Journey to the Mantle of the Earth

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This 1961 effort to drill right through Earth’s crust didn’t succeed. Half a century on, geologists are ready to try again. Journey to the mantle of the Earth On the 50th anniversary of the first attempt to drill into Earth’s mantle, Damon Teagle and Benoît Ildefonse say that what was once science fiction is now possible.

etrieving a sample of Earth’s mantle mantle, now known as the Mohorovičić the Apollo lunar rocks, giving insight into the has been an overarching ambition discontinuity, or ‘Moho’ for short. This origins and evolution of our planet. But this of the geoscience community for boundary marks the start of the bulk of has proved as difficult, perhaps more difficult, Rmore than a century. In 1909, the Croatian Earth’s interior, which extends from the base than going to the Moon. So far, no one has meteorologist Andrija Mohorovičić noticed of Earth’s crust — at 30–60 kilometres under drilled deeper than about 2 kilometres into that seismic waves travelling below about the continents but just 6 kilometres under the ocean crust, or a third of the way to the 30 kilometres underground move faster the thinner crust of the oceans — to the core mantle. The first effort to drill into the mantle, than those above that depth, indicating a 2,890 kilometres below. ‘Project Mohole’, foundered in a geopolitical fundamental change in the composition Drilling down and retrieving samples quagmire and did not achieve its goal. and physical properties of the rocks. He had directly from the mantle would provide A new Mohole campaign is now under discovered the upper boundary of Earth’s scientists with a treasure trove comparable to way, thanks to improved technology,

oceanic crust was booming. Project Mohole proved that scientific drilling into the ocean basement was possible. This contributed to the establishment, and continuance over four decades and running, of international collaboration in scientific ocean drilling. The Integrated Ocean Drilling Program (IODP;

PRINCETON UNIVERSITY LIBRARY PRINCETON www.iodp.org) and its predecessors, the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP), form arguably the most successful, long-term international scientific collaboration in any field. THE DEEP FRONTIER The mantle holds some 68% of the planet by mass. Its sheer volume makes an accurate knowledge of its composition and variability essential for understanding how Earth was formed and has evolved. Almost all of Earth’s surface crust — the material that makes up the sea floor and the continents — originally Harry Hess, a founding father of the theory of plate tectonics, explains Project Mohole. came from the mantle. Some pieces of the mantle have been a better understanding of the rocks far Back then the nascent offshore petroleum thrust up to Earth’s surface during tectonic below our feet and a deeper appreciation of industry had not yet begun to contemplate mountain building, where they are avail- the challenges of drilling through them. Over deep-water drilling. The Mohole project able for study. Other mantle pieces, encased the next few years, geophysical surveys will required the development of new technolo- in lava, have been ejected from volcanoes, be conducted at three Pacific Ocean loca- gies such as dynamic positioning, which and sea-floor spreading has brought some tions that are in contention to be the site of would allow a drill ship to keep its position to the ocean floor. These pieces show that the first deep hole to the mantle (see ‘Drill- steady. The group obtained funding from the the mantle is composed mainly of rocks ing sites’). Drilling down to the mantle will US National Science Foundation and com- called peridotites, made of magnesium- require a huge amount of ship time and will missioned the best ship available for the job: rich, silicon-poor minerals such as olivine be very expensive — far more expensive than the drilling barge CUSS 1, named after the oil and pyroxene. They also suggest, together a current single drilling expedition, although companies that had developed it, Continen- with far-field seismic measurements, that far cheaper than a Moonshot. But if funding tal, Union, Shell and Superior. Within four the mantle’s composition varies from place can be found, and the scientific commitment years of the project’s proposal, propellers had to place, but the extent of this variation maintained, drilling could begin within the been installed on the side of CUSS 1 and a remains unclear. The available samples have decade, and be completed within 15 years. In system developed that allowed these to keep all been chemically the meantime, next month, we will be leading the ship in position. “Drilling to altered by the pro- an expedition to the Pacific to bore further Between March and April 1961 scientists the mantle cesses that brought into the oceanic crust than ever before. took their first core from the uppermost is the most them to the surface hard rock of the oceanic crust, or ‘basement’, or by exposure to sea INSPIRED IDEA challenging off Guadalupe Island in the eastern Pacific endeavour in water. Concentrations The first serious plans to drill down to the Ocean, thanks to the daring and innovative of many of the key ele- mantle were concocted in the late 1950s by engineering efforts of Willard Bascom and the history ments and isotopic a handful of post-war American geoscience his colleagues. From beneath 3,800 metres of Earth tracers (including grandees under the guise of the American of water and 170 metres of sediment, they science.” water, uranium, tho- Miscellaneous Society — an informal group pulled up a few metres of basalt, at a cost of rium, lithium, carbon, of US National Academy of Science mem- US$1.5 million (about $40 million in 2009 sulphur, silicon, potassium, the noble gases bers, sometimes referred to as a ‘drinking dollars, in terms of its share of the total US and the iron oxidation state) that might be club’. The idea came primarily from Harry economy). This remarkable accomplish- useful in reconstructing Earth’s evolution are Hess, one of the founding fathers of the ment was reported in Life magazine (14 April highly labile. A few kilograms of fresh peri- theory of plate tectonics, and Walter Munk, 1961) by the novelist and amateur ocean- dotite from beneath the crust would provide who pioneered studies of how winds drive ographer John Steinbeck, who was aboard a wealth of new information. ocean currents and explained why one CUSS 1 during these first operations. Getting to the mantle requires drilling side of the Moon is locked towards Earth. This was the only ocean core that Project through a full section of oceanic crust, which Frustrated by what they saw as a stream of Mohole succeeded in drilling. After the will also be a boon for geologists. The forma- worthy yet pedestrian research propos- expedition, the management of the pro- tion of crust is the foundation of the plate- als in their field, they sought to undertake ject changed, some poor decisions were tectonic cycle. It is the main mechanism by something more ambitious and innovative. made about which drilling technologies to which heat and material is dredged up from At a wine breakfast at Munk’s home in pursue, and costs spiralled out of control. the interior of Earth, resurfacing some 60% of La Jolla, California, on a Saturday morning In 1966, Project Mohole collapsed when the our planet every 200 million years. Currently, in April 1957, they came up with Project US Congress voted to cancel its funding. the thermal, chemical, and perhaps biological, Mohole, a scheme to drill, for the first time, Nevertheless, the project coincided with a exchanges occurring deep in the oceanic crust right through Earth’s crust and into the growing acceptance of plate-tectonics theory. remain poorly understood because of the lack upper mantle. Interest in the formation and evolution of the of direct observations in situ.

The technology to drill a hole a few inches and close to recent volcanic activity that at least another 400 metres, and recover for wide through 6 kilometres of crust now might have chemically altered the mantle the first time gabbros from the lower crust, exists or is feasible to develop1,2. A promising and perturbed the overlying crust. All the which will be the deepest types of rock ever candidate technology is on the Japanese drill- sites are in the Pacific Ocean because the extracted from beneath the sea floor. (The ing ship Chikyu, launched in 2002. The vessel crust there is formed faster than in other hole itself is not the deepest ever drilled into has a riser system: an outer pipe surrounds oceans, which makes for the simplest, most the sea floor; that was Hole 504B, which the drill string — the steel pipe through uniform basement architecture. Seismic and reached 2,111 metres below the bottom of which cores are recovered. The drilling mud geological studies hint that fast-spreading the eastern Pacific off Colombia.) and cuttings are returned up to the vessel in ocean crust should be relatively uniform and The mission should help to settle many the space between the two pipes. This helps to conform most closely to textbook models. debates: how crust is formed at mid-ocean recycle the drilling mud, control its physical We hope to find material that conforms to ridges; how magma from the mantle is properties and the pressure within the drill these models of a simple crust: a layer cake intruded into the lower crust; the geometry hole and helps to stabilize the borehole walls. of rock types called lavas, dikes and gabbros. and vigour of how sea water can pull heat It also means that cuttings can be evaluated Meanwhile, the ocean-drilling commu- from the lower oceanic crust; and the contri- for scientific purposes. Chikyu is a giant ship, nity continues to core as deeply into the bution of the lower crust to marine magnetic capable of carrying 10 kilometres of drilling crust as is feasible using the conventional, anomalies. We will still be 3.5 kilometres pipes, and is equipped for riser drilling in non-riser technology available on the drill- shy of the Moho, but the project will pro- 2.5 kilometres of water. ing vessel JOIDES Resolution — the recently vide further impetus for, and confidence in, Over the next decade, researchers and rebuilt workhorse of the IODP. deep ocean crust drilling, as well as crucial engineers will have to design and develop information for planning a full penetration new drill bits, lubricants and wireline instru- THE LOWER CRUST through the Moho and into the upper mantle. ments to make coring into the mantle pos- Fifty years after John Steinbeck sailed on Drilling to the mantle is the most sible, at pressures as high as 2 kilobars and CUSS 1 with the pioneers of ocean-crust challenging endeavour in the history of temperatures up to 300 °C, and beneath drilling, we are the co-chief scientists on Earth science. It will provide a legacy of about 4 kilometres of water. In particular, an expedition to obtain for the first time fundamental scientific knowledge, and this will require a riser system capable of a section of the lower oceanic crust — the inspiration and training for the next genera- going deeper than Chikyu’s current equip- material lying just above the mantle. IODP tion of geoscientists, engineers and technol- ment, or a different mud-circulation system, Expedition 335 is due to sail from 13 April ogists. There is a surprising level of interest with part of the equipment installed on the until 3 June3. from the world’s media and engagement by sea floor. The site chosen for this mission is on the the general public in this frontier endeavour. To find the best site for the new Project Cocos plate off the coast of Costa Rica (ODP And it may be only the beginning of a bigger Mohole, a large number of factors need to site 1256). This site is in ocean crust that project. As the crust and the mantle are both be considered. Ideally, it would be in the formed superfast — at more than 20 centi- likely to be different from place to place, we shallowest possible water overlying oceanic metres a year, much faster than any present- will ultimately want a number of such holes. crust, which means going as close as possible day crust formation. That makes the upper That may seem a distant dream, but ultra- to the mid-ocean ridge where new crust is crust there much thinner than elsewhere, deep drilling will only get more routine and formed. It should also be in the coldest pos- so it is possible to reach the lower portions cheaper with time and experience. sible crust, which is away from the ridge. without having to drill very deep. Three pre- As Harry Hess told a US National Acad- These constraints limit the possible sites vious expeditions to Hole 1256D have drilled emy of Science meeting in April 1958, when to three — off the coasts of Hawaii, Baja down to more than 1.5 kilometres below the defending the first Mohole Project against California and Costa Rica, respectively. All sea floor, into the transition zone between detractors: “Perhaps it is true that we won’t have pros and cons. The site near Hawaii, for dikes and gabbros4,5. find out as much about the Earth’s interior example, is the coolest, but also the deepest, This spring we plan to deepen the hole from one hole as we hope. To those who raise that objection I say, if there is not a first hole, there cannot be a second or a tenth or a hun- DRILLING SITES dredth hole. We must make a beginning.” ■ Three areas are under consideration for drilling into the mantle. One includes the original Project Mohole drilling site. Another includes a site (ODP site 1256) where scientists will drill this year into the lower crust. SOURCE: REF. 1 SOURCE: REF. Damon Teagle is at the National Oceanography Centre, University of ORIGINAL PROJECT MOHOLE Southampton, Southampton SO14 3ZH, 30º Proposed Mohole UK, and Benoît Ildefonse is at Géosciences site: Baja Montpellier, CNRS, Université Montpellier 2, 34095 Montpellier cedex 5, France. 20º e-mails: [email protected]; benoit. Proposed Mohole [email protected]

Longitude site: Hawaii 1. Ildefonse, B. et al. Sci. Drill. No. 10, 56–62 (2010). 10º 2. Workshop Report Writing Group Sci. Drill. No. 11, Proposed Mohole 53–57 (in the press). site: Cocos plate ODP SITE 256 3. Teagle, D. A. H., Ildefonse, B. & Blum, P. IODP Exp 335 Scientific Prospectus doi:10.2204/iodp. 0º sp.335.2010 (2010). –160º –150º –140º –130º –120º –110º –100º –90º –80º 4. Wilson D. S. et al. Science 312, 1016–1020 (2006). Latitude 5. Teagle D. A. H. et al. Proc. Integrated Ocean Drilling Program 309/312 Expedition Reports (IODP Management International, 2006). –6,000 –4,500 –4,300 –4,100 –3,900 –3,700 –3,500 –3,000 –2,000 –1,000 0 Water depth (metres) Further reading accompanies this article online at go.nature.com/b2ehus.