constant heat and consequent resettling earth, caused by the magma chambers below the surface, their occurrence is natural. While earthquakes would certainly occur in the leadup to an eruption, there are many other things that scientists think would also happen in concert.
Most importantly, there would be signs—based on the monitoring of volatile gases, ground deformation, and heat measurements, of a massive and shallow pool of a specific kind of magma pressurizing under the surface:
Each of Yellowstone’s explosive caldera forming eruptions occurred when large volumes of “rhyolitic” magma accumulated at shallow levels in the Earth’s crust, as little as 3 miles (5 km) below the surface. This highly viscous (thick and sticky) magma, charged with dissolved gas, then moved upward, stressing the crust and generating earthquakes. As the magma neared the surface and pressure decreased, the expanding gas caused violent explosions.
The process that brings this gassy magma up to the surface is a time consuming one, thanks to the fact that the magma rising from the hot spot is more dense than the continental crust it must push through. The process of rising basaltic magma, which occurs over tens of thousands of years, leads to a buildup of trapped magma pressurized by gases which could trigger a massive eruption. Such an event won’t happen overnight, however.
The most recent characterization of the magma below revealed a shallow magma chamber (which does not have enough material to create a super-eruption) and a second larger, deeper magma chamber below. This lower chamber, which is connected to the shallow one, technically does have enough material to create such an eruption, but this specific two-chamber setup makes for poor apocalypse producing conditions. The authors argue:
Lower-crustal basaltic melts are not expected to contribute to the caldera forming eruptions, and the upper-crustal melts are unlikely to erupt at one time, both [magma chamber] volumes can feed smaller eruptions.
Indeed, there have been a number of minor lava flows since the most recent calamity. Such flows, as well as hydrothermal explosions, are the most likely explosive events to occur at Yellowstone, according to the USGS.
However, the absence of any evidence for large scale trouble brewing in the immediate magma chamber below—shaking ground notwithstanding—has led the USGS, Yellowstone Volcano Observatory, and the volcanology field in general to conclude that the risk of a caldera-forming apocalypse at Yellowstone in the next couple of thousand years is “exceedingly low.”
Our progeny will reassess at a later date.