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Drilling to the Mantle


An international effort to drill holes in the ocean floor has been chugging along since 1966. I simply call it the greatest science program in history because it's had four different names over the years, but maybe its next big initiative will make it a household name, like the Apollo Program that reached the Moon. The plan is to drill a borehole all the way through the Earth's crust and into the mantle beneath. And that's not a stunt.

Why Drill to the Mantle?

The mantle is not an unknown zone like the Earth's core. You could say the mantle was discovered in 1909, when Andrija Mohorovicic published a paper showing that seismic waves were being warped by a strong discontinuity several tens of kilometers deep in the Earth. Mohorovicic's discontinuity, universally known as the Moho (because pronouncing "mo-ho-ROV-it-shitch" can be difficult), is considered the boundary between crust and mantle.

Since 1909 we have carried out thousands of high-temperature, high-pressure experiments on rocks and minerals. We have a hundred years of seismic records that allow us to probe and map the mantle. We have a nice body of theory ready to test, including plate tectonics and detailed knowledge of the world's lavas, which all arose from partial melting of rocks from the mantle.

And we have lots of natural samples of mantle rock that were brought to the surface in various ways, although every one is altered to some unknown extent. The one thing we don't have is a single piece of rock representing what it's truly like down there, one that's guaranteed not to be altered. That is to say, after all this time we still don't really know what the Moho means. Just as genuine Moon rocks allowed us to start making sense of the Moon, a big array of geologic evidence will snap into focus once we can get a few pristine samples of the mantle.

Why Drill in the Ocean?

It is much easier to drill boreholes on land than at sea, and we've been doing that since the 1850s. The deepest hole yet drilled, the Kola Superdeep Borehole, was on land; after almost 20 years it reached 12.3 kilometers below the surface. But that wasn't anywhere near the mantle: the Moho is much deeper on land. The Kola hole got only one-third of the way to the mantle, which in that area is about 35 kilometers down.

Despite the difficulties in taking a drilling rig to sea and making boreholes in the ocean floor from a floating platform, the thinness of the crust makes the offshore option best for reaching the mantle. Unlike the continents, the oceanic crust is only a few kilometers thick in some places. For more detail, see The Earth's Crust.

The oceanic crust has a simple structure, judging from ophiolites, which are considered to be fragments of oceanic crust thrust up on land by plate-tectonic events. And our theoretical picture of the oceanic mantle is much simpler than our ideas about the continental mantle. There are likely to be fewer surprises.

Why Haven't We Done It Before?

The international seafloor drilling program has never tried to reach the mantle. It began in 1966, as the Deep Sea Drilling Project (DSDP), with the much more modest aim of simply mapping the rocks of the seafloor crust and collecting cores of the mud on top of the rocks. Between the DSDP and its successor programs, the Ocean Drilling Program (starting in 1985) and the Integrated Ocean Drilling Program (starting in 2003), thousands of holes have been drilled and their contents retrieved and studied.

It was challenge enough to do that, without trying to pierce through the whole crust. And there was knowledge enough being generated:

  • The sediments are detailed records of ocean environments and ancient climate going back as far as 180 million years.
  • The rocks are records of plate tectonics in action; indeed DSDP cores were a key early proof of plate tectonics.
  • Offshore structures that hold oil and gas were discovered and mapped for the first time.
  • The drillships pioneered the technology used by the petroleum industry and other offshore companies.

But in fact there was an earlier effort to drill to the mantle, before the DSDP. It was an American project, funded by the National Science Foundation at the time of the Cold War space race, called Project Mohole. As a child at the time, I was thrilled to read about it, and I wasn't the only person whose imagination it captured.

Project Mohole used an experimental barge with a drilling rig that was towed to a spot in the Pacific near Guadalupe Island, west of Baja California, in early 1961, for its first phase. It succeeded in drilling almost 200 meters through the seafloor mud and retrieving samples of the rock beneath, which as expected was basalt lava.

That was the proof of concept. The next stages just needed funding from Congress—and you can guess the rest. The whole thing ran aground by 1966. When the DSDP began shortly afterward, the planners stuck to what was feasible, and had lasting success.

Why Do It Now?

As of October 2013, the Integrated Ocean Drilling Program will end and the long-running endeavor will take on its fourth name, the International Ocean Discovery Program (so the abbreviation IODP will not change). The Japanese drillship Chikyu, which began operations for the IODP in 2005, is the world's most advanced ship of its type. Chikyu has done breakthrough work, including drilling through a megathrust fault of the type that caused the 2004 Sumatra and 2011 Tohoku earthquakes. It can handle the conditions of a real Mohole.

Now everything is ready for another try. The new IODP ten-year science plan for 2013–2023 has given a mantle-sampling mission its highest priority. Another high-priority mission will be finding out just how deep life extends into the oceanic crust. Even the deepest rocks we've sampled from below the seafloor have microorganisms living in them—not many, but not none. So we can meet two goals with one project—assuming that life does end somewhere above the Moho. I'm looking forward to what the new IODP will learn.

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