Drilling To The Mantle Of The Earth

Fifty years ago, scientists attempted to drill deep through ocean crust to the Earth's mantle, an endeavor called "Project Mohole." That project failed, but scientists are sharpening their drill bits again. Geologist Damon Teagle talks about what boreholes may reveal about the Earth's formation.

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IRA FLATOW, host:

This is SCIENCE FRIDAY. I'm Ira Flatow.

Remember way back in the days of playing in the sandbox, when you were trying to dig a hole to China? Yeah, you know, you thought if you dug long enough, eventually you'd come out the other side. Now, the kids are curious about this stuff. They want to know what's down there. And as it turns out, scientists want to find out, too.

Some science - some kids just never grow up. They become scientists. And deep under the Pacific Ocean, 500 miles off the coast of Costa Rica, they're drilling - drilling deep down through the Earth's crust into hard, crystalline rocks, layers upon layers of solidified magma. They're headed for the Earth's mantle.

Everyone know what the mantle is? We're going to talk about what the mantle is and why they're headed for that, and what they hope to learn there. They still got a ways to go to reach it. They're only about a third of the way there. What do they want to find out? If oil rigs, for example, can drill eight, nine, 10 miles into the seafloor for wells, what's so challenging about drilling a core just a few miles deep? Well, we're going to find out.

Damon Teagle is a professor at the University of Southampton, based at the National Oceanographic Center in Southampton in the U.K. He joins us by phone.

Welcome to the SCIENCE FRIDAY, Dr. Teagle.

Dr. DAMON TEAGLE (University of Southampton): Yeah. Good afternoon.

FLATOW: Yeah. Good afternoon to you. What is the mantle?

Dr. TEAGLE: The mantle is the next layer down in our planet. We live on the crust, and then the crust encircles our whole planet. But the mantle is a layer that's about 3,000 kilometers thick - so, you know, 2,000 miles thick - principally made of magnesium silicate. And that's actually the largest proportion of our planet.

FLATOW: Is the mantle.

Dr. TEAGLE: Yup.

FLATOW: Wow.

Dr. TEAGLE: Yup.

FLATOW: And so you want to go down there to just sample it?

Dr. TEAGLE: Yes. Well, I mean, I don't think we can actually go and visit it...

(Soundbite of laughter)

Dr. TEAGLE: ...so we're going to have to do this with a drillship and to drill there. But, yes. I mean, it is - all of the rocks from the crust at one stage were produced by partially melting the mantle. So, actually, it's the most important chemical reservoir of our planet.

So if we really want to understand how our Earth has evolved over its history since its formation, then we sort of need to have a very precise view of the chemical composition of the mantle.

FLATOW: Mm-hmm. Now, I remember from my childhood - I remember there was Project Mole Hole that they tried to get down there, but they never did. Was that back in the 1960s?

Dr. TEAGLE: Yes. Well, I mean, one reason for this article we've just published in Nature magazine, which is just a comment, is to mark the 50th anniversary of Project Mole Hole, which was beautifully illustrated by John Steinbeck, actually, in Life magazine, back in 1961.

FLATOW: The author.

Dr. TEAGLE: Yes. Who was - I think a near neighbor in Monterey Bay of Willard Bascom, who was sort of the chief engineer assigned to -scientist involved with the project. And Steinbeck was actually aboard the ship, this drilling barge CUSS I, when they attempted to do the drilling.

But back in 1961, you know, this was before plate tectonics was just a little bit nascent theory at that stage - certainly, before it was widely accepted. And they didn't really have much idea about how this worked, or even understand sort of the layering within the oceans that people had observed from looking at seismic waves, looking at earthquake waves and looking at seismic waves, such as the oil companies do.

FLATOW: Mm-hmm. 1-800-989-8255 is our number, if you'd like to talk about drilling into the mantle. And, in fact, we have a little mini recreation that Flora Lichtman put together, a little mini mole hole on our website, if you'd like to take a look at our video on our website at sciencefriday.com about how you drill into the mantle.

Is - why is it so difficult, and why is it - as I said before, if oil companies can drill so many miles down, what's the big deal about drilling into the mantle?

Dr. TEAGLE: Yeah. This is the big - the big difference that we're, you know, because that scientists who are interested in the formation and evolution of the ocean crust and also about the nature of the mole hole - which is this boundary between the crust and the mantle and the mantle itself - is that we're drilling into very hard, crystalline rocks, rocks that formed from the crystallization of magma. Whereas the oil companies when they're drilling, they're drilling into sedimentary rocks that have sort of been laid down in the oceans over the eons and captured organic material that eventually evolved to form petroleum and gas.

So actually, the rocks we're drilling into are the rocks that sit beneath the sedimentary reservoirs that host the oil and gas. So they can drill deeper holes, but they're not drilling into such hard rocks.

FLATOW: Mm-hmm. And so what does the core look like that you hope to bring up?

Dr. TEAGLE: The core will - when things are going well, we tend to sort of retrieve these effectively long sticks of rock that are about three inches across. And we tend to - we do what's called wireline drillings. So we will advance the drill bit about, say, about 30 feet or so, and then recover the core after 30 feet of advancement. So, you know, when it's going well, it all comes out in big, long sticks. When it's not going so well, it comes out as little sort of crushed up bits of rock.

FLATOW: Mm-hmm. And so this purely basic research, then. There's really nothing practical that's going to come out of this.

Dr. TEAGLE: Well, you never know what's going to be practical, do you? But the - yes, I mean, the fundamental drivers of this are about understanding how the planet works, how plate tectonics work and how our planet has evolved.

FLATOW: Mm-hmm. Why, if we're on a plate and there is plate tectonics, why doesn't the drill shaft shift and shear off that little drill bit that you have on the end?

Dr. TEAGLE: Well, because plate tectonics is operating reasonably slowly. You know, we're talking centimeters a year of movement. And, of course, that's one of the things we don't know, because we don't - if we - we're - aim as to drill through, you know, the six kilometers of the ocean crust into a, you know, a few hundred meters into the mantle - the mantle itself, of course, is solid. But we don't actually know the coupling between the mantle rocks and the crust, whether they move together or whether they've always moved together, or one has moved faster than the other and dragged the other along. That's one thing we'd like to find out.

FLATOW: Let's go to the phones. 1-800-989-8255. Aladdin(ph) in San Francisco.

Hi. Welcome to SCIENCE FRIDAY.

ALADDIN (Caller): Yeah, hi. My question I've always wanted to know is why when you - as you dig into the mantle, as you get deeper, it gets hotter. The temperature gets hotter. Yet when you go into the ocean, the deeper you get, the temperatures get colder.

FLATOW: All right. Good question.

Dr. TEAGLE: I'm sorry. So you're saying why does it get hotter...

FLATOW: Yeah. Why does it get hotter as you get deeper into the mantle?

Dr. TEAGLE: Well, it gets hotter as we go inside the Earth, as well, doesn't it? I mean, this is - I mean, the oceans are relatively well mixed compared to the Earth on the sort of timescales. But, you know, suddenly, the crust gets hotter because there's more radioactive decay, and also there's a lot of heat coming from very, very deep in the Earth from the operation of the geodynamo, and also, through mantle convection, hot rocks, you know, brought up a very slowly from deeper than the Earth to near the - to the bottom of the crust.

FLATOW: Mm-hmm. Let's go to Randy in Tallahassee.

Hi, Randy.

RANDY (Caller): Hi. My question is: How do you know that the mantle is uniform throughout the perimeter of the Earth? How do you know that when you get a core sample, that you won't just be getting a snapshot of that particular area where you took the sample?

FLATOW: Good question.

Dr. TEAGLE: Well, that's an excellent question. Yes, I know. Of course, you're completely right. And it is unlikely to be - I think what I was saying before is that this discontinuity, this seismic boundary called the Moho was all around the planet. And it could be that the Moho was not the same everywhere.

In terms of getting samples of the mantle, well, we sort of had to start somewhere. It is unlikely that it will - the composition will be identical everywhere. And one of the things we would like to know is the, sort of, the scale of the variations of these chemical heterogeneities, because we can - and we have - you know, scientists have dredged rocks from - along the spreading wedges, particularly where the ocean plates are spreading apart very, very slowly. And they see that there is certainly chemical heterogeneities, you know, chemical variations, but we - but they very rarely get those rocks in any context. So we don't actually know the scale of those sorts of variations. So that's, yeah, definitely something we would like to find out.

FLATOW: Good question, Randy. Thanks for calling.

RANDY: Okay. Thank you.

FLATOW: If you've never been down there, how do you know what is down there?

Dr. TEAGLE: Well, we have, you know, snippets of information. You know, we know a lot about the nature of the center of the Earth principally from seismic experiments, from the fact that the Earth has a magnetic field. And also, we do get some small chunks of mantle rocks that are brought to the surface and - by volcanoes called mantle xenoliths or mantle nodules.

And also, there are parts of - you know, many of your listeners might be familiar with serpentinized rocks, these, sort of, green, slimy rocks that you see in mountain belts and also - they also crop out on the ocean floor, where either through tectonics or through a mixture of tectonics and hydrothermal alteration, such their reactions with seawater would brought mantle rocks into the crust. So, you know, we know the basic structure of the crust. What we don't know is the really precise details.

FLATOW: Let's go to Andrea in Overland Park, Kansas. Hi.

ANDREA (Caller): Hi. Good afternoon, Doctor.

Dr. TEAGLE: Hello.

ANDREA: Hi. I was wondering if you could speak to any advancements or innovations that you've made with actually coring this hard, crystalline rock at these depths, and especially at 30-foot length in the wireline system. And I'll take my answer off the air.

Dr. TEAGLE: Yeah.

FLATOW: Okay.

Dr. TEAGLE: Yes, okay. So, you know, there's two projects that we've discussed in our article. One is, you know, a expedition of the Integrated Ocean Drilling Program that's going back to this relatively deep hole in the Eastern Equatorial Pacific next month. The other is this desire to drill all the way to the mantle. So that second project is going to be far more technologically demanding than our current ability. So our sort of understanding - from discussions with engineers is that this is just no longer, you know, sort of, fantasy, and it could be done given, sort of, investment and, sort of, political and scientific will to do it.

So there's things that would need to be done. We would need to have a -some form of riser-like(ph) system - same as, sort of, the oil petroleum companies use - that would work in very, very deep water. And we're talking about 4,000 meters or so. We'd also need to have a drilling equipment, muds and lubricants, as well as wireline geophysical tools that will operate at very, very high temperatures, you know, sort of, 300 degrees or - Centigrade or more, and at these very high pressures, which are probably about 2,000 atmospheres, at least.

So, you know, the geophysical sorry. The geothermal industry already has equipment and has, you know, have actually somewhat accidentally drilled all the way into magma chambers in Iceland. So this technology can work on land. It would need some significant adaptation to work in the oceans. But it's, you know, maybe not completely beyond human endeavor to do this.

FLATOW: We're talking about drilling into the mantel this hour on SCIENCE FRIDAY, from NPR.

I'm Ira Flatow. We're talking with Damon Teagle. Let's see if we can get a phone call or we have a question from Second Life, saying: What materials are the drill bits made of? What do you drill with? What is...

Dr. TEAGLE: The kind of drill bits are it's a little bit - actually adapted industry bits, and they are generally made of hardened steel with, sort of, roller cones of tungsten carbide. Ideally, when drilling hard rock, you'd want to have some and if you were doing this on land, you would have your drill bits impregnated with diamonds.

The difficulty with the use of that in the oceans is that the ship that you're drilling from moves up and down with the waves, and that actually makes the, you know, the motion of the drill bit on where it's trying to cut, so it keeps on moving the bit up and down off the surface and tends to make diamond coring very difficult. So we've have to use these, sort of, industry drill bits. But, certainly, there could be advancement there like, sort of, developing, sort of, substances harder than tungsten carbide.

FLATOW: One last question for you: How do you know when to stop drilling?

(Soundbite of laughter)

Dr. TEAGLE: Probably when the money runs out.

(Soundbite of laughter)

Dr. TEAGLE: Yes, I mean, our aim is to drill, you know, a number of hundred meters into the mantle. And what we would probably try to do is you can kick off the drill holes and direct - you know, do directional drilling, and actually try and penetrate the lowermost crack in the Moho and into the mantle in a number of different places. But, you know, just - but just, you know, one sample would be a revolution.

FLATOW: It would be. So you don't have an aim of how far down you can go, what percentage you want to enter and how deep you'll be satisfied with?

Dr. TEAGLE: Well, yes. It's funny you mention that because, you know, people are already talking, well, if we do this, what's next? I think the core is some distance away, but...

(Soundbite of laughter)

FLATOW: You need a different drill bit for that one, I think.

Dr. TEAGLE. That's right. But also, you know, your earlier caller made a very important point. It's unlikely that this Moho boundary is going to be the same everywhere on the planet. So if we can do one, why not two, you know?

FLATOW: Yeah. As you say, just limited by the amount of money you can get. Who...

Dr. TEAGLE: And the desire and, you know, this would also probably require people spending, you know, one to two years of their life above one spot in the ocean. So...

FLATOW: So who is funding this?

Dr. TEAGLE: Well, it will be, you know - assuming this goes ahead and the planning all works out and technology is developed, it would be under the umbrella of the Integrated Ocean Drilling Program, which is a consortium of 24 or so nations around the world, principally supported by the United States and the Japanese, but with contributions from Europe and - as well as China and India and other places.

FLATOW: Well, good luck to you.

Dr. TEAGLE: Okay, thanks very much. Good to be with you.

FLATOW: Happy drilling.

Dr. TEAGLE: Okay, goodbye.

FLATOW: Damon Teagle is a professor at the University of South Hampton, based at the National Oceanographic Center. That's at South Hampton in the U.K.

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