2011 Research Magazine

"The Great Dying"

By Frank Stephenson

On the trail of a 250-million-year-old catastrophe

It's been called "the mother of all mass extinctions."

It happened about 251 million years ago—20 million years before the first dinosaur drew breath.

Earth scientists know it as the Permian mass extinction (so-named because it occurred at the end of Earth's Permian period). Also called "The Great Dying," the massive cataclysm was far more lethal than four other major mass killings recorded in the geologic record, including the one that doomed the dinosaurs.

The event hardly happened overnight—scientists believe the global carnage unfolded over at least a million years. When it was over, an estimated 96 percent of all the planet's marine life, along with 70 percent of all living things on land, was gone forever, never to be replicated. Some scientists believe that it took 30 million years for Earth to fully recover from its effects.

Exactly what caused Earth's worst environmental apocalypse remains a mystery, although most scientists blame an unusually protracted series of voluminous volcanic eruptions that marked the period. The prevailing theory is that this steady, unrelenting release of noxious gases and sun-blocking ash poisoned the atmosphere and made Earth's climate unbearable for most organisms. Untold thousands of species vanished forever—including the largest insect ever known, a one-pound dragonfly with a thirty-inch wingspan. Fossils are all that's left to prove that such amazing animals ever existed.

If this is the case, for geologists the question then becomes: what triggered the most devastating volcanic convulsion in Earth's history in the first place? Theories range from the dramatic geologic repercussions from the forming of Pangaea—once Earth's only continent, itself a product of the Permian period—to the impact of a giant rock hurled from space.

Aleksey Smirnov is an assistant professor of geophysics at Michigan Tech whose latest research reveals new clues into the mystery of the Permian's unprecedented volcanism. He, along with fellow geophysicist and colleague John Tarduno of Rochester University, are among many earth scientists who are fascinated by the remnants of what may be the largest volcanic eruption ever.

This colossal outpouring of lava—estimated at more than 3 million cubic miles of the stuff—solidified into a rocky landmass the size of Western Europe. Because this titanic eruption happened at the same time as the Permian mass extinction, most scientists—Smirnov and Tarduno among them—suspect it was the primary cause of the catastrophe.

Named for its current location (northern Siberia), this vast area is known to scientists as the Siberian Traps (for "trappa," the Swedish word for "stairs"). During its birth, lava flows waxed and waned over time, thereby creating a distinctive, stair-step pattern in the Traps' solid basalt foundation. Such steps are prominent surface features throughout some of Siberia's most rugged terrain.

Heaving the required amount of molten rock to the Earth's surface to build the Siberian Traps required vast amounts of energy. Smirnov, like most researchers, believes that such huge and prolonged bursts of melted rock could only come from one particular type of volcanic activity. This type is essentially a gigantic plume of abnormally hot, molten rock that boils up from the Earth's core and burns its way straight through Earth's mantle.

Thus known as mantle plumes, these curious heat engines—unlike the shifting tectonic plates above them—stay in one place, just like a burner on a stove. Unlike the well-known volcanoes that occur when landmasses collide (e.g. Washington's Mt. Saint Helens), mantle plumes aren't caused by any geologic event that happens near Earth's surface. They spring from depths of up to two thousand miles, forming veritable pipelines for the release of pressurized, super-heated magma.

Mantle plumes dot the globe, and may be best known for creating the Hawaiian Islands, Smirnov said. A map of the ocean bottom around the islands clearly shows a series of submarine seamounts progressing away from land. This is the key signature of mantle plume activity: the creation of volcanic hot spots that can burn in the same place for millions of years. Over time, these stationary hot spots leave a clear track of eruptions as the plumes periodically burn through the drifting continental landmasses overhead.

While most geologists believe that the Siberian Traps were formed by mantle plume activity, there's still hot debate on the topic, Smirnov said. Some scientists argue that the Traps could have been formed from normal surface volcanism and note that no telltale hot-spot track indicating mantle plume activity has ever been found.

Smirnov and Tarduno came up with the idea of using the Earth's magnetic field as a means of studying the question. Embedded in every grain of mineral on Earth are magnetic clues that can be used to find out where, in relationship to the Earth's magnetic poles, the minerals came from. Geologists have invented ingenious techniques that can ferret out data on both latitude and longitude from these rocks, thereby obtaining fairly precise information on where they originated.

After careful analysis of the magnetic characteristics of rock samples collected from all over the North Atlantic, from Iceland to Siberia, the researchers eventually compiled enough data to reconstruct the ancient path that the Siberian Traps took from the time they erupted to their present-day location.

The data clearly showed that the Traps were blown into existence at a spot that now lies off the Norwegian coast in the North Sea. What's more, they realized that the spot coincides with the place where geologists believe a mantle plume created another massive outpouring of lava some 60 million years ago. The same plume is responsible for Iceland and its volcanism.

But why has no hot-spot track ever been found for the Siberian Traps?

"For at least 125 million years, this hot spot lay under a thick Eurasian lithosphere, and it just didn't have enough power to burn through it," Smirnov said. This is the main reason, he thinks, why no traces of the hot spot have been found in the wake of the Siberian Traps.

So far, at least. Smirnov thinks they exist, submerged in little-studied regions of the Arctic Ocean awaiting discovery. He's already planning a new grant proposal aimed at just that.

Meanwhile, Smirnov's findings better define the role that mantle plumes play in shaping our world. Not only are plumes well known for their island-making abilities, but geologists say that they also play important roles in continental rifting and the formation of ocean basins. As for why they exist, scientists also speculate that the plumes serve to keep the Earth's core cooled to a temperature it can handle.

"What we would really like to know now is exactly what causes these plumes, how they work," Smirnov said. "If our model is correct, it suggests that the deeper parts of the Earth's mantle are able to maintain a large chemical or thermal imbalance for hundreds of millions of years, more than twice as long as was previously thought.

"Obviously, there's some sort of thermal anomaly or imbalance going on down there, but it's a complex system that we don't fully understand."

Smirnov's Siberian Traps research was funded in part by the National Science Foundation.