When I was still in kindergarten, I once asked my teacher why some of the continents looked like they fit together, almost like a jigsaw puzzle. By the logic of a 4-year old, it seemed obvious that the large dent in the western part of Africa was where South America fit, and northwest Africa nuzzled nicely into the eastern seaboard of the United States. “Don’t be silly,” my teacher told me. “The world is not a puzzle.” As a rather naïve child, I took her word for it, having no idea that I’d just stumbled upon one of the most spectacular scientific discoveries of the 20th century.
In 1912, Alfred Wegener, a German meteorologist, proposed the idea of continental drift, which suggested that continents move or ‘drifted’ over timescales of millions of years. He suspected that all continents were originally joined into one supercontinent, which he named Pangaea. In reality, he was not the first to suggest the idea, which dates back as far as the early 1500s, when explorers noted the uncanny fit between South American and African coastlines.
Wegener used several observations to support his case. First, he presented the simple observation of continental fit – the same idea I noted to my kindergarten teacher. It doesn’t take much imagination to fit the continents together like 7 rather large puzzle pieces. It’s important to remember that these particular puzzle pieces are three-dimensional blocks, and near the top are constantly being chipped and molded by ocean and surface processes. If you looked a couple kilometres down, where continents see much less disturbance, the fit is remarkably close.
Wegener’s second observation was the continuity of mountain ranges across continents. The Appalachian Mountains run through the eastern United States, ending near Newfoundland. Similar ranges can be seen through Greenland, the United Kingdom, and Norway. Wegener suggested that these ranges were remnants of what once was once a continuous mountain range, much like the Rocky Mountains, which connect to the Andes in South America. These mountain ranges are all of similar age and made of the same material. The same red sandstone used for some New York buildings was once mined in Scotland to build castles! (Say what you want about the rocks, but those Scots sure build some mighty fine castles.)
Wegener’s final line of reasoning was based on fossil evidence. Fossils of several land-based organisms have been found on multiple continents. Examples are the Mesosaurus (something along the lines of dinosaur meets crocodile) in South America and Africa, and Glossopteris (a fern) in North and South America, Africa, Australia, and Antarctica. This weird distribution could be explained if the continents were once joined together into a single land mass.
The reaction of the scientific community to Wegener was something along the lines of that of the Catholic Church to Galileo. Geologists at the time would have been overjoyed to sentence Wegener to an eternity of geological purgatory. At the time he published his book, much of the scientific community still followed the extensionism theory that proposed that land bridges once connected the continents and then disappeared without a trace. No evidence of these bridges has ever been found—this came much to the disappointment of President Reagan who was forced to bury his ambitions to build a similar bridge to the moon. When Wegener died in 1930, the scientific community still stood firmly against his theory, largely because his hypothesis could not explain how the continents moved.
The modern-day theory of plate tectonics was not born until two decades later, when advances in science and technology allowed further study of Wegener’s hypothesis. The theory of plate tectonics states that the continents and oceans are all embedded on rock ‘plates’ (called tectonic plates) that float on magma (molten rock). Picture the planet as a giant orange. You peel this orange and then carefully put the peeled skin back into its original position. What happens? The peel still forms a sphere, but now it is made out of multiple pieces. The pieces of the peel represent tectonic plates, which together make up the Earth’s crust. The layer underneath the crust is called the mantle, the top of which is composed of thick and gooey magma, much like hot honey. Circulation of the magma (called convection) pulls the crust, causing the tectonic plates to move. These plates play a giant game of bumper cars, sliding past and crashing into one another. Plate boundaries are often marked by volcano and earthquake belts, such as along the west coast of North America, caused by the strain and heat built up as plates collide. While plates move millimeters to centimeters a year, over the course of millions of years, the continents rearrange into new patterns, combining and breaking apart.
The next time you’re looking at a map of the world, imagine what it might look like five or fifty million years from now. Boundaries and borders seemingly set in stone will look nothing like the present, and the puzzle will instead form a new mosaic of mountains, waterways, islands, continents, and oceans.
Image CC NASA via Flickr