Glomar Challenger Begins Collecting Ocean-Floor Samples

Date August 11, 1968

As part of the Deep Sea Drilling Project, the Glomar Challenger was instrumental in retrieving thousands of rock samples from the ocean floor, providing data that brought about a revolution in earth science.

Also known as Deep Sea Drilling Project

Locale World oceans

Key Figures

• Maurice Ewing (1906-1974), American geophysicist
• Bruce Heezen (1924-1977), American geologist
• Alfred Wegener (1880-1930), German geologist

Summary of Event

The August 11, 1968, maiden voyage of the Glomar Challenger was preceded by a revolution in earth scientists’ thinking about how the earth functioned. The roots of the revolution may date back to the 1600’s, when Francis Bacon first wrote about his observation that the recently defined shapes of South America and Africa suggested that they may have been linked in the past.

Similar speculations persisted throughout the ensuing years, supported by an increasing body of geological knowledge from those two continents as well as from India, Australia, and Antarctica. As the evidence mounted, it became clear not only that all these continents were once connected but also that they had moved, in the case of Antarctica, from tropical latitudes to the polar regions.

The theory of continental drift, championed by Alfred Wegener, grew out of a collection of seemingly unrelated observations of geology and fossils on the southern continents. The conclusion seemed inescapable, but there was a major weakness in the theory. In one early conceptualization of the model, the continents were compared to icebergs floating in the “ocean” of denser rocks that make up the earth’s interior. There was no known mechanism whereby massive continental blocks could be moved around on the face of the earth while plowing through the rocks of the denser interior. Thus, continental drift had to wait for another set of seemingly unrelated discoveries.

In the 1920’s, Maurice Ewing was working on his doctorate in physics and covering some of his expenses by working summers for oil exploration teams. He helped explore for oil in the Gulf Coast region of Texas, where prospectors would set off dynamite charges near the surface and record the arrival of the shock waves sent through the ground at listening devices strung out on the ground. Depending on what the sound waves encountered in the subsurface, they either traveled faster or slower than average. By deciphering the various arrival times, a prospector could “see” what structures existed in the subsurface and decide if there was anything interesting or likely to yield oil before embarking on expensive drilling.

During the 1930’s, Ewing was asked to apply some of the same techniques (called seismic exploration) to explore the structure of the continental shelf and ocean basins in the Atlantic. Not only did the results show considerable thicknesses of sediments in the deep ocean but the surveys also suggested that the sediments were covering what once was a very mountainous topography. During World War II, it became increasingly important to map the topography of the ocean basins. Increasing use of submarines made it imperative, and improvements in sonar and seismic technologies made it relatively easy.

In the early 1950’s, Bruce Heezen and colleagues were compiling the results of many topographic studies into one picture of the ocean basin topography. Many surveys had identified a major mountain chain running down the length of the Atlantic Ocean. Heezen’s group, however, noticed that there was a deep valley located in the middle of the mountain chain, and it too ran the length of the Atlantic Ocean. The role of the central valley was unclear, but it was identified as a rift valley based on its morphological similarity to the continental East African Rift Valley. At nearly the same time, Heezen’s group was asked by transatlantic telephone companies to locate areas of earthquake activity in the Atlantic. The thinking was that if the earthquakes were localized, telephone cables could be laid down to avoid those areas and thereby avoid being snapped in the aftermath of a submarine earthquake. Much to Heezen’s surprise, the earthquakes tended to be located in and around the central rift valley.

A number of lines of evidence began to converge such that, by the mid-1960’s, a minority in the earth science community was offering the theory of seafloor spreading and plate tectonics as the answer to the missing-mechanism dilemma of continental drift. In plate tectonics, the earth’s surface may be visualized as an eggshell with cracks. The areas between the cracks are called plates, which can move over the surface and include both continents and ocean basins. The cracks are the boundaries between plates and areas where the plates either collide with or move away from each other.

The driving mechanism is seafloor spreading, wherein some of the cracks separating plates coincide with the rift valleys running the length of submarine mountain chains (for example, Mid-Atlantic Ridge, East Pacific Rise, Indian Ridge). In the rift valley, molten rock from the earth’s interior forces its way to the surface (causing earthquakes) where it cools, forms new oceanic crust, and pushes previously existing oceanic crust to either side. The result in the Atlantic is that the ocean basin is getting larger, while North America moves away from Europe, and South America moves away from Africa (at a rate of about five centimeters every year).

Among the implications of this theory were that oceanic crust near the ridges should be very young and get older as one takes samples nearer the continents. In addition, there should be a very thin sediment cover on oceanic crust near the ridge (reflecting the fact that it has not been there long enough to collect much) and thicker near the continents. All these speculations were testable with the ability to obtain sediment and oceanic crust samples, preferably cores, from any location in the ocean.

The compelling arguments of earth scientists convinced the National Science Foundation to support the voyages of the Glomar Challenger. Glomar Challenger was essentially an oil-drilling platform attached to an oceangoing ship. The crew consisted of the ship’s crew, who were responsible for operations (from cooking meals to operating the drilling equipment), and a scientific crew, who were to establish and oversee the technical objectives of each cruise.

The first cruise in the Atlantic (actually the second leg of a journey that began in the Gulf of Mexico) confirmed the broad outlines of the seafloor spreading hypothesis. Samples of sediments, their associated fossils, and oceanic crust were retrieved and confirmed the basic “aging”-of-oceanic-crust aspect of the hypothesis. The wealth of samples brought back to academic laboratories, however, initiated new studies and spawned a whole new series of questions regarding the details of the historical record preserved in the sediments. In addition, the samples of oceanic crust revealed evidence of reactions between the rocks and hot water (hydrothermal systems similar to hot springs on continents). It became apparent that with each new cruise of the Glomar Challenger, analyses of the data would indicate the need for more voyages.

Over the next fifteen years, Glomar Challenger crisscrossed the oceans, mapping the topography with sophisticated sonar and seismic techniques, collecting core samples, and taking water column samples and measurements. In the early 1980’s, Glomar Challenger was retired and replaced by a larger ship, the Glomar Explorer.

Significance

Each of the cruises of the Glomar Challenger was, for the earth science community, the equivalent of a moon shot. The samples returned to shore-based laboratories were from a remote and largely unexplored portion of the earth’s surface. The technologies needed to retrieve and analyze the samples were continually evolving. In the early days of the project, the samples were so limited and precious that they were doled out in minute quantities to researchers.

As more and more samples were analyzed, subsequent cruises were designed to explore areas of the ocean that would likely provide key pieces of evidence. The skeletons of seafloor spreading and plate tectonics were fleshed out and modified as the results of each cruise were analyzed by the scientific community.

By the time Glomar Challenger was retired, the general outlines of ocean basin topography were well known to most of the public. Maps of submarine topography, illustrating the “alligator back” look of the Mid-Atlantic Ridge, were frequently published in popular scientific publications. Plate tectonics became the accepted explanation for the functioning of the earth. The theory explained the connection between the submarine topography of the relatively shallow ridges and the greatest depths of the oceans in trenches. Both are plate boundaries but with totally different functions. New ocean crust is formed at ridges, and old ocean crust is pushed back into the earth’s interior (and remelted) at trenches. Plate tectonics explained localized areas of earthquake and volcanic activity on land. For example, Japan is both volcanically active and subject to earthquakes because it overlies a plate boundary where old oceanic crust is being forced back into the earth. Similarly active areas are found in the Pacific Northwest of the United States (Oregon and Washington) and along the Pacific coast of South America (Ecuador, Peru, and Chile).

The theories of seafloor spreading and plate tectonics were not developed in a straight line. Many lines of evidence needed to be synthesized over many years. Someone recognized that technologies developed for one application (for example, gravity meters, satellite navigation, heat probes, radiometric dating) could be applied to research-driven ocean exploration. When the theories became testable and the necessary technologies were available, they all came together aboard the Glomar Challenger.

Bibliography

Briggs, Peter. 200,000,000 Years Beneath the Sea. New York: Holt, Rinehart and Winston, 1971. An excellent text that traces the details of the early years of the Deep Sea Drilling Project wherein Challenger became the workhorse. Briggs provides an anecdotal history rich with the details of early successes and the early frustrations and failures. Especially interesting is Briggs’s accounts of how the early results were received by some entrenched interests in the scientific community. There are a few good illustrations, a brief bibliography, and a good subject index. A highly recommended starting point for any interested reader.

Heezen, Bruce C., and Ian D. MacGregor. “The Evolution of the Pacific.” Scientific American, November, 1973, 102-112. A good summary article of early concepts in plate tectonics as applied to the Pacific Ocean. The data used in this analysis are largely derived from Challenger cores. The discussion begins with some historical background but quickly gets into the technical details. The illustrations are helpful, but the lay reader may find the details a bit daunting. The authors call on a wide range of data, including marine chemistry and paleomagnetics, which are handled well but perhaps are beyond many college-level readers. Includes a very brief bibliography.

Hsü, Kenneth J. The Mediterranean Was a Desert: A Voyage of the Glomar Challenger. Princeton, N.J.: Princeton University Press, 1983. An updated and more complete version of the story Hsü began in the Scientific American article (cited below). Hsü paints a vivid picture of life on board Challenger and the trials and tribulations of the chief scientist responsible for bringing this expensive venture into an area to test his hypothesis. Contains a glossary of terms, a brief list of suggested further readings, and a good subject index.

‗‗‗‗‗‗‗. “When the Mediterranean Dried Up.” Scientific American, December, 1972, 26-36. Account of Hsü’s application of the scientific method to evaluate various lines of data and arrive at a testable hypothesis. The test required the use of Glomar Challenger to drill into and retrieve sediments from the floor of the Mediterranean Sea. Those sediments provided the evidence Hsü needed to speculate on the period of time when the sea was closed off from the Atlantic and dried up. The details of the hypothesis are still being debated, but this article presents an excellent account of how science is done.

Sclater, John G., and Christopher Tapscott. “The History of the Atlantic.” Scientific American, June, 1979, 156-174. This article provides an interesting comparison with the Heezen article. The understanding of the functioning of plate tectonics is the foundation of both articles, and it is interesting to compare the level of explanation possible with the advantage of another decade of data from Challenger. Again, the authors call on several lines of data, especially heat flow data, which may be somewhat difficult for some readers to follow. There is a minuscule bibliography that cites technical literature.

Valentine, James W., and Eldridge M. Moores. “Plate Tectonics and the History of Life in the Oceans.” Scientific American, April, 1974, 80-89. The authors extrapolate from plate tectonics and the fact that the sizes and shapes of the ocean basins have changed, to the effect this might have on ocean circulation patterns and biological evolution. A good example of the way plate tectonics and the data from the Challenger voyages required rethinking of the basic understandings in many fields. A good article for the biologically inclined technical reader. Brief bibliography.

Vrielynck, Bruno, and Philippe Bouysse. The Changing Face of the Earth: The Break-Up of Pangaea and Continental Drift over the Past 250 Million Years in Ten Steps. Paris: UNESCO, 2003. Booklet of maps showing the evolution of the Earth’s surface over 250 million years. Invaluable for understanding the effects of continental drift.