Plate Tectonics and the Earth’s Geography

Plate Tectonics and the Earth’s Geography

Lecture 11

You look at a map of the world, and what you see is what looks like a jigsaw puzzle whose pieces have been slowly moved apart.

In the previous lecture, we saw that the early Earth was very different from today’s Earth, and distinctly less friendly to life. How did the Earth acquire today’s geography, a geography that has profoundly shaped the course of human history? Two gures, one German and one American, will play vital roles in our understanding of how the geography of today’s Earth was constructed. What they showed was that the Earth’s surface also has a history and has changed profoundly over time.

To understand modern ideas about the history of the Earth’s surface, it will help to contrast them with more traditional ideas. Traditionally, geologists assumed that, though mountains might rise (through processes such as earthquakes or volcanic activity) and fall (by erosion), the basic geography of the Earth’s surface was xed.

The idea that the Earth’s surface had changed was rst proposed seriously by a German meteorologist, Alfred Wegener (1880–1930). In 1912, Wegener published a book called The Origins of Continents and Oceans, in which he proposed a theory that would come to be known as “continental drift.” Wegener argued that the Earth’s continents had once been joined in a single supercontinent called Pangaea. What evidence did he offer for this revolutionary idea, which contradicted most of the basic assumptions of contemporary geologists?

The rst modern world maps, created early in the 16th century, showed that the continents seemed to t together like pieces of a jigsaw puzzle, particularly across the Atlantic Ocean. Wegener identi ed geological formations of a similar date and composition in West Norway, East Greenland, much of Britain, Northwest Africa, and the eastern seaboard of the U.S. This made sense only if all these regions had once been joined together. An Austrian geologist, Eduard Suess (1831–1914), had already proposed that the southern

continents of South America, Africa, Antarctica, Australia, and India had once been part of a single supercontinent that he named Gondwanaland. Wegener expanded this idea to suggest that all the world’s continents had once been joined together. Wegener showed that fossil species, such as the ancient plant Glossopteris, could be found on most of the continents of Gondwanaland.

He also found evidence of ancient glaciers in tropical regions of Africa, and of ancient tropical forests in Antarctica, which suggested that both continents had traveled huge distances. But how could whole continents possibly move? Wegener could not explain this. As a result, most geologists rejected his ideas from the 1920s to the 1960s. However, they had to get very creative to ignore the powerful evidence he had presented. For example, it became fashionable to argue that there had once been thin land bridges linking the continents.

New research after World War II suggested a solution to Wegener’s dilemma and revived interest in his ideas. In the 1960s, the new theory of plate tectonics emerged as the core idea of modern geology. American geologist Harry Hess (1906–1969), who was a naval commander during World War II, used the technology of sonar (developed to detect submarines) to survey the Paci c sea oor. Like most geologists, he assumed the ocean oors would consist of a huge, at ooze, from sediments washed into the seas. Instead, he was astonished to nd underwater chains of volcanic mountains. These turned out to be one section of a chain linking all the major oceans.

In 1962, studies of the deep sea oor showed that while regions close to the mid-oceanic ridges had normal magnetism, further out, the magnetism was reversed, and further out it was reversed again. In 1966, it was shown that the Earth’s magnetic eld periodically reverses, so that each band represented sea oor laid down in a different period. Improved dating techniques eventually con rmed that the bands closest to the ridges consisted of new oceanic crust and those further away were much older.

This evidence con rmed an idea Hess had proposed in 1962: that molten magma rose from the Earth’s interior at the oceanic ridges, creating new sea oor and pushing existing sea oor aside. Where did the old sea oor end up? Hess suggested that millions of years later it plunged back into the

interior. This idea explained why oceanic crust consisted mainly of volcanic basalts, and why it was so much younger than most continental crust.

These discoveries laid the foundations for the modern theory of plate tectonics. (The term “plate tectonics” is used to distinguish this theory from Wegener’s theory of “continental drift.”) According to plate tectonics, the Earth’s crust consists of a dozen or so plates, like sections of a cracked eggshell.

Convection currents in the semimolten regions beneath them move the plates around. Most geologically interesting events occur at the plates’ edges. At “divergent margins,” such as suboceanic ridges, magma from the interior rises and forces adjacent plates apart. This is why the Atlantic Ocean is widening by about 2 cm each year, about the speed at which your ngernails grow.

Elsewhere, at “convergent margins,” plates are forced together. There are two main types of convergent margins. If both plates are continental, they may buckle up at “collision margins” to form mountain chains such as the Himalayas, which were formed when the Asian and India plates collided. If an oceanic plate, consisting of heavy basaltic materials, meets a lighter continental plate, the oceanic plate will dive under the continental plate at “subduction margins.” As it does so, the plates will grind together and

Harry Hess, a geology professor at Princeton and World War II naval commander, used sonar to detect submarines —and volcanoes on the sea oor.

Corel Stock Photo Library.

generate colossal heat, which can melt rock and drive up mountain chains. The Andes were formed above a subduction margin created as the Paci c plate dove beneath the South American plate.

Where plates slide past each other at “transform margins,” such as the San Andreas Fault in California, friction builds before being suddenly released in earthquakes. Satellite images now enable us to measure tectonic movements with great precision. Driving the entire machinery is heat from the center of the Earth, created when the Earth was rst formed 4.5 billion years ago.

It is now clear that the Earth’s plates have joined periodically to form supercontinents such as Pangaea. Here, we summarize changes in the Earth’s geography over the last two of these cycles. About 540 million years ago, as the rst multi-celled organisms formed in the Cambrian period—the oldest period of the Phanerozoic eon—the ancient supercontinent of Rodinia was breaking up. About 420 million years ago, in the Silurian period, most continental plates were gathered in the south; the rst bony shes and the rst trees appeared. About 300 million years ago, in the Carboniferous period, continental plates were reassembling. This is the period of the rst reptiles, winged insects, and amphibians, and the period in which fossilized trees began to lay down huge beds of coal. About 180 million years ago, in the Jurassic period, most continental plates joined to form the supercontinent of Pangaea. The dinosaurs ourished during this period, as did ferns and conifers—and early forms of mammals, which (disappointingly) probably looked a bit like shrews! About 60 million years ago, Pangaea was splitting into two large continents, Laurasia and Gondwanaland. This is just after an asteroid impact wiped out most species of dinosaurs, allowing the mammals to ourish in the niches they left behind. Today, the Earth is as fragmented as it has ever been, a geographical reality that has had a profound impact on human history.

Essential Reading

Christian, Maps of Time, chap. 3. Redfern, The Earth.

Supplementary Reading

Questions to Consider

Bryson, A Short History of Nearly Everything, chap. 12. Macdougall, A Short History of Planet Earth.

1. What evidence did Wegener have for his idea of continental drift, and why was his idea rejected in the 1920s?

2. What are the main differences between Wegener’s idea and the modern theory of plate tectonics?