It’s no secret that family trips to Yellowstone National Park are likely to involve arguments in the back seat, but you may not know that (adult) scientists find plenty to argue about there, as well.

Yellowstone is actually just the present manifestation of a family of volcanic events going back almost 20 million years. The textbook explanation for this is that Yellowstone sits atop an example of a “mantle hot spot”—a deep plume of hot rock that rises to the surface of a tectonic plate, periodically punching a line of eruptions as the plate moves. But some scientists have proposed more complex scenarios in recent years.

For example, a study we covered just a few months ago concluded that a region of hotter, shallow mantle pulled in from beneath the Pacific by the tectonic collision with North America could explain Yellowstone and other volcanic features in Western North America.

The primary reason it’s difficult to settle this debate is that efforts to produce CT-scan-like imagery of the Earth have struggled in the deeper mantle. Because mantle plumes are fairly slender, it’s possible for them to simply escape detection. So while a textbook Yellowstone mantle hot spot plume has failed to appear in these images, some have argued that shouldn't rule out its existence.

Watching the waves

In a new study, University of Texas at Austin researchers Peter Nelson and Stephen Grand treated this like a resolution problem. Since there is no CT machine big enough to slide our planet into, researchers used seismic waves from earthquakes to do this imaging. By measuring the arrival of seismic waves at many different seismometers, slight differences in arrival times can paint a picture of the material those waves passed through.

The problem with detecting deep mantle plumes is that the seismic waves typically used are travelling horizontally through a thin, vertical plume, which means the seismic waves aren’t affected much by the plume’s presence. To get around this, the researchers used a different type of seismic wave.

There are two different types of seismic waves ("P" and "S" waves) that travel through the interior of the Earth, but those waves bend and reflect as they encounter different layers, resulting in a cacophony of different sets of waves that seismometers pick up after an earthquake. One set of waves (called "SKS" and "SKKS") travels through the liquid outer core of the Earth before bending upward toward the surface. And waves passing more vertically though the deep mantle have a better chance of interacting with a mantle plume strongly enough for it to show up in the imagery.

Using a dense network of seismometers recently installed across much of the US, the researchers used waves generated by 71 earthquakes to image the mantle beneath the Western US. And lo and behold, something popped out beneath Yellowstone: an anomalous cylinder about 350 kilometers across. It stretches all the way from Yellowstone to the bottom of the mantle, directly beneath the California-Mexico border.

That’s actually exactly what you would expect to see from a textbook mantle hot spot plume. Because the rock in the mantle is slowly flowing, a hot spot should be tilted in the direction of the flow, like chimney smoke in a breeze. The cylinder also stands out most clearly at its base, as it is initially around 750 degrees Celsius hotter than the mantle around it; it's less anomalously warm higher up. Put it all together, and the researchers are pretty confident they’ve successfully settled the question of whether a true mantle hot spot lies beneath Yellowstone.

That doesn’t mean all competing explanations should go in the bin, though. While most mantle hot spots are found beneath oceans—like the one responsible for the Hawaiian Islands—the situation beneath Yellowstone is more complex. Where continental and ocean plates collide, the ocean plate dives down beneath the continent. This puts a set of obstacles in the mantle plume’s way. Other studies have explored the effects of those obstacles, which could explain why the pattern of eruptions in the Western US has been so complex.

The location of the mantle hot spot plume imaged in this new study presents its own as-yet-unsolved puzzle, too. Mantle plumes are generally rooted on special structures at the bottom of the mantle called large low-shear-velocity provinces that are mostly found beneath the middle of ocean basins. The location of the Yellowstone plume is nowhere near one of these, so something else must be responsible for its existence.

The researchers are hopeful that their technique could be used to detect other previously invisible hot spot plumes around the world. But since most hot spots are beneath oceans, this may still be held back by a lack of a sufficiently crowded seismometer network in the region to get that high-resolution picture of what lies beneath.

Nature Geoscience, 2018. DOI: 10.1038/s41561-018-0075-y (About DOIs).