Published: 8 December, 1999

Plate tectonics key to continents

What evidence do we have that continents were connected?

This is a very good question, one that leads us into the subject of plate tectonics, the theory that describes how the earth is deforming now, and how it has deformed in the past to make mountains, oceans, earthquakes, volcanoes, and the rocks we see most everywhere.

A wide variety of observational evidence supports the idea that the continents were together a couple of hundred million years ago, that since then they have separated and moved about on the surface of the earth, and, indeed, that they are still moving today. The speeds with which continents move, typically an inch or two per year, or about the rate at which your fingernails grow, may seem slow to a non-earth scientist. But over hundreds of millions of years of earth history, such speeds correspond to movements of a few thousand miles (the distance around the earth is about 25,000 miles), and hence to effects that make our planet into what we observe today. Let's look at some of that evidence for movement, and then at some of the consequences of it.

First, the outlines of the continents around the Atlantic Ocean, that is, Africa, South America, North America, and Eurasia, suggest that those continents once fit together like pieces of a jigsaw puzzle, and hence that they have since moved apart. This idea is rather obvious. It was first reported by an early mapmaker who may have been the very first person to see a reasonably good map of those continents.

But pieces of a jigsaw puzzle have to be fit together so that the picture on top of the puzzle comes out right. And so it is with the continents, but in this case it is the geology, that is the patterns of different kinds of rocks on the surface of the continents, that makes up the picture.

Some features that are especially important in this grand geological picture include: (1) large areas of certain distinct rock types like granite, (2) mountain belts, (3) fossils that record where and when life was found, and (4) rocks that are a consequence of ancient ice ages, ice ages much older than the one so familiar to Ithacans because it formed the Finger Lakes less than a million years ago. Such features are split when the continents break apart, but fit together nicely when they are properly reassembled in our minds.

Additional support for the breakup of the continents comes from the theory of plate tectonics that was discovered by earth scientists in the 1960s. A critical part of this theory says that in certain places great cracks, or rifts, develop at the earth's surface. Molten rock material comes up from below at these rifts and then cools to make, not continental material, but new ocean crust. This new sea floor spreads apart, and still more and still newer sea floor is added. The spreading process may continue for hundreds of millions of years until an entire ocean basin is formed, and the new ocean separates the two pieces of older continental rocks that were once part of a single continent.

Thus Africa and South America were once not distinct, but, more than 100 million years ago, were connected parts of the same large continent. Then a rift formed in that huge continent, and the modern Atlantic Ocean began to form between the two continental pieces we now know as Africa and South America. In fact, on the bottom of the modern Atlantic Ocean there is a large mid-Atlantic ridge with a central rift whose pattern resembles that of the coastlines of those two continents. It is the site of many earthquakes and volcanic eruptions, and it is a so-called spreading center. The Atlantic Ocean, in other words and in geological terms, is a young feature separating two continents made of much older rocks, and the ocean floor is continuing to spread and enlarge today.

I hasten to add that, in order to focus on the question, this brief venture into the theory of plate tectonics discusses just one part of the story, the places where spreading occurs. It omits discussion of many other exciting topics, such as what happens when continents collide and what is known about the continental movements that preceded the Atlantic-centered story emphasized here. Keep asking questions about how the earth got to be the way it is, questions like the one above, and you will learn things about the earth that will make your travels very interesting indeed.