Scientists to drill Earth’s mantle

Richard A Lovett

Miles-deep drilling could yield Earth equivalent of moon rocks, experts say.

It may not be a journey to the center of the Earth, but it could be the closest thing yet.

Scientists are planning to drill all the way through the planet’s miles-thick crust to Earth’s deep, hot mantle and retrieve samples for the first time. The samples, they say, would rival moon rocks for sheer scientific import - and be nearly as hard to get.

“That has been a long-term ambition of earth scientists,” geologist Damon Teagle told National Geographic News.

But a lack of suitable technology and insufficient understanding of the crust have long tempered that ambition.

Now, better knowledge of the Earth’s shell and technological advances - for example, a Japanese drill ship equipped with six miles (ten kilometers) of drilling pipe - have put the goal within reach, according to a commentary in this week’s issue of the journal Nature, co-written by Teagle, a geologist at UK’s University of Southampton.

Even so, drilling into the mantle would be ‘very expensive’ and would require new drillbit and lubricant designs, among other things, according to the paper.

But if all goes as planned, drilling could begin by 2020, Teagle said. As soon as next month, the team will begin exploratory missions in the Pacific, where crews will “bore further into the oceanic crust than ever before,” the paper says.

Mantle holds clues to quakes and Earth origins

Between Earth’s molten core and hard, thin crust, the roughly 2,000-mile-thick (3,200-kilometer-thick) mantle contains the vast bulk of Earth’s rocks. But we don’t know much about them, because all we have are bits that have come to the surface via volcanoes or been trapped in ancient mountain belts.

But all these mantle samples no longer really represent mantle conditions and makeup, since they’ve been altered in the long process of coming to the surface, so they providing only tantalizing glimpses of what lies below, scientists say.

Drilling would tell scientists not only what the mantle is like, but also reveal the nature of the Moho layer, a shadowy transitional layer at the base of the crust.

“We know what happens to seismic waves as they cross the Moho, but we don’t know what it is,” Teagle said.

Scientists would also be able to look for signs of life in the deep crustal rocks.

“Wherever we’ve looked, up to 120 °C (248 °F), we’ve seen evidence of microbial activity,” Teagle said. “We would certainly test that on the way to the mantle.” But the big prize is the mantle itself.

Getting a sample, he said, would tell us much about the Earth’s origins and history. Mantle rocks would also provide insight into how current mantle processes operate - highly important in understanding the plate tectonics that drive many earthquakes, tsunamis, and eruptions, he added.

Deep ocean, shallow crust

The best place to drill, Teagle said, is in the mid-ocean, because that’s where Earth’s crust is thinnest - only about four miles (six kilometers) thick, versus tens of miles deep in continental regions.

But the mid-ocean, is, of course, still deep - about 2.5 miles (4 kilometers) in the targeted areas. That’s nearly twice the depth reachable by today’s offshore drilling techniques, Teagle said. So far, drills have penetrated only about 1.2 miles (2 kilometers) into undersea crust.

And while the seabed is cold, the drill would have to be able to reach into a zone where temperatures would hit 570 F (300 C) and pressures would mount to 2,000 atmospheres - equivalent to more than 4 million pounds per square foot (21 million kilograms per square meter).

“There are deeper drill holes than this,” Teagle said, “but they have been done on land or into relatively soft sediments.”

There’s no danger of a blowout, such as the Gulf oil spill, because there are no oil and gas deposits in the mid-ocean for the drill to accidentally penetrate, he added. Nor would the mantle rocks suddenly erupt out of the hole, since the channel would be narrow and mantle rocks aren’t molten.

“There is a risk of failure in that the hole could collapse,” he said, “but there is no perceived environmental risk.”

National Geographic News