2 From continental drift to plate tectonics
2.1 Continental drift
The remarkable notion that the continents have been constantly broken apart and reassembled throughout Earth's history is now widely accepted. The greatest revolution in 20th century understanding of how our planet works, known as plate tectonics, happened in the 1960s, and has been so profound that it can be likened to the huge advances in physics that followed Einstein's theory of relativity. According to the theory of plate tectonics, the Earth's surface is divided into rigid plates of continental and oceanic lithosphere that, through time, move relative to each other, and which increase or decrease in area. The growth, destruction and movement of these lithospheric plates are the major topics of this course, but it is first worth considering how the theory actually developed from its beginnings as an earlier idea of 'continental drift'.
The German meteorologist Alfred Wegener (1880-1930) is largely credited with establishing the fundamentals of the theory that we now call plate tectonics. The idea that continents may have originally occupied different positions was not a new one (Box 1), but Wegener was the first to present the evidence in a diligent and scientific manner.
Box 1: Continental drift to plate tectonics: the evolution of a theory
1620 Francis Bacon commented upon the 'conformable instances' along the mapped Atlantic coastlines.
1858 Antonio Snider-Pellegrini suggested that continents were linked during the Carboniferous Period, because plant fossils in coal-bearing strata of that age were so similar in both Europe and North America.
1885 Austrian geologist Edward Seuss identified similarities between plant fossils from South America, India, Australia, Africa and Antarctica. He suggested the name 'Gondwana' (after the indigenous homeland of the Gond people of north-central India), for the ancient supercontinent that comprised these land masses.
1910 American physicist and glaciologist Frank Bursley Taylor proposed the concept of 'continental drift' to explain the apparent geological continuity of the American Appalachian mountain belt (extending from Alabama to Newfoundland) with the Caledonian Mountains of NW Europe (Scotland and Scandinavia), which now occur on opposite sides of the Atlantic Ocean.
1912 Alfred Wegener reproposed the theory of continental drift. He had initially become fascinated by the near-perfect fit between the coastlines of Africa and South America, and by the commonality among their geological features, fossils, and evidence of a glaciation having affected these two separate continents. He compiled a considerable amount of data in a concerted exposition of his theory, and suggested that during the late Permian all the continents were once assembled into a supercontinent that he named Pangaea, meaning 'all Earth'. He drew maps showing how the continents have since moved to today's positions. He proposed that Pangaea began to break apart just after the beginning of the Mesozoic Era, about 200 Ma ago, and that the continents then slowly drifted into their current positions.
1920-1960 A range of geophysical arguments was used to contest Wegener's theory. Most importantly, the lack of a mechanism strong enough to 'drive continents across the ocean basins' seriously undermined the credibility of his ideas. The theory of continental drift remained a highly controversial idea.
1937 South African geologist Alexander du Toit provided support through the years of controversy by drawing maps illustrating a northern supercontinent called Laurasia (i.e. the assembled land mass of what was to become North America, Greenland, Europe and Asia). The idea of the Laurasian continent provided an explanation for the distribution of the remains of equatorial, coal-forming plants, and thus the widely scattered coal deposits in the Northern Hemisphere.
1944 Wegener's theory was consistently championed throughout the 1930s and 1940s by Arthur Holmes, an eminent British geologist and geomorphologist. Holmes had performed the first uranium-lead radiometric dating to measure the age of a rock during his graduate studies, and furthered the newly created discipline of geochronology through his renowned book The Age of the Earth. Importantly, his second famous book Principles of Physical Geology did not follow the traditional viewpoints and concluded with a chapter describing continental drift.
1940-1960 The complexity of ocean floor topography was realised through improvements to sonar equipment during World War II. Accordingly, there was a resurgence of interest in Wegener's theory by a new generation of geophysicists, such as Harry Hess (captain in the US Navy, later professor at Princeton), through their investigations of the magnetic properties of the sea floor. In addition, an increasing body of data concerning the magnetism recorded in ancient continental rocks indicated that the magnetic poles appeared to have moved or 'wandered' over geological time. This apparent polar wander was explained by the movement of the continents, and not the magnetic poles.
1961 The American geologists Robert Dietz, Bruce Heezen and Harry Hess proposed that linear volcanic chains (mid-ocean ridges) identified in the ocean basins are sites where new sea floor is erupted. Once formed, this new sea floor moves toward the sides of the ridges and is replaced at the ridge axis by the eruption of even younger material.
1963 Two British geoscientists, Fred Vine and Drummond Matthews, propose a hypothesis that elegantly explained magnetic reversal stripes on the ocean floor. They suggested that the new oceanic crust, formed by the solidification of basalt magma extruded at mid-ocean ridges, acquired its magnetisation in the same orientation as the prevailing global magnetic field. These palaeomagnetic stripes provide a chronological record of the opening of ocean basins. By linking these observations to Hess's sea-floor spreading model, they lay the foundation for modern plate tectonics.
1965 The Canadian J. Tuzo Wilson offered a fundamental reinterpretation of Wegener's continental drift theory and became the first person to use the term 'plates' to describe the division and pattern of relative movement between different regions of the Earth's surface (i.e. plate tectonics). He also proposed a tectonic cycle (the Wilson cycle) to describe the lifespan of an ocean basin: from its initial opening, through its widening, shrinking and final closure through a continent-continent collision.
1960s-present day. There was an increasingly wide acceptance of the theory of plate tectonics. A concerted research effort was made into gaining a better understanding of the boundaries and structure of Earth's major lithospheric plates, and the identification of numerous minor plates.
Evidence for Wegener's ideas on continental drift is presented on the pages that follow, and remains the root of modern continental reconstructions.