Plate Tectonics: Prevailing working theory in Geology that states that the external layer of the Earth, the lithosphere, is broken into large rigid slabs called plates, which move independently from one another. Energy released at plate boundaries is the main cause of earthquakes.
Like most great unifying scientific ideas, the plate tectonics theory is simple. Briefly, it states that the lithosphere is a layer of hard, strong rock about 100 kilometers thick that floats on the hot, plastic asthenosphere. The lithosphere is broken into seven large (and several smaller) segments called tectonic plates. They are also called "lithospheric plates" and simply "plates"-the terms are interchangeable. The lithospheric plates move slowly over the asthenosphere at rates ranging from less than 1 to about 16 centimeters per year, like sheets of ice drifting across a pond. Continents and ocean basins make up the upper parts of the plates. As a tectonic plate glides over the asthenosphere, the continents and oceans move with it.
A plate boundary is a fracture that separates one plate from another. Neighboring plates can move relative to one another in three different ways. At a divergent boundary, two plates move apart from each other. At a convergent boundary, two plates move toward each other, and at a transfonn boundary, they slide horizontally past each other. Table 5-2 summarizes characteristics and examples of each type of plate boundary.
The great forces generated at a plate boundary build mountain ranges and cause volcanic eruptions and earthquakes. These processes and events are called tectonic activity, from the ancient Greek word for "construction." Tectonic activity "constructs" mountains and ocean basins. In contrast to plate boundaries, the interior portion of a plate is usually tectonically quiet because it is far from the zones where two plates interact.
The Anatomy of a Tectonic Plate
The nature of a tectonic plate can be summarized as follows:
I. A plate
is a segment of the lithosphere; thus, it includes the uppermost mantle
and all of the overlying crust.
2. A single plate can carry both oceanic crust and con tinental crust. The average thickness of lithosphere covered by oceanic crust is 75 kilometers, whereas that of lithosphere covered by a continent is 125 kilometers. Lithosphere may be as little as 10 to 15 kilometers thick at an oceanic spreading center.
3. A plate is composed of hard, mechanically strong rock.
4. A plate floats on the underlying hot, plastic asthenosphere and glides horizontally over it.
5. A plate behaves like a slab of ice floating on a pond. It may flex slightly, as thin ice does when a skater goes by, allowing minor vertical movements. In general, however, each plate moves as a large, intact sheet of rock.
6. A plate margin is tectonically active. Earthquakes and volcanoes are common at plate boundaries. In contrast, the interior of a lithospheric plate is normally tectonically stable.
7. Tectonic plates move at rates that vary from less than 1 to 16 centimeters per year.
Why Plates Move: Mantle Convection
Two associated processes facilitate the movement of tectonic plates. Notice that the base of the lithosphere slopes downward from a spreading center; the grade can be as steep as 8 percent, steeper than most paved highways. Calculations show that even if the slope were less steep, gravity would cause the lithosphere to slide away from a spreading center over the soft, plastic asthenosphere at a rate of a few centimeters per year.
Also, the lithosphere becomes denser as it moves away from a spreading center and cools. Eventually, old lithosphere may become denser than the asthenosphere below. Consequently, it can no longer float on the asthenosphere, and sinks into the mantle in a subduction zone, dragging the trailing portion of the plate over the asthenosphere. Both of these processes may contribute to the movement of a lithospheric plate as it glides over the asthenosphere.
To summarize the processes that drive plate tectonics, hot mantle rock flows upward from the core boundary. As the upper portion of the rising mantle nears the surface, it cools to become new lithosphere at a spreading center. The new lithosphere glides over the asthenosphere away from the spreading center. At the same time, the older, leading portion of the same plate sinks deeply into the mantle, replacing the rock that is rising beneath the spreading center. Thus, the mantle convects in huge elliptical cells that extend from the deepest mantle to the Earth's surface. The convecting system includes the rising mantle, the lithosphere sliding over the asthenosphere, and the lithosphere sinking deeply into the mantle.
In contrast to the huge curtain-shaped mass of mantle that rises beneath
a spreading center, a mantle plume is a relatively small rising column
of hot, plastic mantle rock. Many plumes rise from great depths in the
mantle because rock near the core-mantle boundary becomes hotter and more
buoyant than surrounding regions of the mantle. Others may form as a result
of heating in shallower portions of the mantle. As pressure decreases in
a rising plume, magma forms and rises to erupt from volcanoes at a hot
spot on the Earth's surface. The Hawaiian Island chain is an example of
a volcanic center at a hot spot. It erupts in the middle of the Pacific
tectonic plate because the plume originates deep in the mantle, away from
a plate boundary.
How Plate Movements Affect Earth Systems
of tectonic plates generate volcanic eruptions and earthquakes. They also
build mountain ranges and change the global distributions of continents
and oceans. Other less obvious impacts of plate tectonics on Earth systems
are also important. For example, changes in the distribution of continents
and oceans alter global and regional climate, which, in turn, affect the
hydrosphere, atmosphere, and biosphere. The opening essay to this Unit
describes many of these Earth systems interactions.