Until the late 1960s, most geologists held the view that the ocean basins and continents had fixed geographic positions and were of great antiquity. Over the following decade, scientists came to realize that Earth’s continents are not static; instead, they gradually migrate across the globe. These movements cause blocks of continental material to collide, deforming the intervening crust and thereby creating Earth’s great mountain chains (Figure 1). Furthermore, landmasses occasionally split apart. As continental blocks separate, a new ocean basin emerges between them. Meanwhile, other portions of the seafloor plunge into the mantle. In short, a dramatically different model of Earth’s tectonic processes emerged. Tectonic processes (tekto = to build) are processes that deform Earth’s crust to create major structural features, such as mountains, continents, and ocean basins.

This profound reversal in scientific thought has been appropriately called a scientific revolution. The revolution began early in the twentieth century as a relatively straightforward proposal termed continental drift. For more than  years, the scientific community categorically rejected the idea that continents are capable of movement. North American geologists in particular had difficulty accepting continental drift, perhaps because much of the supporting evidence had been gathered from Africa, South America, and Australia, continents with which most North American geologists were unfamiliar.

After World War II, modern instruments replaced rock hammers as the tools of choice for many Earth scientists. Armed with more advanced tools, geologists and a new breed of researchers, including geophysicists and geochemists, made several surprising discoveries that rekindled interest in the drift hypothesis. By 1968, these developments had led to the unfolding of a far more encompassing explanation, known as the theory of plate tectonics.

In this chapter, we will examine the events that led to this dramatic reversal of scientific opinion. We will also briefly trace the development of the continental drift hypothesis, examine why it was initially rejected, and consider the evidence that finally led to the acceptance of its direct descendant—the theory of plate tectonics.

• Sixty years ago, most geologists thought that ocean basins were very old and that continents were fixed in place. Those ideas were discarded with a scientific revolution that revitalized geology: the theory of plate tectonics. Supported by multiple kinds of evidence, plate tectonics is the foundation of modern Earth science.

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