For a strong dose of humility, consider that not even the land beneath our feet can be taken for granted. For example, geological data indicate with considerable certainty that between 300 and 200 million years ago all of the Earth's continental land masses were assembled 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

continental fragments.

For the past 20 years, however, evidence has been amassing that Pangea itself was only the latest in a series of supercontinents that have assembled and dispersed over 3 billion years. Although the mechanisms responsible are controversial, many geoscientists agree that repeated cycles of supercontinent amalgamation and dispersal have not just taken place, but also have had a profound effect on the evolution of the Earth's crust, atmosphere, climate and life over billions of years.

The amalgamation of Pangea appears 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 Rodinia about one billion years ago. Another 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 exists 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 accordion; the other proposes that the continents 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 revolutionized our understanding of the Earth by providing a comprehensive explanation of the forces that shape it.

Plate Tectonics

According to the theory of plate tectonics, the Earth has a rigid outer layer, known as the lithosphere, which is generally 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 centimeters 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 interactions have caused mountains to rise where plates collide and continents to break apart where plates diverge.

The continents are embedded...