Continental Drift and Plate Tectonics: Theory and History

Introduction

It is now universally recognized that the continents and other landmasses on Earth are constantly moving — albeit at a very slow rate — and have been doing so for millions of years. These landmasses have collided, broken apart, and drifted across the planet while floating on the fiery mantle beneath the outer layer of Earth's crust. The Continental Drift and Plate Tectonic theories indicate that about 250 million years ago there was only one continent on Earth, named Pangaea (Greek for "all the Earth"). This great landmass fragmented, and its parts began to move away from one another, forming the great oceans between the continents. As an extension of these theories, it can now be predicted with a fair degree of confidence that the moving continents will one day come together again to form a single giant landmass. This paper describes the Continental Drift and Plate Tectonic theories, traces their history, discusses the key figures who developed them, and examines the current state of scientific knowledge about Earth's geology and paleontology.

The idea that Earth's continents have drifted has a long history. As far back as 1596, the Dutch mapmaker Abraham Ortelius, in his work Thesaurus Geographicus, suggested that the Americas were "torn away from Europe and Africa . . . by earthquakes and floods." He was the first — though certainly not the last — to notice the apparent jigsaw fit of the eastern bulge of South America into the bight of Africa (Kious and Tilling, 1999).

Theory of Continental Drift

Around 1850, the French scientist Antonio Snider-Pellegrini, while researching the similarity of fossil plants and coal deposits in North America and Europe, concluded that the phenomenon could only be explained if the two continents had once been connected. In 1908, Frank B. Taylor of the United States invoked the notion of continental collision to explain the formation of some of the world's major mountain ranges.

Alfred Wegener (1880–1930), a German meteorologist, was a brilliant interdisciplinary scientist who first formally proposed the theory of Continental Drift. Though ridiculed at the time, the theory was later accepted by the scientific community and gave rise to one of the most important geological theories in history: the theory of plate tectonics. Wegener obtained his doctorate in planetary astronomy, carried out most of his research in meteorology, and yet his most notable contribution was in geology — a testament to his interdisciplinary approach.

Alfred Wegener and His Landmark Proposal

Wegener believed that only by combining the findings and evidence of all earth sciences could one learn the truth about Earth's past. In 1911, he came across a scientific paper listing fossils of identical plants and animals on opposite sides of the Atlantic. The accepted explanation at the time was that land bridges — now sunken — had once connected the continents. But Wegener was intrigued by the close fit between the coastlines of Africa and South America and became convinced that the continents had been joined together at one time. He proceeded to gather scientific evidence in support of his view. He soon found that several geographical features on either side of the Atlantic matched closely — for example, the mountains of eastern North America and the Scottish Highlands. Moreover, he discovered that fossils found in certain locations often indicated that an animal or plant had lived in a climate entirely different from that of the region where its remains were found (Waggoner, 1996).

Wegener published his theory of Continental Drift in his book The Origin of Continents and Oceans in 1915. The theory held that about 300 million years ago all the continents had formed a single mass, called Pangaea, which later split into pieces and began to move apart — a movement continuing to this day. Despite presenting considerable evidence in support of his theory, Wegener was met with derision from the scientific community. Beyond a natural resistance to revolutionary ideas, opposition arose because Wegener could not adequately explain how the continents moved. He argued that centrifugal and tidal forces drove the continents through Earth's crust like icebreakers plowing through ice. This explanation was flawed: centrifugal and tidal forces were far too weak to move entire continents, and some scientists demonstrated that it was physically impossible for a large mass of rock to plow through the ocean floor without breaking apart. Consequently, despite scattered support, the majority of scientists continued to believe in the old land-bridge theory.

While most scientists had rejected Wegener's theory, a few began to build upon his ideas. In 1929, Arthur Holmes — an English geologist — suggested that convection currents within Earth's mantle, driven by radioactive heat, might provide the mechanism that continental drift theory lacked. The idea rested on the principle that as a substance is heated its density decreases and it rises to the surface, until it cools and sinks again. Holmes proposed that this repeated heating and cooling produces a thermal convection capable of breaking apart a continent and forcing its pieces in opposite directions, carried by the currents. The idea attracted little attention at the time but would be revisited later (Weil, 1997).

Beginning in the 1950s, new evidence emerged to revive interest in Wegener's long-discredited theory. British geophysicist Stanley K. Runcorn showed that the north magnetic pole had wandered from its original position in the past, and continental drift offered a reasonable explanation for this fact.

Support for the Continental Drift Theory

Other developments added further support. More accurate mapping of the ocean floor revealed the presence of a great underwater mountain range — named the global mid-ocean ridge — that virtually encircled the Earth "like the seam on a baseball" (Kious and Tilling, 1999). It was also found that the sediment layer on the floor of the Atlantic was far thinner than originally believed, indicating that the ocean floor could not have been forming for the 4 billion years previously assumed.

How the mid-ocean ridge formed was an intriguing question. In the 1960s, evidence began to emerge that the ridge marks structurally weak zones in the ocean floor from which new magma rises from deep within Earth and settles along the crest of the ridges, creating new oceanic crust. This process was later called seafloor spreading — a term coined by American scientist Robert S. Dietz, who collaborated with Harry H. Hess in his research.

American geologist Harry H. Hess was the first to grasp the full implication of seafloor spreading. He drew on Holmes' convection theory to explain how Earth's crust could expand along oceanic ridges without an increase in the planet's overall size. Hess reasoned that the new oceanic crust, continuously spreading away from the ridges in a conveyor-belt-like motion, eventually descends — millions of years later — into the oceanic trenches: the deep, narrow canyons along the rim of the Pacific Ocean basin. This reasoning was further supported by the discovery that the youngest regions of ocean floor lie along mid-oceanic ridges, with age increasing with distance from the ridges, and that the oldest seafloor is typically found near the deep-sea trenches. According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. His theory explains why Earth does not grow larger through seafloor spreading, why so little sediment accumulates on the ocean floor, and why oceanic rocks are much younger than continental rocks (Kious and Tilling, 1999).

The developments described above eventually evolved into the Plate Tectonic Theory, the main features of which are as follows:

Earth's surface (called the lithosphere) is covered by a series of crustal plates. The ocean floors are continually moving — spreading from the center, sinking at the edges, and being regenerated. Convection currents beneath the plates move the crustal plates in different directions. The heat driving these convection currents derives from radioactivity deep in Earth's mantle (Weil, 1999).