PLATE TECTONICS: THE UNIFYING THEORY

Earth is an extremely dynamic planet that constantly recycles its surface. Its lithosphere consists of about a dozen major plates and some smaller plates. Thes plates move around on the surface, creating new lithosphere and destroying old. Realization that the plates are constantly shifting led to the development of the paradigm of Plate Tectonics in the 1960's. The new paradigm explained the character and distribution of most large-scale features of the planet. The events and investigation that led to consensus among geologists about the new paradigm are a splendid example of the way science works.

The Discovery of Plate Tectonics

Plate tectonic theory explains nearly all of Earth's major features. It quickly became the unifying principle of geology after the fast-paced developments of the model in the 1960's. The seeds of the idea, however, go back several centuries. These earliest observations were only possible due to improvements in mapmaking.

In 1596, geographer Abraham Ortelius first noted the close fit of the shorelines of the Americas with those of Africa and Europe. In 1620, Sir Francis Bacon, Renaissance author and father of deductive reasoning, noted that the coastlines of South America and Africa might fit together like pieces of a jigsaw puzzle. His observation was quickly lost only rediscovered in the 1990's. The similarity of the shorelines is so striking that the idea of a jigsaw puzzle fit was proposed again several times. Your book provides an excerpt from Benjamin Franklin's 1782 letter in which he speculates that the planet's surface could be floating on a dense liquid and subject to movement as the liquid flowed.

In the mid 1800's, Antonio Snider-Pelligrini, a French scientist, noted the similarty between fossil plants found in the coal beds of Europe and North America. He speculated that the two continents had once been together. In 1858 he published a map showing how Africa and South America might have once formed a supercontinent. Austrian geologist Eduard Suess made a similar reconstruction and designated the southern supercontinent as Gondwanaland, usually referred to as Gondwana, today.

In 1908 F. B.Taylor and H. Baker noted that mountain belts tend to be arcuate and occur on the edges of continents. They also made correlations between mountain belts that are separated by great distances today, suggesting that their present positions could be due to motion of the continents.

Continental Drift

Alfred Wegener, a German meteorologist, surveyor, and balloonist, was a busy man intrigued by the fit of the continents. He found time to research for further evidence and write his ideas while recuperating from wounds he suffered as a German soldier in World War I. By this time coal, bearing tropical fossil plants, had been discovered in Antarctica. In 1915, he wrote a book about the breakup of the supercontinent Pangæa, composed of all of the present continents.

Wegener presented a number of lines of evidence to support his hypothesis, known as continental drift. They are:

1. The jigsaw fit of the continents
2. Similar plant fossils on distant continents: Glossopteris, a Permian seed fern genus, has been found on all of the southern continents, including Antarctica.
3. Identical reptile fossils in South America and Africa: Mesosaurus was a freshwater reptile and Lystrosaurus was a strictly terrestrial mammal-like reptile. Neither could have crossed the wide ocean. They must have walked or swum through fresh water.
4. Indian mammals: Prior to Eocene time, ~40 million years ago (Ma), all mammals in India were marsupials. Around 40 Ma the marsupials abruptly disappeared and were replaced by placental mammals.
5. The presence of abrupt structural terminations: Some mountain chains, such as the Appalachians extend for thousands of kilometers and then end abruptly. The Appalachians terminate off of the north coast of Newfoundland. Interestingly, western Europe's Calidonide Mountains pass along the length of Norway and then cross Scotland and northern England over to Ireland where they terminate abruptly at the island's southern coast. Curiously, these two mountain belts are the same age and the abrupt terminations are at almost the same latitude.
6. Location of Paleozoic climatic zones: Certain sedimentary rock types are associated with distinct climatic zones. Limestones, for instance, are deposited in abundance only at tropical latitudes. Desert sandstones typically accumulate 30º N or S of the equator. Coal is most likely to form in subtropical to mid-latitude deltaic environments. Glacial sediments are only abundant at high latitudes.

Earth's climatic zones often correspond to the types of sedimentary rocks found within the zones. Topographic features and the positions of the continents can cause variations in this pattern. Wegener argued that if the continents were fixed, the position of the Paleozoic climatic zones should be the same as they are shown on the present world map. They are not!

1. • If the continents were in fixed positions, then ancient rocks should correspond to modern climatic zones.
   • Looking at just Paleozoic coals, however, we see them at many different latitudes, including Antarctica's Queen Maud Mountains at 85º S and on the island of Spitzbergen (Svalbard) at more than 70º N.
   • When the position of the other climatically-determined Paleozoic sedimentary rocks are investigated they produce a similar chaotic disarray.
2. Permian glacial striations: When a glacier moves over bedrock, rocks carried in the base of the ice scratch the underlying bedrock, leaving a record of the direction in which the glacier was moving. Striations that resulted from a glaciation that took place during Pennsylvanian-Permian time (about 300 Ma) appear to radiate out of central Africa. Contemporaneous striations found in Australia, India, and South America are oriented in an east-west direction.

Permian glacial striations appear to radiate out of central Africa on today's map. They are seen in a roughly east-west orientation in South America, India, and Australia.

Wegener's continental drift hypothesis sparked a lot of lively debate in the 1920's. But he had two fundamental problems in trying to convince the geological community that his ideas were valid. The major problem was that he didn't have a mechanism or reason why the continents were drifting. He envisioned the continents to be ships of granite plowing through a sea of basalt. No one could see where the tremendous amount of energy needed to do this could come from.

The second problem was that Wegener envisioned the drifting process to take place at a much faster rate than it actually occurs. This was based largely on the assumption that Pangæa had done all of its spreading since the last ice age (< 20,000 years). His wishful thinking blinded him from a surveying error he had made during field work in northern Greenland.that corroborated an extremely fast rate of continental drift. He returned to Greenland in 1930 but perished on the ice cap at age 50. Had he not had an untimely death he would undoubted have continued his journey of discovery. He was one of the great unsung minds of the 20th Century.