F or a strong dose of humility, consid- er that not even the land beneath our feet can be taken for granted. For exam- ple, geological data indicate with con- siderable certainty that between 300 and 200 million years ago all of the Earths continental land masses were assem- bled into a supercontinent, which has been named Pangea (meaning all lands), surrounded by a superocean known as Panthalassa (meaning all seas). Indeed, the evolution of the Earth over the past 200 million years has clearly been dominated by the breakup of Pangea and the resulting formation of new oceans, such as the Atlantic, between the dispersing conti- nental fragments. For the past 20 years, however, evi- dence has been amassing that Pangea itself was only the latest in a series of supercontinents that have assembled and dispersed over 3 billion years. Al- though the mechanisms responsible are controversial, many geoscientists agree that repeated cycles of supercon- tinent amalgamation and dispersal have not just taken place, but also have had a profound effect on the evolution of the Earths crust, atmosphere, cli- mate and life over billions of years. The amalgamation of Pangea ap- pears to have been preceded by that of Pannotia about 650 to 550 million years ago, and, although its configuration is debated, there is general acceptance of the existence of the supercontinent Ro- dinia about one billion years ago. An- other supercontinent, variously termed Nuna or Columbia, is thought to have amalgamated about 1.8 billion years ago. Two others, Kenorland and Ur, are believed to have assembled 2.5 and 3.0 billion years ago, respectively. Since the expression the past is the key to the present is one of the basic tenets of geology, a strong probability ex- ists that another supercontinent will form in the future. But how would such a supercontinent form, and what would it look like? There are two competing models: One has the continents drift apart and back together again like an ac- cordion; the other proposes that the con- tinents break apart and march all the way around the Earth to reunite on the other side. To determine which is correct, we must first review the basic principles of plate tectonics, the theory that revolu- tionized our understanding of the Earth by providing a comprehensive explana- tion of the forces that shape it. Plate Tectonics According to the theory of plate tecton- ics, the Earth has a rigid outer layer, known as the lithosphere, which is gen- erally 100 to 150 kilometers thick and rides atop a hot, plastic layer in the Earths mantle called the asthenosphere. Like a cracked eggshell, the lithosphere is broken up into a mosaic of about 20 slab-like fragments, or plates, which move relative to one another at rates that are typically less than 10 centime- ters per year. As they move, the plates interact along their boundaries, where they may converge and collide, diverge and separate, or slide past one another. Over millions of years, such interac- tions have caused mountains to rise where plates collide and continents to break apart where plates diverge. The continents are embedded in the plates and drift passively with them. Over millions of years, this motion of the continents is sufficient to open and close entire oceans. For example, over the past 180 million years, the diver- gence between Europe/Africa and North/South America has opened the Atlantic Ocean. The plate boundary along which these continents diverge takes the form of a mid-ocean ridge running the length of the ocean basin. From the crest of this ridge, new ocean floor spreads in both directions as up- welling hot magma from the underly- ing mantle cools and solidifies, generat- ing new lithosphere between the diverging plates. On an Earth of constant radius, the creation of new lithosphere in this fash- ion must be balanced by lithospheric destruction. Over the same period that the Atlantic Ocean has been opening, for example, the convergence of Africa with Europe and of India with Asia has closed an ancient ocean known as Tethys, while the westward motion of the Americas has consumed much of the Pacific. When continents converge, the inter- vening oceanic lithosphere re-enters the mantle and is consumed in a process known as subduction. In general, oceanic crust is denser than continental crust, so 324 American Scientist, Volume 92 How Do Supercontinents Assemble? One theory prefers an accordion model; another has the continents travel the globe to reunite J. Brendan Murphy and R. Damian Nance J. Brendan Murphy is a native of Birr, Ireland, and came to Canada after completing his B.Sc. in geol- ogy at University College Dublin. He received a Ph.D. in geological sciences at McGill University in 1982 and joined St. Francis Xavier University, where he is now a professor in the Department of Earth Sciences. His research interests include mountain-building processes and the relation be- tween tectonic activity and igneous processes. R. Damian Nance is a native of Cornwall, U.K., and received his B.Sc in geology at the University of Leicester in 1972. He moved to Canada in 1976 after completing his Ph.D. in geology at the Uni- versity of Cambridge, and joined Ohio University, where he is now professor of geological sciences, in 1980. He shares with Dr. Murphy a research inter- est in tectonic activity and large-scale geodynamic processes, and the two have been collaborating since 1985. Address for Murphy: Department of Earth Sciences, St. Francis Xavier University, P.O. Box 5000, Antigonish, Nova Scotia B2G2W5, Canada. Internet: bmurphy@stfx.ca ' 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact perms@amsci.org.