“The revolutions and changes which have left the earth as we now find it, are not confined to the overthrow of the ancient layers” - Georges Cuvier, 1831.

Our planet Earth has extinguished large portions of its inhabitants several times since the dawn of animals. And if science tells us anything, it will surely try to kill us all again. Working in the 19th century, paleontology pioneer Georges Cuvier saw dramatic turnovers of life in the fossil record and likened them to the French Revolution, then still fresh in his memory.

Today, we refer to such events as “mass extinctions,” incidents in which many species of animals and plants died out in a geological instant. They are so profound and have such global reach that geological time itself is sliced up into periods—Permian, Triassic, Cretaceous—that are often defined by these mass extinctions.

Debate over what caused these factory resets of life has raged ever since Cuvier’s time. He considered them to be caused by environmental catastrophes that rearranged the oceans and continents. Since then, a host of explanations have been proposed, including diseases, galactic gamma rays, dark matter, and even methane from microbes. But since the 1970s, most scientists have considered the likely root cause to be either asteroid impacts, massive volcanic eruptions, or a combination of both.

Those asteroid (or comet) impacts have captured the public imagination ever since 1980, when Luis and Walter Alvarez found global traces of iridium, which they inferred to be extraterrestrial, at the geological boundary that marked the disappearance of the dinosaurs. The identification of the Chicxulub impact crater in Mexico soon after sealed the deal. Impacts have been proposed to explain other mass extinctions, but there’s very little actual evidence to support those links. In the words of researchers David Bond and Stephen Grasby, who reviewed the evidence in 2016: “Despite much searching, there remains only one confirmed example of a bolide impact coinciding with an extinction event.”

Not just a random series of unfortunate events

Volcanism, on the other hand, has coincided with most, if not all, mass extinctions—it looks suspiciously like a serial killer, if you like.

This isn’t your regular Vesuvius/St. Helens/Hawaii style volcanism. It’s not even super-volcanoes like Yellowstone or Tambora. I’m talking about something far, far bigger: a rare, epic volcanic phenomenon called a Large Igneous Province or “LIP.”

LIPs are floods of basalt lava on an unimaginable scale: the Siberian Traps LIP, which erupted at the end-Permian extinction, covers an area the size of Europe. It’s estimated that over 3 million cubic kilometers of rock were vomited onto the planet’s surface, The end-Triassic Central Atlantic Magmatic Province, stretching from Canada to Brazil into Europe and West Africa, was just as large. Others are similarly gigantic.

In the words of Bond and Grasby, “Four of the ‘Big Five’ extinctions are associated with LIPs—too many to be mere coincidence —implying that large-scale volcanism is the main driver of mass extinctions.”

Even the extinction of the dinosaurs at the end of the Cretaceous was simultaneous with the Deccan Traps LIP in India. It’s possible that the combination of the Chicxulub asteroid impact and the Deccan eruptions, rather than just the impact, pushed life over the edge. And recent evidence points to a LIP trigger for the second phase of the end-Ordovician extinction, the one missing from Bond and Grasby’s quote. If confirmed, that would link LIPs to all five of the Big Five extinctions.

For decades, the sheer size of LIPs and the wide error margins in attempts to put dates on rock formations led geologists to suspect that LIPs erupted slowly over millions of years; any associated extinctions could easily be just coincidence. But in the last four years, improved rock dating techniques have shrunk those error margins, revealing two important things: LIPs erupt in intense pulses that are geologically fast (tens of thousands of years), and they often coincide precisely with mass extinctions.

Seth Burgess, a geochronologist from the US Geological Survey, told me about his observations while dating part of the Karoo-Ferrar LIP in Antarctica:

“Every single rock I dated from the Ferrar—and we’re talking up the mountain, down in the ravine, from one side of the continent to the other along the Transantarctic Mountains—they’re all 182.6 million years old. It's every single rock the same. It gives me a great sense of it’s all in one shot. It’s not a big slow prolonged event.”

Burgess used the new dating techniques to show that the Siberian Traps LIP was also quick, and it happened at precisely the same time as the end-Permian mass extinction—Earth’s most severe. “We dated the first magmas to spread laterally into the shallow Siberian crust and think these magmas are the culprit,” he said. “This spread happened fast and at precisely the same time as the extinction.”

As someone told me years ago, there’s a lot of time in deep time. Yet the LIP and the extinction happen at exactly the same time, even though the gaps between these eruptions are millions or tens of millions of years. That seems enough to declare the LIP a smoking gun behind that extinction.

This is true for multiple LIP-extinction links. Precise matches have been confirmed for the mid-Cambrian, the end-Triassic, the Toarcian, and others. And it isn’t just a date match. Volcanic nickel and mercury have been found at several extinction-aged locations, including for the Ordovician and Cretaceous events.

So if our serial killer is the volcanism associated with an LIP eruption, when will it strike again?

To answer that, we need to find what causes the planet to hemorrhage lava on such a scale. And for that, we need to look deep into Earth’s mantle.

Bond & Grasby Palaeo3 2017

Kaiqing Yuan, UC Berkeley

Chimneys of apocalypse

Seismologists like Barbara Romanowicz and Scott French of UC Berkeley do exactly that—look deep into the mantle. They use the vibrations from large earthquakes around the world to illuminate the inside of our planet and take pictures, rather like a medical ultrasound.

Their images reveal fat mantle plumes, regions of hot rock as wide as France, rising like chimneys through the mantle. Today, they fuel relatively benign hotspot volcanoes like Hawaii and Iceland—tourist attractions rather than global apocalypses. But evidence suggests that LIPs were also fed by mantle plumes. The plumes responsible for LIPs must have been something far more potent.

In their quest to understand what could switch these plumes into killers, seismologists and mineral physicists are searching for the driving force that produces mantle plumes. The Earth’s molten core supplies heat that drives the motion of mantle material, like a burner heats a pot of water, so it makes sense to focus on the roots of plumes at the core-mantle boundary. There, seismologists have discovered blister-like patches with properties that hint that molten metal might be leaking from the core.

Earthquake waves passing through those patches slow dramatically, giving them their name: “Ultra-Low Velocity Zones” or “ULVZs.” As a result, the seismic waves are bent, like light through thick glass. The patches seem to be confined to the roots of plumes and have been confirmed to reside beneath Iceland, Hawaii, and Samoa so far. Their seismic slowness suggests they might contain molten rock. While the mantle behaves a bit like a fluid, the pressures there ensure that rock stays solid until relatively shallow depths.

“What’s special about these ULVZs is they are also very fat!” Romanowicz told Ars. “They seem to be 800km in diameter at the core-mantle boundary—we can’t say very precisely. It’s still a mystery what they are. I think [it] is partial melting, but exactly what their role is, how long they have been there, this is something we need to investigate further.”

Catherine McCammon, of the University of Bayreuth in Germany, and Razvan Caracas, a mineral physicist from the University of Lyon, have been investigating the properties of ULVZs by looking at how rocks behave under the conditions that are thought to be present at the core-mantle boundary. “There are not too many people that do this type of experiment,” explained McCammon. “You need a synchrotron, so this makes it a rather exclusive group of people.”

The synchrotron that she is referring to is a particle accelerator three times the size of a football stadium, which generates X-rays 100 billion times brighter than those from a hospital X-ray machine. The X-rays are blasted through mineral samples compressed and heated to recreate conditions at the core-mantle boundary. Data from the X-rays track the vibrations of the materials’ atoms, which allows us to measure the seismic wave speed through those samples. Razvan, by contrast, uses quantum mechanics to calculate the theoretical seismic wave speed of those same materials. The difference between the theoretical and measured results suggests there’s molten material in ULVZs. “We think it’s some degree of melt,” said Catherine. “Either partial melt, or metallic iron melt that came from the core.”

Other scientists have seen hints of liquid moving in ULVZs, and a core-derived melt might explain why some diamonds contain microscopic traces of iron-nickel alloy—the material that makes up the core. If ULVZs are indeed patches where molten core leaks into the mantle, perhaps Earth’s core has a role in turning plumes into mass killers. But core leakage is not supported by hot-spot lava chemistry, and there is no clear evidence for any material from the core ever making it to Earth’s surface in a plume, so ULVZs remain an enigma for now.

Perhaps the ‘special sauce’ that turns plumes into killers is much closer to the Earth’s surface.