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5.13 Plate Tectonics

For centuries, geologists studied observable characteristics of earthquakes, mountain building, and volcanic eruptions without understanding the underlying geophysics creating these features. It wasn't until the last century that progress was made thanks to one simple observation on a map. If you stare at a globe long enough you may notice that the continents seem appear to be puzzle pieces that could be fit together. Sir Francis Bacon first noted this in the 17th century, but it wasn't until the early 1900s that the implications of this observation were realized by German geophysicist Alfred Wegener. He pointed out that the coastlines of some continents, such as South America and Africa, could fit together, and in fact have matching geology and biology on either side of the sea. Wegener boldly proposed that those two continents once were joined, and had split and drifted apart in the ancient past. He called this phenomenon continental drift.

Portrait of Alfred Wegener. Click here for original source URL

Few scientists accepted this seemingly crazy notion that continent-sized blocks of the Earth's surface could move across the face of the globe. Wegener and other proponents of continental drift could match up distinctive rock formations and fossils on opposite sides of the Atlantic Ocean, but could suggest no reasonable mechanism for the motion of the continents. Finally, in the 1960s, scientists used new techniques to study the sea floor. First, using sonar developed from military technology, they mapped the ocean floors. The ocean floors are not in fact featureless plains, but they have mountains and canyons as impressive as those on the continents. When scientists used radioactive dating to measure the age of the underwater mountains that bisect the Atlantic Ocean, they found them to be relatively young. Finally, they used patterns of magnetized rock on the sea floor to show that the Atlantic sea floor was slowly spreading over time. All of this evidence provided support for Wegener's hypothesis.


Continental drift?fossil evidence. As noted by Snider-Pellegrini and?Alfred Wegener, the locations of certain?fossil?plants and animals on present-day, widely separated?continents?would form definite patterns (shown by the bands of colors), if the continents are rejoined. Click here for original source URL.

The theory that Earth's surface features are molded primarily by moving plates, dragged along by currents in the underlying plastic asthenosphere, is called the theory of plate tectonics. This theory is now universally accepted among geologists. Its acceptance was the major revolution in geological science in the 20th century.

Portrait of Francis Bacon. Click here for original source URL.

The blocks of the lithosphere that move are called plates, and they include both continental masses and large regions of the sea floor. Where two plates move apart, fresh basalt wells up from the mantle and forms new crustal rock to fill in the gaps. This is called a spreading center, one example of which is the Mid-Atlantic Ridge, a mostly underwater seam where young lava extrudes. Iceland, a highly volcanic island, is an exposed section of the Mid-Atlantic Ridge where volcanoes rise above the sea.

The collision of two continental plates can compress the lithosphere laterally and crumple it to form mountain ranges. For example, the Himalayas are young mountains formed where the Indian plate has pushed into the southern Asian plate. In places where a continental plate collides with an oceanic plate, the denser, basaltic ocean floor is forced below the less dense, granitic continent. This process is called subduction, and the surface expression is a deep trench where the oceanic plate is driven downward. Eventually, the oceanic plate is forced deep enough to melt. This cycle of new rock being created at mid-ocean ridges and destroyed at subduction zones ensures that the ocean floors are much younger than the continents, which are never subducted.

Plates can also slip along next to each other at transform boundaries. The San Andreas Fault in California is a transform fault, and the source of many earthquakes. In fact, a global map of seismic activity reveals an outline of all of the Earth's tectonic plates. This is because earthquakes are concentrated along plate boundaries, where stresses build up from plates moving against each other. An earthquake is the sudden release of that stress. Likewise, volcanic activity is clustered along mid-ocean ridges and subduction zones, where molten mantle material has access to the surface. Unfortunately, over half a billion people live on the "ring of fire" that traces the plate boundaries at the rim of the Pacific Ocean.

The motions of the plates might be imperceptible, but they are real. The techniques of radio astronomy have been used to measure directly the motion of continents from space. Europe and North America are moving apart at a rate of about 5 centimeters per year, as fast as a fingernail grows. The Nazca plate in Peru is racing along at 16 centimeters per year, as fast as hair grows. About 150-200 million years ago, Europe and North America were locked in a geological embrace. In fact, all the continents were joined in a primeval landmass called Pangaea.

Earth is the only planetary body in the solar system to have this type of large-scale motion of lithospheric plates, although the moon Europa has a form of ice tectonics. One of the results of plate tectonics on Earth is a bimodal distribution of elevations. This is another way of saying that the continents are distinctly higher in altitude than the ocean floors. Even though we can't look at fossil records or detailed seismic data from other planets, we can look at their topography and see if there is any evidence of this type of separation. There is not. The reasons for Earth's unique tectonics are not fully understood, but other planets like Mars may have lithospheres that are too thick to break into plates. Other planets also lack the liquid water necessary to lower the melting point of mantle rock and provide lubrication for motion.

We can get some insights into how science works from the story of plate tectonics. At a personal level, the story was not a happy one for Alfred Wegener. He only had fragmentary evidence to support his radical hypothesis, and some of his ideas were wrong. He also had no idea what force moved the continents. Wegener was ridiculed by some of his colleagues; he died young, without seeing his hypothesis accepted. This parallels the story of Aristarchus, who had the correct idea of a heliocentric universe but very little evidence to support it. Over the longer term, scientists do accept new ideas, and theories that were once radical become conventional wisdom. The evidence for plate tectonics eventually became so overwhelming that geologists embraced the idea of an active Earth.