Early Evidence for Plate Tectonics

Magma generation, igneous intrusions, metamorphism, volcanic action, earthquakes, faulting, and folding are usually the result of plate tectonic activity. The earth's crust is divided into six large pieces, and about twenty smaller pieces, by deep fault systems. These crustal plates include both oceanic and continental crust. Underlying convection currents in the mantle and lower crust are thought to create forces that push and pull these plates at the surface. Intense geologic activity occurs where plates move apart (divergent boundaries), collide (convergent boundaries) or slide past one another (transform boundaries). About 200 million years ago, it is thought, plate tectonic forces began to break a single continental land mass into pieces that spread apart to form the continents as we know them today.

Continental drift. As world maps began to improve in the 1600s and 1700s, scientists noticed that the continents, especially South America and Africa, would roughly fit together like the pieces of a jigsaw puzzle were they in contact with one another (Figure 1). The idea that the continents were once joined together and somehow split apart was originally called continental drift and was the precursor to modern‐day plate tectonic theory. As more was learned over the centuries (especially the existence of deep midoceanic rifts that parallel the outlines of the continents), the idea of plate tectonics became more and more plausible to geologists.

Figure 1

Formation of the Continents

The work of Alfred Wegener. Scientists began to talk seriously about continental drift in the mid 1800s. Alfred Wegener, a German climatologist, noticed that certain mountain belts, rock formations, strikes and dips, and fossil remains were nearly identical in parts of South America, Australia, India, and Africa. He reasoned that if a shared species such as Mesosaurus survived swimming the oceans between the continents, their remains should be widely distributed in oceanic sediments—yet they have been found only in eastern South America and southern Africa. Considering the distance between the continents, Wegener concluded that to have the same unique fossil assemblages they had to have been part of the same larger landmass. He named this theoretical supercontinent Pangaea, which, he suggested, split to form Laurasia and Gondwanaland. Laurasia, the northern part, later fragmented again to form North America and Eurasia. Gondwanaland broke apart to form South America, Africa, India, Australia, and Antarctica.

Wegener's studies also revealed that a well‐defined period of late Paleozoic glaciation affected the southern Gondwanaland continents. If the continents had been at their present positions and covered by the same ice sheet, the weather would have been cold enough to result in glaciation of the northern continents; however, late Paleozoic climates in North America and Europe were actually warm and humid. The occurrence of glacial striations (and the directions of ice movement) on the southern continents strongly suggest Gondwanaland was a single landmass toward the end of the Paleozoic era. The ice sheet was centered over present‐day Antarctica and spread westward over part of South America, north and westward into Africa, and eastward into India and Australia, forming a radial pattern.

Further intrigued, Wegener studied rocks around the world to reconstruct the climate zones for each geologic time period. For example, limestones and reefs indicate warm ocean waters near the equator, and glacial deposits would indicate colder climates. Wegener discovered that the positions of the North and South Poles in the geologic past were quite different from their positions today, at least in their relationship to the positions of the continents. For example, fossil trees from coal fields in frozen terrain like Siberia contain no growth rings, indicating they grew very rapidly in a tropical climate.

This evidence for polar wandering meant that either the geographical poles moved and the continents were stationary or the continents moved and the geographical poles remained stationary.