How Push Comes to Shove
A massive blob of heated rock is rising beneath Africa, pushing the huge continent upward as if it were a floating cork.
Across the globe, beneath the Pacific, another monster seethes. This one poured out so much lava in a massive burp 120 million years ago that it raised the ocean floor by a mile and a half.
What is causing these strange dips and swells, these huge, odd bulges in the Earth? “It’s really one of the grand mysteries of geology,” said Michael Gurnis, a Caltech geophysicist who has spent two decades studying the Earth’s interior.
Now, through the use of a global network of seismic sensors and high-powered computers, scientists are focusing on one possible explanation: “superplumes”--rising, continent-size bubbles of scalding rock so massive they make the volcanic plumes creating Iceland and Hawaii look like dripping faucets.
By tracking earthquake waves as they travel through the Earth, scientists have been able to peer deeper and with far greater detail into the innards of the planet--a kind of geological CT scan. Earthquake waves travel differently through areas of different temperature and pressure. They slow in hotter, less dense materials and speed up in cold, dense rock. Some pass through molten rock; others do not.
The latest scans have provided the sharpest pictures yet of the strange structures lurking beneath Africa. They reveal a rising column of rock stretching over thousands of miles and reaching to the planet’s core.
The structures are so large that their existence could recast some basic aspects of geology. Not only do they explain why the Earth is shaped the way it is, they also could help geologists better understand the beating heart of the planet: the heat engine that fuels the shifting plates on the surface that build mountains, shake cities during earthquakes and uncork volcanic flows.
Powerful as it is, the brute geological force of plate tectonics cannot explain all of the Earth’s ups and downs, and why the continental crust under Africa is so much hotter and higher than it should be. The superplumes also may help explain how planets without tectonics, such as Mars and Venus, spawn massive volcanoes.
But the closer they look, the stranger superplumes get.
One of the newest snapshots, published this month in the journal Science, shows that the African superplume is not an amorphous blob but has surprisingly sharp edges.
“The fact that it’s almost like a wall really was quite different than what we expected,” said Gurnis, who co-wrote the paper with Donald Helmberger, who is the director of Caltech’s seismological laboratory, staff scientist Sidao Ni and graduate student Eh Tan.
Although scientists don’t know for sure, the sharp sides suggest that the plume is not merely made of hotter material but may be an entirely different type of rock surging its way to the surface.
Is it left over from the origin of the Earth? A byproduct of plate tectonics? A result of violent interactions between the metallic core and the rocky mantle?
“To be honest with you, we really don’t know what it is,” Gurnis said.
The fact that such colossal objects could go so long without being well described illustrates just how shallow our knowledge of the deep Earth remains. “Right now we know embarrassingly little about why the plates are doing what they’re doing,” Helmberger said. “We haven’t progressed beyond the cartoon stage.”
The problem is that even now there is no way to peel back the Earth’s skin and look through the 4,000 miles of rock and metal layers below. “It’s not easy knowing all this from being so far away,” Helmberger said.
The seismic method for scanning the Earth’s interior has been used for a century. Early results were too crude to provide much in the way of detail. For years, many findings were derided as “fuzzy business” and attacked as artifacts--byproducts of the complex statistical work used to analyze the results.
In the last five years, an expanded network of global seismic stations has helped cut through the fog, although scientists still have to wait for earthquakes to occur in just the right spots. “In medical tomography, you put sensors all over,” said Barbara Romanowicz, director of UC Berkeley’s seismological lab. “We sometimes wait decades for an appropriate data set.”
It can be worth the wait. In April, Romanowicz and graduate student Yuancheng Gung published a paper in Science suggesting that the structure under Africa was a coherent superplume. Previous work had shown a structure deep within the Earth and one near the surface but could not prove they were the same body.
“Even though the resolution is not that great, [seismic snapshots are] giving us a clearer picture,” Romanowicz said.
The images however, still are not clear enough to resolve the controversies concerning these stone behemoths of the deep.
Gerald Schubert, an expert on planetary interiors who chairs the department of Earth and space sciences at UCLA, remains skeptical about whether the “super-size structures” exist.
While he lauds the work of the seismic imagers, he thinks the hot regions that have been detected could be caused by something else, such as a cluster of many smaller plumes. He added that there is no way to tell if the rock is rising, and he questions those who see the superplumes as powerful sculptors of the surface.
“I believe in mantle plumes,” he said. “I’m not so sure I believe in what you’d call a superplume.”
Kent C. Condie, a geochemist at the New Mexico Institute of Mining and Technology, also disputed the interpretation of the pictures as superplumes. Condie, the author of “Mantle Plumes and Their Record in Earth History,” said the so-called superplumes do not have the characteristic wide-headed, thin-tailed shape of geological plumes.
They probably are simple upwellings, he said, large areas of rising rock that are a normal result of convection in the planet’s hot interior. The implication is that they would be a largely benign force.
Even Romanowicz concedes that superplumes may not turn out to be so super after all. “A big debate in the community is whether all plumes are created equal,” she said.
But there is evidence beyond our planet weighing in favor of some massive force beyond the twisting and shifting of surface plates. The evidence? Olympus Mons on Mars, the largest volcano in the solar system. Plate tectonics couldn’t have created the volcano--the size of Arizona--because Mars has no moving plates. Victor Baker, a planetary scientist at the University of Arizona thinks the volcano could be born of a Martian superplume.
Venus is another planet covered with a scarred and volatile volcanic surface. Scientists who examined the planet by radar in the early 1990s were stunned to discover that it too had no plate tectonics. “This was a surprise. It made people think of plumes,” Baker said.
The strange activity on our neighbor planets suggest there may be more than one way to sculpt a planet.
“Maybe plate tectonics isn’t the final answer in understanding how our planet behaves,” Baker said.
