2.9 The Great Cycle

Our world map has inspired human imagination for centuries. Alfred Wegener was not the first to notice that the South American and African coastlines could align together like puzzle pieces or conclude that they might have once been connected as one mass. However, in his book over a century ago, Wegener was the first to publish the words “Ur-Kontinent” and “Pangäa”, which we now popularly refer to as a supercontinent or Pangaea.

When the ideas of seafloor spreading and continental drift were unified into the theory of plate tectonics by J. Tuzo Wilson in the late 1960s, questions remained over how Earth’s appearance evolved over its 4.54 billion-year history. If the oceans and continents shifted and change configuration as a result of tectonic forces, then what might our world map looked like millions or billions of years ago and what will it look like in the future?

In the decades following the acceptance of plate tectonics until present, geologists have examined the rock and fossil record for clues to describe the appearance of Earth throughout the eons. Many of these techniques were similar to those used by Alfred Wegener, Marie Tharp, and Harry Hess. Today, there are still many questions about the processes behind plate tectonics and Earth’s history, but we have pieced together paleomaps that illustrate Earth’s past appearance.

Video 2.9.1. Paleogeography and ice ages. Christopher Scotese has worked for decades reconstructing paleomaps and the evolution of the continents (1:39).

Throughout Earth’s history, there have been several episodes of supercontinent assembly and breakup. The last supercontinent on Earth was called Pangaea, and it existed around 300 million to 180 million years ago. The formation of a supercontinent is driven by convergent boundaries. However, the breakup of these landmasses requires a more complicated mechanism. The extremely thick crust from the convergence of land will insulate the lithosphere and eventually allow magma to rise from the mantle; thus, divergent boundaries will form throughout the supercontinent. It took approximately 250 million years for our world map to arrive at its current, broken-up, configuration from Pangaea.

The pattern of tectonic forces causing supercontinents to form, breakup, then form once more is called the Wilson Cycle (named for J. Tuzo Wilson). This cycle has been operating for at least a billion years; before Pangaea, a supercontinent called Rodinia had assembled around 1 billion years ago. Current modeling suggests that the Wilson Cycle will continue well into the future. Hundreds of millions of years in the future – about 250 million to be precise – a new supercontinent will form on Earth. We are currently referring to it as “Pangaea Proxima” or “Pangaea Ultima”.

All of this shows that our planet has been undergoing a tremendous cycle that continuously reshapes its surface over billions of years, and we are just beginning to understand this process!