Plate Tectonics

Earth Structure

The Earth's internal structure can be viewed in two ways: either in terms of compositional layers, or in terms of layers of varying strength. There are three main compositional layers: the crust, mantle, and core. The crust, the outermost layer, is relatively buoyant and very thin compared to the mantle and core.

Beneath the oceans, the oceanic crust varies very little in thickness, generally extending only about 5 kilometers (3.1 miles), and is composed of basalt . The crust beneath the continents, however, is much more variable in thickness, averaging about 30 kilometers (18.6 miles); under large mountain ranges it can extend to depths of up to 100 kilometers (62.1 miles).

The San Andreas fault is the border between two tectonic plates—the North American Plate and Pacific Plate. Los Angeles is located on the Pacific Plate, and San Francisco is on the North American Plate. In a few million years, the two geographic areas will be right next to each other because the western side of the fault (the Pacific Plate) is moving northward with respect to the rest of the state. The fault is moving at about 2 centimeters (just under an inch) per year.

Below the crust is the mantle, a dense, hot layer approximately 2,900 kilometers (1,802 miles) thick. At the center of the Earth lies the core, which is composed of an iron–nickel alloy. It is divided into two regions—a liquid outer core and solid inner core. As the Earth rotates, the liquid inner core spins, creating the Earth's magnetic field.

Within the crust and mantle, there also are two important mechanical layers—the lithosphere and asthenosphere. The lithosphere is the outermost of these layers, and comprises the crust and uppermost mantle. The lithosphere is relatively cool, making the rock strong and resistant to deformation. The lithosphere is broken into the moving tectonic (or lithospheric) plates.

Below the lithosphere is a relatively narrow, mobile zone of the mantle called the asthenosphere. The asthenosphere is a weak zone, formed of mostly solid rock (with perhaps a little magma mixed in), and flows very slowly, in a manner similar to the ice at a bottom of a glacier. The rigid lithosphere is believed to "float" or move about on the slowly flowing asthenosphere.

Plate Tectonic Theory is Developed

The plate tectonic theory known today evolved in the 1950s, owing to four major scientific developments:

1. Demonstration of the young age of the ocean floor;
2. Confirmation of repeated reversals of the Earth's magnetic field in the geologic past;
3. Emergence of the seafloor-spreading hypothesis and associated recycling of the oceanic crust; and
4. Precise documentation that the Earth's earthquake and volcanic activity was concentrated along subduction zones and mid-ocean ridges.

Youthful Seafloor.

Before the nineteenth century, the depth of the open ocean was a matter of speculation, although most scientists believed it to be flat and featureless. Only in 1855 did the first bathymetric maps reveal the first evidence of underwater mountains in the central Atlantic. In 1947, seismologists found that the sediment layer on the floor of the Atlantic was much thinner than previously thought. Scientists believed that the oceans were over 4 billion years old, and were perplexed by the distinct lack of sediment cover. The answer to this question would prove vital to advancing the theory of plate tectonics.

Magnetic Field Reversals.

In the 1950s, scientists began recognizing magnetic variations in the rocks of the ocean floor. This was not entirely unexpected, since it was known that basalt contained the mineral magnetite, and this mineral was known to locally distort compass readings. In the early part of the twentieth century, geologists recognized that oceanic rocks had normal or reverse polarity (i.e., in normal polarity, the rocks have the same orientation of today's magnetic field). This can be explained by the ability of the magnetite grains to align themselves in the molten basalt with the Earth's magnetic field. When the rock cools, these grains are "locked" in, recording the magnetic orientation or polarity (normal or reversed) at the time of cooling. As more of the ocean floor was mapped, patterns of alternating stripes of normal and reverse polarity were noted; this became known as magnetic striping.

Seafloor Spreading.

With the discovery of magnetic striping at mid-ocean ridges, scientists began to theorize that mid-ocean ridges mark structurally weak zones where magma from deep within the Earth rises and erupts at the surface. This theory, called seafloor spreading, quickly gained acceptance, but raised an additional question: If new crust is continually being formed at mid-ocean ridges, and the Earth is not increasing in size, what is happening to the old crust? Harry Hess and Robert Dietz postulated that the old crust must be destroyed in the deep canyon-like oceanic trenches, while new crust if formed at the mid-ocean ridges. This theory explained why the Earth is not expanding, there is little sediment on the ocean floor, and oceanic crust is much younger than continental rocks.

Subduction Zones.

The final scientific discovery that cemented the theory of plate tectonics occurred with improvements in seismic detection in the 1950s. Seismologists identified regions of earthquake activity that coincided with Hess's predicted areas of ocean crust generation (mid-ocean ridges) and oceanic lithosphere destruction (subduction zones). Today scientists know that tectonic plates move, because they can measure their motion directly using the global positioning system (GPS).

Plate Tectonic Boundaries

Plate tectonic boundaries are regions where lithospheric plates meet. There are three types of plate tectonic boundaries: divergent, convergent, and transform.

Divergent.

Divergent boundaries occur along spreading ridges where plates are moving apart and new crust is being created by ascending magma from the mantle. An example of a divergent plate boundary is the Mid-Atlantic Ridge. This submerged mountain chain extends from the Arctic to the southern tip of Africa, and is one part of the global ridge system that extends around the Earth. ^* The Mid-Atlantic Ridge spreads at a rate of approximately 2.5 centimeters (1 inch) per year.

Convergent.

Convergent boundaries are regions where lithospheric plates collide. The type of convergence depends on the types of plates involved: namely, (1) oceanic–oceanic convergence; (2) oceanic–continental convergence; (3) continental–continental convergence (see figure).

• Oceanic–Oceanic. When two oceanic plates collide, one plate is subducted beneath the other. This occurs as one lithospheric plate becomes older, colder, and denser than the underlying hot, weak asthenosphere. As the lithosphere sinks slowly through the asthenosphere, the uppermost sediments are melted, and the resulting magma reaches the surface to form volcanoes. As a result, subduction zones are marked by an arc of volcanoes parallel to and about 150 kilometers (93 miles) from the plate margin. An example of oceanic-oceanic collision is the Marianas Trench and the Aleutian Islands in the Pacific Ocean.
• Oceanic–Continental. When oceanic and continental plates collide, the oceanic plate is the one that is subducted beneath the continental plate, because the continental crust is lighter and less dense. An example of oceanic-continental collision is seen at the Cascadia Subduction Zone, where the Pacific Plate is being subducted beneath the North American Plate.
  

  This schematic illustrates the three types of plate convergence.
• Continental–Continental. Continental–continental convergence results in spectacular mountain ranges such as the Himalayas, the Alps, and the Appalachians. Because continental crust is buoyant, neither plate will subduct, and a collision zone is the result.

Transform.

Transform boundaries mark regions where plates slide past one another. Transform boundaries are great vertical fractures that extend down through the lithosphere. An example of a transform boundary is the San Andreas Fault in Southern California (see the photograph of the fault on page 202).

Plate Movements

The lithospheric plates do not randomly meander about the Earth's surface, but are driven by internal forces. The mantle is believed to move in circular motions rather like soup boiling on a stovetop, wherein the heated soup rises to the surface, cools, and sinks back to the bottom of the pot, where it is heated and rises again. This cycle is called convective flow, and it is the same process that occurs in the mantle today. However, the heat source within the Earth is radioactive decay of minerals and residual heat from the formation of the Earth.

Until the early 1990s, scientists believed that mantle convection, seafloor spreading, and magma intrusion at mid-ocean ridges (called "ridge push") were the predominant mechanisms that drove plate motion. However, in recent years, the significance of subduction mechanisms over mid-ocean ridge processes has taken precedence. The gravity-controlled sinking of a cold, dense, oceanic slab into a subduction zone (called "slab pull") now is considered the driving mechanism behind plate tectonics.

Although scientists know that forces deep within the Earth drive plate motion, they may never know the exact details, because no mechanism can be directly tested. The fact that lithospheric plates have moved in the past and are still in motion today is beyond dispute, but the exact mechanisms of how and why they move will continue to challenge scientists in the future.

SEE ALSO Geothermal Energy ; Hot Springs on the Ocean Floor ; Mid-Ocean Ridges ; Ocean Basins ; Ocean-Floor Bathymetry ; Volcanoes, Submarine .

Alison Cridland Schutt

Bibliography

Skinner, Brian J., and Stephen C. Porter. This Dynamic Earth. New York: John Wiley & Sons, 1992.

Internet Resources

This Dynamic Earth. U.S. Geological Survey. <http://www.pubs.usgs.gov/publications/text/> .