by Anne E. Egger, Ph.D.
The Himalayas are often referred to as the "roof of the world" because they host the highest peaks on earth, most famously Mt. Everest at 8,848 meters above sea level. The rock that caps Mt. Everest is limestone, which forms at the bottom of warm, shallow seas and is composed entirely of fossilized marine creatures, everything from plankton to clams and fish. For years, geologists struggled to explain how the hardened remains of tiny sea organisms could exist at the top of a mountain range.
Into the 1900s, many scientists believed that as the earth cooled after the Big Bang, the planet's surface contracted and wrinkled like the skin of a raisin. The "raisin" theory implied that mountain ranges like the Himalayas were forced up by the wrinkling process. This theory assumed that all of the features on the earth had formed during one cooling event and that the planet was relatively static, changing little as the cooling (and wrinkling) slowed to a halt over billions of years.
Alfred Wegener, a German geophysicist and meteorologist, was not satisfied by this explanation. His ideas drew on the widely recognized fact that Africa and South America appeared to fit together like jigsaw puzzle pieces. He collected data from the continents on both sides of the Atlantic, finding that fossils and rock types along the eastern coast of South America matched those on the western coast of Africa. When he added the northern continents to the puzzle, Wegener realized that the chain of Appalachian Mountains in North America continued as the Caledonian Mountains in northern Europe.
Figure 1: Map showing distribution of fossils on the southern continents. These distributions caused Wegener to join the continents as shown in the map. Image from the USGS publication This Dynamic Earth
To explain these data, Wegener proposed the theory of continental drift in his book The Origins of the Continents and the Oceans, published in Germany in 1915. His theory stated that all of the continents had originally been joined together in a supercontinent called Pangaea. About 200 million years ago, the theory continued, Pangaea broke apart and the continents slowly drifted to their present positions.
When Wegener's book was translated into English, French, Spanish, and Russian in 1924, he was widely ridiculed for his suggestion that the continents had moved. One of the main problems with his theory was that he did not propose a driving mechanism for the motion of the continents. What was the force that moved the continents? Where did it come from? How much force was needed to move a continent?
The driving mechanism, an important key to the continental drift theory, lay out of reach until the 1960s. Wegener had made his claims based on data from the continents, but the oceans cover 70 percent of the earth's surface – a vast area hidden from view under kilometers of water. After Wegener published his theory, major technical and scientific developments allowed scientists to map the ocean floor and detect paleomagnetic reversals in the rocks. These two sets of data provided geologists with additional evidence for the process of continental drift.
Before ships equipped with sonar started mapping the ocean floor in the 1920s, the crust below the seas was thought to be flat and featureless. The sonar maps, however, showed the seafloor to be anything but - the ocean floor hosted tremendously deep valleys, mountain chains rivaling the North American Rockies, and vast plains. Most remarkably, a long ridge was found to run down the middle of the Atlantic Ocean, rising 1 to 2 km above the surrounding ocean floor and paralleling the continental coasts on both sides. Similar mid-ocean ridges were mapped in the eastern Pacific Ocean and the western Indian Ocean. Volcanic activity occurred along these ridges. Clearly, the ridges had something to do with continental drift, but what?
In a 1962 paper entitled "History of Ocean Basins," Harry Hess, a geologist at Princeton University, proposed that the mid-ocean ridges marked regions where hot magma rose close to the surface. Further, he suggested that the extrusion of magma at the ridges pushed the ocean floor away from the ridges like a conveyor belt. In deep trenches like those found off the coast of South America and Japan, the spreading ocean floor was forced down below the thick continents into subduction zones. Hess's theory of "seafloor spreading" offered a compelling driving mechanism for Wegener's continental drift, but it needed more proof.
Figure 3: Magnetic stripes.
The same year that Hess proposed his theory, the U.S. Navy published a report that summarized findings concerning seafloor magnetism. During World War II, ships dragging devices to measure magnetism found bands of alternating strong and weak magnetism in the rocks of the seafloor (originally these magnetometers had been designed to locate submarines). These bands, irregular in width, not only paralleled the mid-ocean ridges, but were patterned symmetrically about the crest of these ridges.
The magnetism was caused by the presence of magnetic minerals in the rocks. Scientists were not surprised to learn that seafloor rocks contain the magnetic mineral magnetite. When magma from the ridges rises and cools, it crystallizes, locking the magnetite crystals into alignment with the earth's magnetic field like the needle of a compass (see our Earth Structure module). The existence of the earth's magnetic field had been known since ancient times, but only after World War II did scientists realize that the magnetic field is not constant - it fluctuates in intensity and occasionally even reverses itself. Today we consider the field to have "normal" polarity, and our compass needles point to the north. But at various times in the past, the polarity has been reversed - any compass needles would have instead pointed to the South Pole. This phenomenon of magnetic reversals had previously been observed in continental rocks, and was clearly the case for oceanic rocks as well.
In 1963, Fred Vine and Drummond Matthews, two British geologists, joined the map of the Mid-Atlantic Ridge with the symmetric bands of magnetism on the seafloor. Where the navy ships mapped strong magnetism, rocks showed normal polarity; where they mapped bands of weak magnetism, the rocks showed reversed polarity. These strongly patterned paleomagnetic reversals recorded on the seafloor provided the necessary proof of Hess's seafloor spreading. Specifically, they proved that new crust was continuously being generated at the mid-ocean ridges, where magma cooled and magnetite crystals "locked in" according to the orientation of the earth's magnetic field at that time. The continents no longer had to "drift" to their present locations-they were driven by the slow and steady magma "conveyor belts" at the mid-ocean ridges.
Concept simulation - Recreates activity at Plate Boundaries.
The work of Hess, Vine, and Matthews resulted in a new map of the earth, one that included plate boundaries in addition to coastlines. Boundaries were drawn at mid-oceanic ridges and subduction zones.
Figure 4: Blue = mid-ocean ridges, Red = subduction zones.
Today, much of the evidence concerning plate tectonics is acquired with satellite technology. Through use of the global positioning system (GPS) and other satellite-based data collection techniques, scientists can directly measure the movement and speed of plates on the surface of the earth. Speeds range from 10 to 100 mm per year, confirming the long-held belief that plates move at a slow but constant rate.
The Himalayas, as it turns out, started forming about 40 million years ago when the Indian Plate collided head-on with the Eurasian Plate, shoving and folding rocks that had formed below sea level into lofty peaks. Because the Indian Plate is still moving northward, the Himalayas are still rising at a rate of about 1 cm per year. We no longer need to invoke a shrinking, wrinkled earth to explain the marine fossils at the top of the world.
The earth is incredibly dynamic - mountain chains build and erode away, volcanoes erupt and go extinct, seas advance and recede - and these changes are all a result of the processes of plate tectonics. Before Wegener, few had conceived of such a world. His continental drift theory was the first step in the development of plate tectonic theory, the foundation upon which modern geology is built.
Anne E. Egger, Ph.D. "Plate Tectonics I: The Evidence for a Geologic Revolution," Visionlearning Vol. EAS-1 (1), 2003.