Supercontinent cycles

Supercontinent cycles, which recur over periods of 400 million to 440 million years, help explain the distribution of certain natural resources, fluctuations throughout geologic time in sea level and climates, the process of mountain building, and the evolution of life.

Theory

One of the most persistent questions relating to the historical development of the Earth concerns the processes whereby extensive mountain chains, such as the Himalayas, Appalachian, and Ural Ranges, have been formed. During the nineteenth century, the planet-contraction hypothesis, supported by the doctrine of permanence of continents and ocean basins, suggested that such linear features of the Earth's crust were comparable to the wrinkles within the skin of a dried apple. After the discovery of radioactivity in 1896, studies suggested heat formed by radioactive decay of rock minerals approximately equaled heat lost to the atmosphere by the gradual cooling of the Earth from its supposed original liquid stage. This balance of heat gain and loss did not support a shrinking-Earth concept. With the additional knowledge that linear mountain ranges had formed during different stages of geologic history rather than simultaneously, the contraction hypothesis gradually lost favor.

Between the publication of his 1915 book Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans, 1924) and his death during an expedition to the Greenland icecap in 1930, German meteorologist and geologist Alfred L. Wegener gained international repute as the father and chief advocate of the theory of continental drift. Wegener postulated that mountain ranges were created by the collision of large blocks of continental crust moving through the oceanic crust, following fragmentation and dispersion of the vast supercontinent he called Pangaea (Greek for “all lands”), surrounded approximately 200 million years ago by the universal ocean Panthalassa (from thalassa, Greek for sea). An intriguing set of evidence supports this theory, including the unusual degree of geometric fit of present-day continents (especially South America with Africa) and, through the reassembly of Pangaea, the reconstruction of truncated salt, fossil-reef, glacial-deposit, and mountain-range trends. While many scientists at the time became “pro-drift,” many others questioned what possible mechanism could displace solid continental crust through equally solid oceanic crust. Various Earth forces, ranging from centrifugal to tidal to rotational axis wobble, were investigated and rejected as inadequate by physicists and mathematicians. By the 1930s, continental drift as a theory was no longer considered viable and was increasingly mentioned only within the context of the history of science.

Exploring the Ocean Floor

Following World War II, a new era of Earth investigation began, focused primarily on the ocean basins. Employing newly developed military technology, including methods of water-depth sounding (bathymetry) and magnetic-body detection, surplus military aircraft and surface-vessel equipment began to collect a wide array of information. By the International Geophysical Year of 1957-1958, ocean-floor depth-profile analyses confirmed the existence of a previously unknown, 65,000-kilometer-long global submarine mountain range system traversing the Atlantic, Pacific, Arctic, and Indian Oceans. This feature defied immediate explanation.

During the same period, ocean evaluation programs sponsored by both private and US government interests began measuring the ocean floor's magnetism. By the mid-1950s, sufficient data had been collected off the West Coast of the United States to reveal a repetitive north-south pattern representing alternate zones of above-average and below-average magnetism. Soon similar patterns were shown to exist in the Atlantic and Indian Oceans.

The discovery of these unexplainable phenomena prompted the collection of any form of additional data that would help explain the existence of ocean floors dissected by a universal mountain range and masked by symmetrical magnetic patterns. New oceanographic programs gathered information on bedrock temperature, radiometric age, and ocean sediment thickness and studied the worldwide distribution of earthquakes and volcanic eruptions.

Theory

By the early 1960s, broad-based analyses of these various forms of oceanographic data were being conducted in Canada, the United Kingdom, and the United States. Gradually, consensus began to form that perhaps at least some of the ideas of Alfred Wegener were worthy of reconsideration. Paramount among these resurrected ideas was that of the assembly of the supercontinent Pangaea. Rather than postulating continental crust as floating through oceanic crust, the revised continental drift theory, termed “plate tectonics,” envisioned Earth's outer layer as divided into a series of major plates, each composed of both continental and oceanic bedrock.

For example, the North American plate comprises the continent of North America (including Greenland), the western half of the North Atlantic Ocean, and eastern Siberia. In contrast, the Indian-Australian plate comprises the continent of Australia, the country of India, and portions of the Pacific and Indian Oceans. Major plate displacement was considered possible because of the movement of global thermal-convection cells, which form within the Earth’s mantle and rise toward the surface until the presence of a supercontinent blocks them. The blockage of further heat transmission, caused by the insulating nature of the continental rock, divides the convection current into lateral, horizontally directed segments, which gradually dome by thermal expansion and then dissect the supercontinent.

As the new subcontinent begins to diverge, the separating void fills with high-density gabbro and basalt-type rock, which forms the floor of a newly developed ocean. Because the Earth is neither expanding nor contracting in size, continuing divergence cannot proceed indefinitely without experiencing resistance caused by the convergence of antipodal plates. The effect of plate convergence depends on whether such plate margins are oceanic (basaltic) or continental (granitic) in nature. Where margins are oceanic, the denser margin will subduct, or plunge under, the less dense margin. Where margins are both continental, subduction is unlikely, resulting in massive folding and faulting (earthquaking). Finally, where a continental margin converges with an oceanic margin, the latter, being denser, will subduct beneath the former. In all three possible convergence cases, crust shortening is accomplished. Rock volume harmony results as the formation of new oceanic crust through plate divergence is matched by the destruction of the older crust through subduction.

Processes of divergence and convergence are believed to have been ongoing throughout a large portion of geologic time and continue today. Divergence, accompanied by new-ocean development in region A, continues simultaneously with convergence in region B, resulting in the gradual destruction of region A ocean through subduction. Eventually, region A ocean will cease to exist, and the ocean birth and death cycle will be complete. This sequence of events, during which it is estimated that 2.6 square kilometers of ocean floor rock are created and destroyed each year, is termed a “supercontinent cycle” (also known as a Wilson cycle, after the Canadian geologist J. Tuzo Wilson, an early advocate of the plate tectonics theory). Conversely, the dispersion and amalgamation of continental crustal masses, as opposed to oceanic masses, constitute the principal phases of what has been termed the Pangaean cycle. Plate tectonics continuously creates and recycles ocean basins, while continental regions increase in age geologically even as they are agglomerated and dissected by ongoing seafloor spreading.

Future Supercontinent Cycles

A maxim of geology states that the validity of any hypothesis or theory is determined by the degree to which that concept can be examined through the analyses of extant geology. Where, then, might there be modern-day examples of a supercontinent cycle in its various stages of tectonic development?

The Great Basin of the western United States has been portrayed as a model of very early continental rifting that may, in the future, separate the North American plate from a newly constituted Pacific plate. The approximately one hundred block-faulted mountains composing this geographic terrane are caused by the same extensional forces responsible for the early continental-rift stage of dissection of a supercontinent.

Similar forces have formed the more structurally advanced rift systems of East Africa, the most illustrative of which are those broad-basin and steeply dipping escarpment topographies of Tanzania, Kenya, and Ethiopia, which continue to yield a record of mammalian and hominid evolution. The fresh waters that partially cover these rift valleys, such as Lake Tanganyika, are evidence of a late stage of continental rifting. To the north, the central valley or rift of the Red Sea is filled with salt water, which is characteristic of an incipient ocean that develops during an early stage of oceanic rifting, separating the Arabian from the African plate.

Finally, the Atlantic Ocean is a mature example of oceanic rifting representative of the midpoint of a supercontinent cycle. According to the concepts of plate tectonics, the Atlantic Ocean has been created through some 200 million years of seafloor spreading driven by the extensional rifting of Pangaea. This stage, representing the first half of a supercontinent cycle, will, in theory, be followed by assembly of the world's continents over the next 200 million years into a new supercontinent. This assembly may have already begun, as India, an island subcontinent up to approximately 35 million years ago, has since that time been colliding with the Eurasian plate, resulting in the formation of the Himalayan Mountains.

Evidence of Past Supercontinent Cycles

Following the general acceptance of the plate tectonics theory in the 1960s, many ideas were advanced regarding the existence and nature of pre-Pangaea supercontinents. Since the supercontinent cycle not only creates but also destroys oceans, pre-Pangaea supercontinents must be reconstructed from continental geologic data that become more difficult to interpret with increasing geologic age. Certain criteria have been developed to discern a pattern of supercontinent cycling since the earliest periods of geologic time.

The convergence of continents will largely eradicate any intervening ocean. Such loss by subduction is seldom complete, as attested by remnants of preexisting ocean floor rock contained within the deformed (suture) zone caused by plate collision. Basalt, gabbro, and olivine-rich rocks, termed an “ophiolite suite,” are scraped off, or “obducted” from, the subducting ocean floor and thus preserved in the developing mountain belt. Obducted ophiolitic rock from the former Tethys Sea, which lapped onto the eastern shores of Pangaea, is present in the Alps and the Himalayas. At the same time, ophiolites within the Ural Mountains of Central Asia are evidence of an ocean destroyed by the collision of Baltica and Siberia, continental masses that preceded the formation of Pangaea. The presence, location, and age dating of ophiolite suites are helpful in the identification of former supercontinents.

Paleontological and high-pressure rock evidence collected from the Appalachian Mountains—created by the convergence of proto-Africa (the earliest form of Africa) with proto-North America—suggests the existence of a proto-Atlantic Ocean approximately 500 million years ago. The existence of this body of salt water, the Iapetus Ocean, attests to the existence of an associated supercontinent.

The presence of exotic terranes forming the collisional edge of former crustal plates in Japan, New Zealand, and the Apennines of northern Italy is further evidence of former supercontinents and their cycles. These terranes, packages of rock possessing similar mineral and fossil character, are accreted to enlarging supercontinents by the same obduction process as ophiolites.

Two Schools

There are two major contemporary schools of thought regarding supercontinent cycles. The oldest Earth specimen found to date is the 4.28-billion-year-old metamorphic continental rock forming a portion of Canada's Northwest Territories. This and slightly younger rock terranes from Greenland, Antarctica, and Australia may have combined to form the first amalgamated continental masses. John Rogers, a professor of geology at the University of North Carolina, proposed the existence 3 billion years ago of an early subcontinent he calls “Ur” (from the German for “original”). Approximately 500 million years later, a second subcontinent, Arctica (predecessor to Canada, Greenland, and eastern Russia), formed, followed another 500 million years later by Baltica (proto-Western Europe) and Atlantica (eastern South America and western Africa). Then, 1.5 billion years ago, plate tectonic forces formed the sub-supercontinent Nena (from the Russian for “motherland”) by merging Baltica and Arctica. This lengthy chain of events culminated 1 billion years ago in forming the first supercontinent, Rodinia (also known as proto-Pangaea), due to the joining of Ur, Atlantica, and Nena. After a period of stability lasting some 300 million years, Rodinia subdivided, forming numerous proto-continents and Iapetus, the proto-Atlantic. Finally, the reassembly of these proto-continents and subsequent subduction of Iapetus brought about the creation of the second supercontinent, Pangaea, about 250 million to 300 million years ago.

A second school of thought differs principally in suggesting that plate tectonic processes did not begin until the first distinct oceanic and continental crust developed some 2.5 billion years ago. Prior to that time, the Earth's very high temperature and the relative thinness of the primordial crust forestalled the onset of supercontinent-cycle processes such as divergence, convergence, and subduction. While differing in detail, both schools of thought generally recognize Rodinia and Pangaea as supercontinents.

The estimated length of a typical supercontinent cycle varies from 400 million to 440 million years. Such a cycle would constitute three phases. Once formed, a supercontinent would exist for 100 million to 120 million years before the accumulation of thermal convection heat initiated crustal dissection. During the second phase, lasting from 150 million to 160 million years, maximum dispersal of subcontinents would occur, resulting in new ocean development. Finally, subduction would gradually destroy the intervening oceans over a period of 150 million to 160 million years, creating a new supercontinent and terminating the cycle.

A study published in 2023 looked into the next supercontinent, Pangaea Ultima, expected to form in 250 million years. From this, scientists have created dire climate models for the future supercontinent, suggesting that its harsh conditions will make it incompatible with mammalian life. Pangaea Ultima is expected to experience high temperatures and increased volcanic activity. Although other models exist for alternative supercontinents, studying all possibilities aids twenty-first-century climatologists in better understanding contemporary global climate change. 

Principal Terms

continental drift: the theory that continental fragmentation and displacement caused the creation of new ocean basins and the formation of mountain ranges

convergence: the process that occurs during the second half of a supercontinent cycle, whereby crustal plates collide, and intervening oceans disappear as a result of plate subduction

divergence: the process of fracturing and dissecting a supercontinent, thereby creating new oceanic rock; divergence represents the initial half of the supercontinent cycle

ophiolite suite: a unique vertical sequence of peridotite rock overlain by gabbro, basalt, and oceanic sediments representative of ancient seafloor material

plate tectonics: a theory describing the Earth's surface as composed of rigid plates continually in motion over the interior, causing earthquakes, mountain building, and volcanism

seafloor spreading: the continual creation of new seafloor bedrock along mid-ocean ridges through the process of ascending thermal currents

supercontinent: a single vast continent formed by the collision and amalgamation of crustal plates

Wilson cycle: the creation and destruction of an ocean basin through the process of seafloor spreading and subduction of existing ocean basins

Bibliography

Dalziel, Ian W. D. “Earth Before Pangaea.” Scientific American, vol. 272, Jan. 1995, pp. 58-63.

Davidson, J. P., W. E. Reed, and P. M. Davis. Exploring Earth: An Introduction to Physical Geology. 2d ed., Upper Saddle River, N.J.: Prentice-Hall, 2001.

Fisher, Richard. “How the Next 'supercontinent' Will Form.” BBC, 3 Apr. 2022, www.bbc.com/future/article/20220401-how-the-next-supercontinent-will-form. Accessed 27 July 2024.

Nance, R. Damian, T. R. Worsley, and J. B. Moody. “The Supercontinent Cycle.” Scientific American, 259, July 1988, pp. 72-79.

Nicolas, A. The Mid-Ocean Ridges. New York: Springer-Verlag, 1995.

Nield, Ted. Supercontinent: Ten Billion Years in the Life of Our Planet. Cambridge, Mass.: Harvard University Press, 2007.

O'Callaghan, Jonathan. “Pangaea Ultima, the Next Supercontinent, May Doom Mammals to Far-Future Extinction.” Scientific American, 27 Sept. 2023, www.scientificamerican.com/article/pangaea-ultima-the-next-supercontinent-may-doom-mammals-to-far-future-extinction. Accessed 27 July 2024.

Rogers, John J. W., and M. Santosh. Continents and Supercontinents. New York: Oxford University Press, 2004. .

Sullivan, W. Continents in Motion. 2d ed., New York: American Institute of Physics, 1993.

Wicander, Reed, and James S. Monroe. Historical Geology. Belmont Calif.: Brooks/Cole, Cengage Learning, 2010.