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PLATE TECTONICS

In 1912 a German meteorologist named Alfred Wegener proposed the theory of continental drift. He based his theory in part on the matching coastlines of South America and Africa as well as on similarity of plant and animal fossils found on different continents. Wegener’s view of the drifting continents was controversial. Not until geomagnetic surveys showed a pattern of matching strips of newly formed ocean floor on either side of the mid-Atlantic ridge could scientists begin to understand how tectonic processes from deep inside Earth could drive the continents apart. Wegener’s work and the discovery that lava emerging at mid-oceanic ridges creates new oceanic crust led to the development of the theory of plate tectonics, now one of the most important theories in earth sciences. We now understand that the plates do not wander randomly on Earth’s surface, so the term “continental drift” has been retired.

Earth is made up of three main layers, the crust, the mantle, and the core, (moving from the outside in). Continental crust, which is about 30 kilometers thick, and oceanic crust, which is only about five kilometers thick, make up part of the lithosphere. (The two types of crust differ in their mineralogic and chemical composition.) The lithosphere also includes the uppermost part of the mantle, which is a dense, hot layer of semisolid rock about 2900 kilometers thick and is composed of iron, magnesium, and calcium. The core, making up Earth’s center, is nearly twice as dense as the mantle because it is made of a metallic iron-nickel alloy. The core has outer and inner portions. The outer core is a liquid, 2200 kilometers thick. The mechanism that generates Earth’s magnetic field, the geodynamo, originates in the electrically conducting outer core. The solid inner core, 1250 kilometers thick, is roughly the size of the moon, but it is as hot as the surface of the sun.

The theory of plate tectonics states that Earth’s lithosphere consists of plates that ride on hotter, more mobile material. In geologic terms, a plate is a large rigid slab of solid rock. The word tectonics is derived from a Greek word meaning “to build” and refers to the deformation of Earth’s crust and the structural features that result.

Earth has seven large plates and a dozen or so smaller plates. The plates are generally internally rigid and move relative to each other. Each plate is about 80 kilometers (50 miles) thick and consists of a shallow part that deforms by elastic bending or by brittle breaking and a deeper part made up of material that deforms without rupturing (plastic deformation). By using the Global Positioning System (GPS), we can determine the direction and rate of movement. Plates move from one to 13 centimeters per year.

Tectonic plates can move into, away from, or past one another. These three relative motions correspond to the three major types of plate boundaries, or margins: convergent, spreading, and transform, respectively.

Two of the most common forms of convergent margins are subduction zones and collision zones. Subduction occurs when a higher-density plate moves beneath a lower-density plate and plunges as much as several hundred kilometers into Earth’s interior. On the west coast of South America, for example, the Nazca Plate dives underneath the South American Plate. Collision occurs when two plates of relatively low density converge, and neither can be subducted into the denser, deep Earth. Collision leads to orogenesis, or the creation of mountain ranges, one of the most dramatic effects of tectonic forces. Beginning around 40 to 50-million years ago and especially within the past 10 million years, for example, the Himalaya Mountains and the Tibetan Plateau were forced up when the Indian Plate moved northward and collided with the Eurasian Plate. The collision created the Earth’s tallest mountains, more than 8800 meters high.

Most spreading margins are deep under the oceans. They occur where hot magma from Earth’s interior spews into the mid-oceanic ridges between plates and forms new oceanic plate material. The Mid-Atlantic Ridge, the boundary between the North American and Eurasian Plates, is a spreading margin. Sometimes spreading margins are on continents, as in the Red Sea/African Rift Valley system at the boundary between the African and Arabian Plates.

Transform margins occur when one plate slides past another. One of the most familiar examples is the San Andreas Fault Zone on the west coast of North America where the Pacific Plate slides northward relative to the North American Plate.

Plate margins are some of the most geologically active parts of Earth’s surface. Volcanoes, for example, are not randomly distributed; they are normally concentrated on the edges of the continents, on island chains, or along large mountain ranges beneath the sea. Their distribution usually coincides with plate boundaries. More than half of the world’s active volcanoes above sea level encircle the Pacific Plate to form the circum-Pacific “Ring of Fire.” In addition, 90% of the world’s earthquakes occur along boundaries where plates are colliding or scraping past one another.

Because the movement of plates correlates directly with the prevalence of geologic hazards, it is generally riskier to live near plate boundaries than on stable areas of the continents. People living along active plate boundaries, where most earthquakes and volcanic eruptions occur, need to be aware of the potential hazards. Major earthquakes that occur along subduction zones can trigger tsunamis that can affect coastal areas all around major ocean basins. These seismic sea waves have historically been devastating for communities along coasts and on islands.

The developing technologies of ocean-floor mapping, including exploration of the ocean floor with deep-sea submersibles and compilations of Earth’s paleomagnetic record, have proven invaluable to improving our understanding of plate tectonics. The study of earthquakes and advances in seismic instrumentation also provide important information on Earth’s interior.

The forces that drive the movement of the plates are not precisely known, but they likely relate to convective movement within Earth’s mantle. One hypothesis holds that upward convection, or flow, of hotter material within Earth’s mantle drives the plates apart along mid-oceanic ridges. A different hypothesis proposes that gravity pulls down on the older, colder, and heavier ocean floor at subducting plate margins with more force than on the newer, lighter seafloor along the mid-oceanic ridges, pulling the plates apart and allowing lava to emerge at the mid-oceanic ridges.

In addition to driving plate tectonics, the dynamics of Earth’s interior are also believed to produce Earth’s magnetic field through the flow of liquid metal in the outer core. The paleomagnetic record indicates that the geomagnetic field has existed for at least three billion years. This record shows that at numerous times in Earth’s history the polarity of the magnetic field has switched—the north magnetic pole became the south magnetic pole, and the south became north. This reversal of Earth’s magnetic field does not occur at predictable or regular intervals, and its cause is not fully understood, but the record provides a valuable dating tool for earth scientists. For more information on paleomagnetics, read about it in the “Dating Methods” section of this web site.


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