“We live on a hunk of rock and metal that circles a humdrum star,” Carl Sagan declared in 1996, in one of the astronomer's last interviews.
It’s a simplistic description of Earth, but for a long time many scientists would have gone along with it. There’s no way to explore our planet’s interior directly; the deepest hole ever drilled, the Kola Deep borehole in the Russian Arctic, reaches only 0.2 percent the way to the center. So even the best scientific maps didn’t look much better than your middle-school textbook cartoon showing an outer crust, an inner core and a thick layer called the mantle in between.
But that picture is changing. Researchers such as Barbara Romanowicz at the University of California, Berkeley are using seismic (earthquake) waves to scan our planet’s innards, much like doctors use ultrasound to peer inside patients. What they’re seeing is full of complex detail — not a bland mineral hunk, but a rich and dynamic inner landscape.
The mantle appears to be layered like an onion, with major transitions 250 miles and 410 miles down. At the 410-mile level, researchers recently identified a tremendous interior mountain range, with peaks perhaps even taller than Mount Everest. “Recently, we’ve discovered another change at about 1,000 kilometers [600 miles] depth,” Romanowicz says.
The mantle blobs are of special interest because of their impact on life on the surface. Recent work by Maria Tsekhmistrenko, a seismologist at the University of Oxford, confirms the blobs as a source of the hot mantle plumes — and those plumes can trigger devastating supervolcanic eruptions when they surface.
Her research has found detailed connections between the African blob and the La Reunion mantle plume (currently under Reunion Island, east of Madagascar) that unleashed a wave of eruptions in what is now India 67 million years ago, delivering a one-two punch with the big asteroid impact to kill off the dinosaurs.
The nature of the mantle blobs is a persistent mystery, however, largely because the seismic waves studied by Tsekhmistrenko and her colleagues reveal our planet’s inner structure only indirectly. Waves move faster or slower depending on the temperature and composition of the material they pass through. The waves can then be detected by instruments that measure ground motion at the surface, and analyzed to interpret the structures the waves passed through.
Unfortunately, this approach cannot easily distinguish warm, dense material from cooler, lighter stuff. That leaves a lot of room for interpretation.
Sanne Cottaar, a University of Cambridge geophysicist who has mapped the blobs extensively, thinks they’re masses of high-density rock that sank to the bottom of the mantle early in Earth’s history. “They sit on top of the hotter core and will thus heat up over time. The heat then makes them less dense,” she said in an email.
The blobs act like a lid on a pot — until heat from the stove causes what’s inside to boil and spill out, sometimes catastrophically.
That conclusion jibes with Romanowicz’s emerging view of the inner Earth as a series of overlapping, onion-like layers. Scientists used to picture the mantle as a single unit, slowly churning under the influence of heat from the core. A tidy, up-and-down circulation tugged at the crust, they concluded, moving continents and reshaping oceans.
“There is more and more a kind of understanding in the community that it's not just that simple,” Romanowicz says. Instead, each layer of the onion seems to have its own history and evolution. The 410-mile transition is associated with an abrupt change in the structure of the rocks in the mantle. At the 600-mile zone studied by Romanowicz, rising mantle plumes seem to bend and deflect, as if they’ve run into a wall: “That one’s yet to be understood.”
The layers below the mantle show crazy complexity, too. The outer core is a molten iron alloy that flows as readily as water in the ocean. “It also circulates at speeds similar to the ocean and is very turbulent,” Cottaar says.
At the very center of Earth is the inner core, a 760-mile-wide ball of iron. Seismic studies show that it consists of iron crystals that are kept solid by tremendous pressure even though it is hotter than the surface of the sun. It’s the youngest major structure inside the planet, having begun freezing out of the liquid part of the core less than a billion years ago. Adding to the weirdness, the inner core rotates slightly faster than the rest of the planet.
“There are many, many questions remaining about the structure of the inner core,” Romanowicz says.
The ultimate goal is to connect all of Earth’s onion layers to the way our planet formed, which in turn will help explain how it became the life-friendly world it is today. David J. Stevenson, a planetary scientist at the California Institute of Technology, traces the story back to a moment 4.5 billion years ago when Earth was struck by a planetary body the size of Mars.
The debris kicked up by that collision is thought to have made the moon, but it also remade our planet.
“The giant impact may have melted or stirred part of the deep Earth. The resulting separation of liquid from solid, and core material from mantle material, set up the state that has evolved to what we see,” Stevenson says. Core, mantle blobs and layering may all derive from that early state.
Romanowicz wants to explore more of our planet’s inner history by building the Pacific Array, an ocean-based network of motion detectors that would provide a new way to watch seismic waves passing through Earth’s insides. “It’s something that some of us have been working on for 30 years,” she says. “We need it to illuminate the lower mantle.”
For Stevenson, the ultimate dream is to explore downward in the style of Jules Verne. At one point, he even floated the idea of building a probe that could travel all the way to the core. “It was a tongue-in-cheek idea, but it sure would be nice to have something to sample the mantle properly,” he says. “I subscribe to the view that you don't know a place until you go there. ‘You’ being a robot, of course.”