Compare the theory of continental drift with the theory of plate tectonics.

Some 300 million years ago, Earth would hardly have been recognizable to us from space. At that time, Earth had only one ocean and one supercontinent, called Pangaea (meaning “whole land”), formed by the fusion of all the continents into a single large landmass (Figure 12.2). The ocean that surrounded Pangaea, called the Panthalassic Ocean, was much larger than the Pacific Ocean is today.

Pangaea was rifted (split apart) as a result of convection in the asthenosphere beneath it. The transition from Pangaea to today’s continental configuration occurred in two major steps: the opening of the Tethys Sea and, later, the opening of the Atlantic Ocean. The opening of the Tethys Sea, which began about 200 million years ago, created two large landmasses: Laurasia to the north and Gondwana to the south. Laurasia consisted of the landmasses that would become North America, Greenland, and Eurasia. Gondwana consisted of the landmasses that would become South America, Australia, Africa, India, and Antarctica. The opening of the northern Atlantic Ocean started about 180 million years ago, and by 65 million years ago, the Atlantic was fully opened. As the Atlantic Ocean basin developed, the shapes of the continents began to take their modern form (Figure 12.3).

Pangaea is only the most recent supercontinent. Rodinia formed about 750 million years before Pangaea (more than 1 billion years ago). There were other supercontinents before Rodinia, each of which broke apart and rearranged into a new supercontinent.

It took a long time and much scientific evidence for scientists to accept that continents move. As recently as the late 1950s, most scientists thought that the continents were fixed and immovable. How could entire continents made of solid rock move thousands of miles? As we will see, the growing body of evidence supporting the idea of a dynamic lithosphere eventually became indisputable.

The first person to propose a theory of continental movement was the German meteorologist Alfred Lothar Wegener (1880–1930; his name is pronounced VEGG-en-er). Wegener’stheory of continental drift proposed that continents move slowly across Earth’s surface. One piece of evidence that motivated Wegener to pursue his theory was the jigsaw-puzzle fit between the east coast of South America and the west coast of Africa (Figure 12.4).

388

Wegener was not the first to be struck by how the continental outlines fit together, but he was the first to assemble a formal theory proposing that the continents shift. Wegener supported his theory using evidence from matching rock types on separated continents, deposits of glacial gravels, and fossil remains of organisms (Figure 12.5).

Wegener formally proposed his theory in 1912 and published it in his book The Origins of Continents and Oceans. Wegener’s theory was dismissed by his professional colleagues because no forces sufficiently powerful to move continents such great distances were known.

Wegener’s biography reveals that the process of science and scientific revolutions are not neat and tidy. Established views are hard to change. Some scientists rejected Wegener’s idea because it conflicted with their paradigm (conceptual model) of the crust being fixed and impossible to move. Wegener’s theory was forgotten for five decades.

Scientific evidence supporting Wegener’s theory would come later, decades after his death. With the discovery of that evidence, Wegener’s theory was embraced by scientists and given a new name. Plate tectonics is the theory describing the origin, movement, and recycling of lithospheric plates and the resulting landforms.

Continental drift theory states that continents are moving. Plate tectonics theory addresses how the lithospheric plates on which continents rest move and interact.

389