Ocean drilling program

The series of ocean-drilling efforts called the Ocean Drilling Program (ODP) revolutionized understanding of the Earth's structure, climate, and available minerals. It also allowed researchers to collect data about the Earth's cosmological history.

Mohole Project

Scientific drilling on the ocean floors began almost as a stunt, but it evolved into a long-term research program that revolutionized geology, contributed vital clues about climate change and about the extinction of the dinosaurs, discovered a major new type of subsea hydrocarbon deposit that may fuel the world, and aided in the study of rich metal ores that may also be tapped. Along the way, scientific drilling pioneered many techniques that have been used in offshore drilling for petroleum and natural gas.

Drilling samples have been a major part of geology since the second half of the nineteenth century. Drillers seeking water or oil could sample pieces of rock drilled from varying depths and log them into their drill records. For scientific purposes, drilling with circular drills allowed the cutting of a long cylinder or “core” that could be pulled up and carefully measured for position and composition. By the middle of the twentieth century, hard-rock miners were using cores to sample for minable ores. Data from these drillings were correlated into three-dimensional maps of distinctive strata showing dips and faults.

In the late 1950s, a number of geologists envied the space program with its romantic goal of flight to the moon. In 1959, they proposed a similarly dramatic program called the Mohole Project. In 1909, Andrija Mohorovičić had analyzed seismic waves from earthquakes and concluded that the rock of the crust changed significantly about 16 to 40 kilometers below the surface as it changed to partially melted mantle. There was speculation that rocks might be different at this so-called Mohorovičić discontinuity, or Moho, and researchers proposed drilling all the way to the Moho for samples. Drilling such a tremendously deep hole would be expensive and maybe impossible. Several marine geologists suggested that oceanic drilling to the Moho would be cheaper because the crust is thinner under the ocean floor. Also, a drill core through the sea floor might yield a complete fossil record of tiny marine shells.

Drilling into the ocean floor required several major innovations. The drilling locations were in deep water, so the drilling platform had to be an oceangoing vessel rather than a tower resting on the bottom. Anchoring in those depths would be difficult, so the vessel had to actively maintain position. This “dynamic positioning” had to keep the drill ship within two ship lengths of straight, or the drill string would break. While out of sight of land, the drilling vessel crew had to navigate within this small area with no landmarks whatsoever. This was managed originally by taut, moored buoys and later by satellite position-finding and acoustic beacons on the sea floor. Since the drill bit would hang as much as several thousand meters below the drill ship, bottom-hole assemblies were required for getting the drill started and for returning to the same hole. Finally, because the drill platform rose and fell with the waves, the rig needed a heave compensator.

Project Mohole started with engineering tests in 1961 in waters off California by Cuss I, a drilling barge developed for offshore oil drilling and built by Global Marine. (A number of famous drill ships from this company begin with the abbreviation “Glomar.”) Initial tests off Baja California, Mexico, were promising, but the project eventually ended when technical difficulties and political management problems caused projected drilling costs to increase significantly. It was later discovered that the Moho outcrops at the Earth's surface in certain areas and that most areas of the ocean are comparatively young geologically, so continuation of the Mohole project would have been of less use than its backers had hoped.

However, the possibility of deep-ocean drilling had been demonstrated. Oil drillers, who had previously worked in depths of fewer than 100 meters, saw new possibilities. Project Mohole stirred interest in dynamic positioning for oil and gas exploration; the offshore drilling industry subsequently developed many technologies that passed back to the scientific drillers. Oceangoing rigs allowed exploratory drilling in deep areas of the continental shelf before making multibillion-dollar investments in production platforms. Thus, the Mohole project was a major factor in opening many offshore oil fields.

Furthermore, the Mohole demonstration had obtained one major data point. A mysterious lower layer visible on sonar graphs was found to be not another layer of sedimentary rock but rather basalt, an igneous rock formed by recrystallization of molten rock and often associated with volcanoes. This discovery suggested that the oceans were younger than expected and that the Mohole project, as originally planned, would have been able to gather minimally useful data about the geological history of the Earth. Conversely, it gave support to the then-radical theory of continental drift.

Testing continental drift required not one deep hole to the Mohorovičić discontinuity but many shorter holes surveying many areas. These shorter holes were still a tremendous advance. Before oceanic drilling, the only data about the sea floor came from dredge hauls and piston cores. Dredges only pull a jumbled mass of material from the first few centimeters of the sea floor. A piston core is essentially a weighted pipe that is allowed to fall as fast as possible to the sea floor; its weight and momentum drive it into soft sediment. Its limitations are that it penetrates only a few meters and that it cannot penetrate hard surfaces.

Joides

In 1964, the Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES) was formed; it has become an international organization that includes universities and government research organizations. In April and May of 1965, JOIDES used the drill ship Caldrill I to test upgraded methods by drilling six holes on the Blake Plateau off the coast of Florida to sub-bottom depths of more than 1,000 meters. Based on that success, JOIDES proposed an eighteen-month program of scientific drilling in the Atlantic and Pacific Oceans to the US National Science Foundation. The resultant Deep Sea Drilling Project was operated by the Scripps Institution of Oceanography. The DSDP drill ship Glomar Explorer was capable of drilling 760 meters in 6,100 meters of water. It began operations in July 1968 and ultimately made ninety-six voyages (or drilling legs) for JOIDES that focused on sites on or near the mid-ocean ridges. The cores retrieved revolutionized geology by proving the theory of continental drift.

In 1985, the Glomar Explorer was replaced by the JOIDES Resolution run by Texas A&M University as part of the one-third internationally funded Ocean Drilling Program (ODP). The ship was designed to drill in 8 kilometers of water for a total drill-string length of 9 kilometers, and it can drill 2 kilometers into the sea floor in shallower waters. Both the Glomar Explorer and the JOIDES Resolution worked in concert with remote instruments and piloted instruments in a number of revolutionary developments. By 2003 the JOIDES Resolution had conducted 110 ODP expeditions at 2,000 ocean-basin drill holes around the world.

Building on the achievements of DSDP and ODP, the Integrated Ocean Drilling Program ((I)ODP) started operations in 2003. Comprised of twenty-six participating nations, the IODP financed drilling platforms, including a revamped JOIDES Resolution, the Japanese deep sea drilling vessel Chikyu, and specialized, mission-specific platforms. These platforms conducted fifty-two expeditions in previously undrilled areas of the Earth’s subsurface. In October 2013, the IODP changed its name to the International Ocean Discovery Program: Exploring the Earth Under the Sea, but kept its acronym, IODP, unchanged. As of 2025, twenty-one nations were funding the organization’s Science Plan,Illuminating Earth’s Past, Present and Future. The plan’s scope of inquiry included climate change, deep life, planetary dynamics, and geohazards.

Drilling in marine strata has yielded tremendous advances in geological knowledge for several reasons. First, oceans cover more than 70 percent of the Earth, so a proportionate number of discoveries should be expected from oceanic data. Second, tens of thousands of drill cores have been logged on land, so many of the land discoveries have already been made. Third, land is often subject to erosion, so there are more gaps than in marine strata. Fourth, land strata have been subjected to more compression, heat, and chemical attack—all of which could confuse possible geological data—than seafloor strata. Fifth, some of the mechanisms that happen under several kilometers of water in the oceans happen under several kilometers of rock on land, so these mechanisms are easier to study with marine drilling. Finally, drilling on land requires disassembly and reassembly of equipment, often in difficult terrain; a drill ship simply sails to the next drill site.

In 1620, Francis Bacon noted similarities between the coastlines of South America and Africa and suggested that they had once been interconnected. In the early twentieth century, Alfred Wegener proposed the former supercontinent of Pangaea to explain similar fossils from areas now widely separated. However, these theories lacked a provable mechanism for moving entire continents through the rock underlying the ocean floor.

It was then noted that the Mid-Atlantic Ridge had bands of different magnetic orientation on either side. These bands corresponded to reversals of the Earth's magnetic field that occur every several thousand years. It appeared that new melted rock was crystallizing (and thus freezing a weak magnetic field) at the ridge and was then shoved away from the ridge by new rock. The crystallizing rock that cooled and hardened out of magma would quite likely be basalt. The Mohole finding of basalt in its deeper cores was encouraging, but conclusive proof required correlating the age from a large number of drill samples. The initial Glomar Explorer drill cores allowed geologists to concentrate on that dating using marker fossils and radioisotopic dating. Data from the cores showed that the ocean floor approaching the Mid-Atlantic Ridge was progressively younger. Thus, it appeared that Europe, North America, Africa, and South America were joined until sometime between 200 and 170 million years ago when the Atlantic Ocean began opening.

This revolutionary discovery led to the development of plate tectonics theory, which proposes that large plates of connected rock exist on the Earth's surface. Plate tectonics has many implications. Because the Earth cannot expand indefinitely, the idea that new rock accretes along mid-ocean ridges means that plate material must be disappearing elsewhere. That explains the existence of deep ocean trenches (such as the Mariana Trench) where oceanic strata are diving steeply down toward the mantle, and some of the heated rock spurts up, forming island arcs. One plate riding over another provides the mechanism for raising up mountains, such as the Andes Mountains, which are riding up over the edge of the Pacific plate. A more extreme example is the Indian subcontinent, which is burrowing under the Tibetan Plateau and lifting the Himalayas.

Continued study of plate tectonics along the mid-ocean ridges and other plate boundaries led to the unexpected discovery of hydrothermal vents, which are somewhat like geysers and hot springs but which are found on the ocean floor; however, the scale and results of the activity surprised researchers. In 1965, a sample of rocks dredged from the sea floor yielded rock samples of two types. One was depleted in certain chemicals that were present in the other type. It was also noted in the 1960s that thermal readings in sediments near plate boundaries did not have as much heat as expected. Hydrothermal vents were suggested as the mechanism for moving the large amounts of heat. Seawater percolating down to the molten rock beneath the sea floor would be heated and return to the ocean as hot springs. The heated water would be much less dense than the near-freezing seawater near the ocean floor, so convection would drive the process.

In 1977, researchers in a piloted submersible discovered an active field of hydrothermal vents with “black smokers,” where plumes of superheated water meet colder waters and dark minerals begin to precipitate out and form “chimneys.” This discovery revealed the importance of such vents in circulating heat from the Earth's core, circulating minerals into and out of the ocean waters, and sustaining life driven by chemosynthesis (chemical energy) rather than photosynthesis (light energy).

However, only drilling could supply data on subsurface events. For example, bacteria living by chemosynthesis have been found several hundred meters below the ocean floor, suggesting that such life-forms may be more biomass-based than photosynthesis-based life. Also, drilling in mounds built by successive chimneys revealed large amounts of carbonate salt that eventually dissolves when the area cools. Thus, old, inactive hydrothermal fields might have rich mining potential. Finally, the sulfide-enriched upper layer may be underlain by a copper-enriched layer, also with good mining potential. More importantly, comparable areas of former seabed have been lifted up and become land. These areas, such as the Klamath Mountains of northwestern North America and areas of Cyprus, have already served as mining areas, and better knowledge of the mineral-formation processes will probably improve land mining long before any ocean mining.

Marker Fossils

Probably the most important dividends from the ODP are carefully logged and partially analyzed cores from every ocean except the Arctic. By the end of the twentieth century, there were more than 240 kilometers of cores available for new studies and new instruments. These cores have helped provide increasingly fine chronological correlation among strata from various locations. Oceanic core data allow the identification of “marker fossils” that (ideally) lived for a short time but over a wide area. Those marker fossils can then be used to date match with other oceanic cores and land cores. This correlation is crucial for tracking individual layers of rock through folds and faults. The dated and mapped layers allow one to calculate what might have happened in the past, what might happen in the future, and where ores may exist at present.

The types of fossils in the cores are generally tiny microfossil remains of plankton that lived in the surface waters. The types of fossils indicate the climate in those waters when they were deposited. Likewise, percentages of carbon and oxygen isotopes vary with sea surface temperature. Data from isotopes and microfossils can be checked against one another and against climate data from land to calculate past climates (paleoclimatology). With better estimates of past climate, climatologists may better project possible climate change in the future.

Ocean drilling provides another climate indicator: sediments from land. Drilling confirmed massive catastrophic flows when ice dams broke at the end of the last ice age, releasing torrents of water down the Columbia and St. Lawrence River Basins. Drilling also shows the timing, direction, and quantity of windborne sand and dust from places such as North Africa and Central Asia. Cores around Antarctica have corroborated that Antarctica's continental ice sheet has existed for millions of years; however, continued studies are needed to detail fluctuations of ice level.

Drill cores also confirmed that turbidity currents (dense masses of sediment-laden water) are a major mode of deposition in the ocean. (The low relief of the abyssal plains in the Atlantic Ocean, which has a high amount of sediment compared to total area, is caused by turbidity flows filling in low areas.) Turbidities indicate past ocean currents, another paleoclimate factor.

Petroleum and natural gas traces in deep waters are another major discovery of ocean drilling. However, the Glomar Challenger and JOIDES Resolution had to stop drilling whenever they encountered such traces. These craft have room for laboratories because they were not fitted with a “riser,” which collects anything coming out of the drill hole and pumps down drilling mud. The riser can be used to stop a flow of fluid from a borehole. Therefore, the riserless drill ships risked causing an uncontrolled leak into the ocean if they continued drilling in areas with any sign of petroleum or natural gas.

Methane Hydrates

Still, another hydrocarbon deposit researched by ocean drillers is methane hydrate. Scattered reports of flammable ice and fizzing material brought up by piston cores led to a theory that methane (natural gas) might be frozen into the structure of ice in deposits beneath the deep ocean floors. These deposits could form because the waters are only slightly above the freezing point of fresh water, and they have high pressure—the two requirements for methane hydrate. The methane source would probably consist mostly of the decay of biological material in sediments. Sonar readings even showed layers in seafloor sediments that might be such hydrates.

Drill cores from several sites confirmed extensive methane hydrate deposits, and their analysis led to several conclusions. Hydrates may contain several times as much energy as all other fossil fuel deposits combined. Hydrates might act as a cap rock to contain conventional gas and petroleum. It might be some time before hydrates are exploited because they are often spread thinly in sub-seafloor ocean strata. Hydrates might be a major factor in climate change because disturbances on the ocean floor or lowered sea levels might release methane, a greenhouse gas; conversely, raised sea levels would increase the area of high pressure, allowing more hydrates to form, thus decreasing the flow of greenhouse gases to the atmosphere. Finally, hydrates are stronger than unfrozen seafloor muds and oozes. Consequently, a hydrate-melting event could cause a major collapse of sediments on the continental slope and result in a major turbidite flow.

Drill cores also corroborated the asteroid-impact explanation for the extinction of the dinosaurs. A thin dead zone marked the end of the Cretaceous period, the last great age of dinosaurs, and the beginning of the Tertiary. This Cretaceous-Tertiary (K-T) boundary zone was noted in a few rock outcrops in Europe, and rock from this layer had elevated levels of platinum-group metals (similar to elevated levels in asteroids) in that strata. However, the theory was doubted until multiple drill cores closed in on the probable impact site on the Yucatán Peninsula of Mexico with thicker corresponding deposits off Florida and in the Caribbean Sea. Besides the platinum-group-enriched dead layer, those deposits had large amounts of shocked quartz, suggesting cosmic impact.

Sea levels over time have also been deduced from oceanic cores. One major sea-level surprise was massive evaporite deposits in the Mediterranean Sea from when sea levels were low enough to make the Strait of Gibraltar dry land and the Mediterranean a salty inland sea. Finally, remote stations have been implanted in several oceanic boreholes. These stations take seismic data to augment land seismic stations, and they also have temperature sensors.

Principal Terms

abyssal plains: flat areas that make up large areas of the ocean floor

basalt: rock formed from recrystallization of molten rock; most of the rock in the mid-ocean ridges and underlying the abyssal plains is basaltic

hydrothermal vents: areas on the ocean floor, typically along fault lines or in the vicinity of undersea volcanoes, where water that has percolated into the rock reemerges much hotter than the surrounding water; such heated water carries various dissolved minerals, including metals and sulfides

mantle: the thick rock layer between the Earth's crust and the core below

marker fossil: a species that existed in a wide area but died out in a short time; finding such a fossil fixes the date of the strata in which it is found

methane hydrate: mineral formed when methane (natural gas) is trapped within the structure of water ice crystals; extensive ocean-floor deposits of methane hydrate might influence climate and could become a major resource

plate tectonics: mechanism that allows continental drift; the Earth's crust consists of individual shifting plates that form at mid-ocean ridges and other locations and are destroyed where plates collide and send material back into the mantle

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