Tectonic Plate study is a Fantastic topic of modern Earth Science. When looking at the shape of the ocean between the continents of Africa and South America, one naturally wonders if the two continents were closer together in the past. What evidence would you need to be convinced that the two were at one time closer together and that the continents are still moving? What would you think if there were similar fossils on both sides of the Atlantic? What would you think if you could take GPS satellite measurements and see the two moving apart? Indeed, this is exactly what we observe—the Atlantic Ocean is widening.
Close inspection of Earth’s landforms, both above and beneath the ocean, reveal that Earth’s crust is divided into huge tectonic plates whose motions produce earthquakes, volcanoes, mountain ranges, and oceanic trenches. This picture of Earth’s crust, now known as plate tectonics (from the Greek tekton, meaning “builder”), has come to be the central unifying theory of geology, much as the theory of evolution has become the centerpiece of modern biology.
The Tectonic Plate
Based on the apparent fit of the continents, the German meteorologist Alfred Wegener advocated “continental drift”—the idea that the continents on either side of the Atlantic Ocean have simply drifted apart. In 1915 he published the theory that there had originally been a single gigantic supercontinent, which he called Pangaea (meaning “all lands”). Based on how fast South America and Africa are separating today, this separation probably began roughly 200 million years ago, during what geologists refer to as the early Jurassic period, when dinosaurs dominated the land.
Over time, geologists have refined this theory, arguing that Pangaea must have the first split into two smaller supercontinents, which they called Laurasia and Gondwana, separated by what they called the Tethys Ocean. Gondwana later split into Africa and South America, with Laurasia dividing to become North America and Eurasia. According to this theory, the Mediterranean Sea is a surviving remnant of the ancient Tethys Ocean.
The actual mechanism that was driving the continents’ motions wasn’t evident until the mid-1950s when geologists discovered that material is being forced upward to the crust from deep within Earth. Bruce C. Heezen of Columbia University and his colleagues began discovering long mountain ranges on the ocean floors, such as the Mid-Atlantic Ridge, which stretches all the way from Iceland to Antarctica (Figure 5-10). During the 1960s, Harry Hess of Princeton University and Robert Dietz of the University of California carefully examined the floor of the Atlantic Ocean. They concluded that rock from Earth’s mantle is being melted and then forced upward along the Mid-Atlantic Ridge, which is, in essence, a long chain of underwater volcanoes.
The upwelling of new material from the mantle to the crust forces the existing crust to separate, causing seafloor spreading. For example, the floor of the Atlantic Ocean to the east of the MidAtlantic Ridge is moving eastward and the floor to the west is moving westward. By explaining what fills in the gap between continents as they move apart, seafloor spreading helps to fill out the theories of continental drift. Because of the seafloor spreading from the Mid-Atlantic Ridge, South America and Africa are moving apart at a speed of roughly 3 cm per year. Working backward, these two continents would have been next to each other some 200 million years ago.
Tectonic Plate Motion and Convection
What makes tectonic plates move, anyway? The high temperatures within Earth cause energy in the form of heat to flow outward from Earth’s hot core to its cool crust. Hot material deep in the Earth is less dense than cooler material farther away from the core and tends to rise, much the same way heated air in a giant hot air balloon will rise high above Earth’s surface. As hot mantle material rises, it transfers heat to its surroundings. As a result, the rising material cools and becomes denser. It then sinks downward to be heated again, and the process starts over. This up-and-down motion is called convection. You can see convection currents in action by heating water on a stove. Figure 5-11 shows that convection occurs when a fluid is heated from below. The heat that drives convection in Earth’s outer layers actually comes from very far below, at the boundary between the outer and inner cores. As material deep in the liquid core cools and solidifies to join the solid portion of the core, it releases the energy needed to heat the overlying mantle and cause convection.
The asthenosphere, or upper level of the mantle, is hot and soft enough to permit an oozing, plastic flow. Atop the asthenosphere is a rigid layer, called the lithosphere (from the Greek word for “rock”). The lithosphere is divided into tectonic plates that ride along the convection currents of the asthenosphere. The crust is simply the uppermost layer of the lithosphere, with a somewhat different chemical composition than the lithosphere’s lower regions.
The Picture shows how convection causes plate movement. Molten subsurface rock seeps upward along oceanic rifts, where tectonic plates are separating. The Mid-Atlantic Ridge is an oceanic rift and seafloor spreading is occurring there. Where tectonic plates collide, cool crustal material from one of the tectonic plates sinks back down into the mantle along a subduction zone. One such subduction zone is found along the west coast of South America, where the oceanic Nazca Plate is being subducted into the mantle under the continental South American plate at a relatively speedy 10 centimeters per year. As the material from the subducted plate sinks, it pulls the rest of its plate along with it, thus helping to keep the plates in motion. New material is added to the crust from the mantle at the oceanic rifts and is “recycled” back into the mantle at the subduction zones. In this way, the total amount of crust remains essentially the same.
Geologists today realize that earthquakes, and the vast majority of volcanoes, tend to occur at the boundaries of Earth’s crustal plates, where the plates are colliding, separating, or rubbing against each other. The boundaries of the tectonic plates, therefore, stand out clearly when earthquake locations are plotted on a map as in the picture below.
The boundaries between tectonic plates are the sites of some of the most impressive geological activity on our planet. Great mountain ranges, such as the Sierras and Cascades along the western coast of North America and the Andes along South America’s west coast are thrust up by ongoing collisions between continental tectonic plates and the tectonic plates of the ocean floor. Subduction zones, where the old crust is drawn back down into the mantle, are typically the locations of deep oceanic trenches, such as the Peru-Chile Trench off the west coast of South America.
The Supercontinents & Cycle of Supercontinents
Plate tectonics theory offers insight into geology on the largest of scales, that of an entire supercontinent. In recent years, geologists have uncovered evidence that points to a whole succession of supercontinents that once broke apart and then reassembled. Pangaea is only the most recent supercontinent in this cycle, which repeats about every 500 million (5 108 ) years. As a result, intense episodes of mountain building have occurred at roughly 500-million-year intervals.
Apparently, a supercontinent sows the seeds of its own destruction because it blocks the flow of heat from Earth’s interior. As soon as a supercontinent forms, temperatures beneath it rise, much as they do under a book lying on an electric blanket. As heat accumulates, the lithosphere domes upward and cracks. Molten rock from the overheated asthenosphere wells up to fill the resulting fractures, which continue to widen as pieces of the fragmenting supercontinent move apart.
It can take a very long time for the heat trapped under a supercontinent to escape. Although Pangaea broke apart some 200 million years ago, the mantle under its former location is still hot and still trying to rise upward. As a result, Africa—which lies close to the center of this mass of rising material—sits several tens of meters higher than the other continents.
The changes wrought by plate tectonics are very slow on the scale of a human lifetime, but they are very rapid in comparison with the age of Earth. For example, the period over which Pangaea broke into Laurasia and Gondwana was only about 0.4% of Earth’s age of 4.56 109 years. (To put this in perspective, 0.4% of an average human life is about 4 months.) The lesson of plate tectonics is that the seemingly permanent face of Earth is in fact dynamic and ever-changing.