How Old is the Continental Crust?

Although we now know that the Earth formed roughly 4.5 billion years ago, the next 600 million years of Earth history is essentially blank. The oldest rock samples yet identified on Earth(from the Northwest Territories in Canada) have an age slightly greater than 3.9 billion years. These rocks have undergone strong metamorphism (changes to the form of the existing rock due to heat and pressure), and it is difficult to tell much about their origin. However, they are not very different from many other typical continental rocks that are much younger. Thus we know that there were at least some fragments of continental crust in existence 3.9 billion years ago.

The question of when the first continents formed is one that has long intrigued geologists, because it is evident that the continental crust has grown and evolved over geologic time. It is probable that there were small continents even before the creation of the 3.9-billion- year-old rocks. The clues that lead to this conclusion are rare and minute and difficult to discover. Where would one look for such evidence? The answer provides a good example of how geologists often work: by using the present as a window into the past. We know that the products of erosion accumulate on the edge of continents today, and there is no reason to think that the situation was different in the past. Even the earliest continents must have had beaches. There is a chance that if some of those very old sediments have been preserved, they might contain mineral grains from even older continents.

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