Plate tectonics

Plate tectonics is a theory that explains the origin of most of the major features of Earth’s surface. For example, the theory tells us why most volcanoes occur where they do, why there are high ridges and deep trenches in the oceans, and how mountains form.

According to this theory, Earth has an outer shell made up of about 30 rigid pieces called tectonic plates. Some of these plates are gigantic. For instance, most of the Pacific Ocean covers a single plate.

The plates move about on a layer of rock that is so hot it flows, even though it remains solid. The plates are moving very slowly relative to one another. They move at speeds up to about 4 inches (10 centimeters) per year.

Plates have been moving about for hundreds of millions of years. So, in spite of their very low speeds, some of them have moved vast distances. In fact, over the past several hundred million years, plate movement has changed the map of Earth drastically. Earth scientists have determined that before about 200 million years ago, all the continents were part of a supercontinent called Pangaea << pan JEE uh >> .

Structure of tectonic plates

Tectonic plates are made up of Earth’s crust and the outermost part of its mantle. The crust is the outermost layer of Earth. It is thin and rocky. All the dry land, all the ocean floors, and the beds of all the other bodies of water on earth are part of the crust. The mantle is a thick layer of hot rock under the crust and above the core, a dense sphere at Earth’s center.

The continents are embedded in the tops of plates, so as these plates move, they carry the continents along with them. The plates that carry continents do not have the same boundaries as their continents; they include both continents and ocean floor.

Plates are typically about 60 miles (100 kilometers) thick. However, they may be less than 5 miles (8 kilometers) thick at certain places in the oceans and as much as 120 miles (200 kilometers) thick under parts of continents.

The plates as a whole make up Earth’s lithosphere. The layer of mantle rock under the plates is the asthenosphere << as THEHN uh sfihr >> . This rock reaches temperatures between about 2400 and 3600 °F (1300 and 2000 °C).

Plate interactions

As the tectonic plates move about on the asthenosphere, they interact with one another at their boundaries. There are three types of boundaries: (1) divergent, where plates move apart from each other, (2) convergent, where plates move toward each other, and (3) transform, where plates slide alongside each other.

Divergent plate boundaries

are mostly on ocean floors. There, the separation of plates, or rifting, creates lithosphere. Rifting on continents creates gaps into which water flows to form major river systems, lakes, and even oceans.

The rifting of the ocean floor

enlarges the floor. Magma (liquefied rock) rises from the asthenosphere, filling the gap between the separating plates. The magma hardens, creating equal amounts of new crust on the edges of the two plates. The process of separation of plates and formation of new crust is called sea-floor spreading. This process creates about 1 square mile (2.4 square kilometers) of ocean crust a year.

The build-up of ocean crust on plate boundaries generates long underwater mountain ranges called ocean ridges. Some of these mountain ranges occur along the center of ocean basins and are called mid-ocean ridges. One such mid-ocean ridge, called the Mid-Atlantic Ridge, extends from waters east of the Canadian island of Newfoundland to an area off the southern tip of South America.

Earthquakes occur at ocean ridges when one plate edge drops down and grinds against the edge of a neighboring plate. These earthquakes occur a short distance beneath the surface of the plates, indicating that newly formed plate edges are very thin. See Earthquake (How an earthquake begins).

The rifting of continents

creates new seas as ocean waters fill a gap in continental crust. The Red Sea region, for example, is in an advanced stage of rifting. The rift is already flooded by ocean waters—the Red Sea, an extension of the Indian Ocean.

The East African Rift, a unit of the Great Rift Valley that extends from Eritrea to Mozambique and connects to the Red Sea, is in an early stage of rifting. There, the gap is not yet deep enough to become filled with ocean water from the Indian Ocean. However, scientists believe that in 50 million years an extension of that ocean may cut into southeastern Africa.

Convergent plate boundaries

are places where lithosphere created at divergent boundaries is destroyed by recycling into the mantle. At a convergent boundary, the edge of a plate sinks, thrusting under the margin of its neighboring plate. This process is called subduction. The sinking plate can create deep ocean trenches where it plunges into the asthenosphere. Because Earth is not changing in size, scientists believe that subduction zones consume the same amount of ocean crust as ocean ridges create.

The subducting plates generate powerful earthquakes and usually create a line of volcanoes along the overriding plate boundary. A volcano forms when magma, hot gases, and fragments of rock burst through the surface. Subduction zones generate magma at a depth of about 60 to 90 miles (100 to 150 kilometers) by melting three kinds of material: oceanic crust at the top of the descending plate, ocean sediment dragged to great depths, and asthenosphere caught in the corner between the converging plates.

At some convergent plate margins, the overriding plate scrapes a thick mass of sediment off the descending plate. This process of subduction accretion << uh KREE shuhn >> , adds material to the edge of the overriding plate. In California, for example, subduction accretion formed a large part of the coastal mountain ranges.

At other convergent plate boundaries, the edge of the descending plate, all its cover of sediment, and even pieces from the edge of the overriding plate disappear beneath the overriding plate. This process, subduction erosion, causes continents to shrink. Such erosion is occurring in the Pacific Ocean along the coasts of Peru and Chile and east of the Mariana Islands.

At boundaries where plates carrying continents collide, layers of rock in the overriding plate crumple and fold like a tablecloth that is pushed across a table. About 45 million years ago, a plate that includes what is now the country of India collided with the southern edge of the Eurasian Plate, which includes Europe and most of Asia. The Indian-Australian Plate began to push beneath the Eurasian Plate, causing rock in the Eurasian Plate to crumple and fold. Over millions of years, the Himalaya, the world’s highest mountain system, was formed.

Transform plate boundaries,

where plates slide horizontally against each other, neither create nor destroy lithosphere. However, at these boundaries, or transform faults, powerful earthquakes can occur. For example, devastating earthquakes have occurred in California along parts of a transform plate boundary known as the San Andreas Fault.

The San Andreas Fault forms part of the boundary between two large plates—the North American Plate and the Pacific Plate. The fault connects a spreading ridge in the Gulf of California to a trench off the coast of northern California. The parts west of the fault are attached to the Pacific Plate and are moving northwest.

Plate movement

Rate.

Earth scientists measure the speed of plate movement by monitoring how rapidly a plate moves relative to the plate next to it. Today, plates move about 4 inches (10 centimeters) a year—about as rapidly as human hair grows. In the past, plates may have moved as fast as 6¹/₄ inches (16 centimeters) per year.

The overall pattern

of movement of the tectonic plates is a widening of the Atlantic Ocean and a shrinkage of the Pacific Ocean. The Atlantic is widening because sea-floor spreading at the Mid-Atlantic Ridge continues to create lithosphere. The Pacific is shrinking because much of it is ringed by convergent plate boundaries that are consuming its lithosphere.

Scientists have traced the movements of tectonic plates millions of years into the past. According to the commonly accepted description of plate movement, all the continents once formed part of an enormous single land mass called Pangaea. This mass was surrounded by a giant ocean known as Panthalassa.

About 200 million years ago, Pangaea began to break up into two large masses called Gondwanaland and Laurasia. These masses, in turn, broke up into the continents, which drifted to their present locations.

Evidence of plate movement.

Earth scientists find much evidence of plate movement at the boundaries of plates. They study surface features, such as mountains and ocean trenches, and investigate the frequencies and locations of earthquakes and volcanic eruptions.

Volcanoes that rise within plates are also evidence of plate movement. Scientists believe that these volcanoes are caused by mantle plumes, columns of very hot mantle that rise from deep inside Earth to the base of the lithosphere. These plumes generate magma that rises through the lithosphere and erupts in places called hot spots.

As a plate moves over a hot spot, the spot can generate a chain of volcanoes. For example, a hot spot under the Pacific Plate generated volcanoes that became the Hawaiian islands.

Paleomagnetism (the study of magnetism in ancient rocks) also provides evidence of plate movement. The evidence is in rocks that contain magnetic particles.

When such a rock was hot and liquid, the magnetic particles moved too rapidly to be influenced by Earth’s magnetic field. But as the rock cooled and solidified, the particles aligned themselves with Earth’s magnetic field, like tiny compass needles. Thus, the particles continue to point in the direction of the magnetic field that was present during the time that the rock cooled.

So when the plate containing the rock either drifts to a different latitude or rotates, the particles no longer align with Earth’s magnetic field. A comparison of the direction in which the particles now point in the rock with the direction of Earth’s present magnetic field provides information about where the plate was when the rock solidified.

Causes of plate movement.

Tectonic plates slide mostly because of temperature changes and gravity. As an edge that has formed on the ocean floor cools, it shrinks, becoming denser. After about 25 million years of cooling and shrinking, the edge becomes so dense that gravity can pull it down into the asthenosphere. There, the intense heat and increased pressure due to the great depth change the crust of the sunken plate edge into even denser rock. Because of the additional density, gravity pulls the plate edge into the asthenosphere even more strongly.

This sinking action is known as slab-pull because the sinking edge pulls the remainder of the slablike plate behind it. Many scientists believe that slab-pull is the main action driving the motion of plates with sinking edges.

Gravity also causes plates to slide downhill away from ocean ridges. This sliding force is called ridge-push.

Another cause of plate movement is the simple pushing of plates against one another. Scientists believe that large plates shove some small plates about.

The rise of mantle plumes and other movements of mantle rock may also affect the motion of tectonic plates slightly. The circulation of mantle rock as it rises to the top of the asthenosphere, cools, and then sinks is known as a convection current.

Earth scientists once thought that convection currents caused continental drift. Today, however, most earth scientists believe that such currents are primarily a result of the sinking of plates, rather than the cause of plate motion.

Maintaining tectonic activity.

The interior of Earth has generated enough heat energy to keep the planet tectonically active since it formed at least 4.5 billion years ago. This energy has maintained tectonic activity by keeping the asthenosphere so soft that the lithosphere can sink into it.

The interior of Earth generates heat energy mainly through the radioactive decay of atoms in the crust and mantle. In radioactive decay, radioactive atoms release energetic particles and rays. Material near these atoms absorbs energy from the particles and rays, becoming hotter.

The production of heat within Earth is declining, mainly because decay is decreasing the number of radioactive atoms. As Earth’s heat production is slowing, its interior is cooling. During perhaps the next 5 billion or 10 billion years, this cooling will harden the asthenosphere so much that plate motion will cease. After that occurs, volcanic eruptions will stop and earthquakes will become infrequent. Earth will be tectonically inactive.

History of tectonic theory

The theory of plate tectonics developed from a theory of continental drift, presented in 1912 by German meteorologist Alfred Wegener. Wegener’s theory proposed that the continents move about the surface of Earth. It explained why the shape of the eastern coast of the Americas and that of the western coast of Africa seem to fit together like pieces of a jigsaw puzzle. Evidence for the drift came from the presence of certain rock deposits that indicate the continents have changed position over time. For example, rock deposits from glaciers that existed hundreds of millions of years ago are found in India, Australia, Africa, and South America, indicating that these continents were once in a very cold climate, probably near the South Pole. Fossils of tree ferns and other tropical features in North America indicate that it was once at the equator.

Wegener was not sure what caused continents to drift, however. His theory of continental drift became a subject of much debate among scientists. Then, in the 1920’s, British physicist Harold Jeffreys proposed that the deep interior of Earth was very strong and therefore could not flow. As a result, most scientists rejected Wegener’s theory.

However, evidence supporting the theory of continental drift gradually accumulated. In the late 1930’s, American geologist David Griggs demonstrated that apparently solid rock can flow slowly when subjected to high temperatures and pressures. In the 1940’s and 1950’s, other researchers showed that the ocean floor contains much less sediment than would be expected if the floor were a permanent depression. A permanent sea floor would have accumulated more sediment due to the erosion of soil from the continents. The oldest sea-floor rocks that could be found were less than 150 million years old.

In the 1950’s, scientists developed techniques for studying rock magnetism that enabled them to determine the positions of the continents millions of years ago. By the late 1950’s, scientists completed mapping a system of ocean ridges extending for about 37,000 miles (60,000 kilometers) and reaching nearly around the world.

In the late 1950’s, scientists discovered that most earthquakes occur along lines parallel to ocean ridges and trenches. In 1960, Harry H. Hess, an American geologist, proposed a theory of what came to be called sea-floor spreading. In 1967, American geophysicist Jason Morgan and British geophysicist D. P. McKenzie independently proposed the idea that Earth’s surface consists of a number of movable plates. The following year, American earth scientists Bryan L. Isacks, Jack E. Oliver, and Lynn R. Sykes combined the idea of sea-floor spreading with new results from earthquake detection and proposed that rigid plates of lithosphere move about on a soft, flowing asthenosphere.

In 1969, the drillship Glomar Challenger completed its first scientific cruise. Material drilled from various locations on both sides of the Mid-Atlantic Ridge indicated that the age of the ocean crust was exactly as predicted by the analysis of paleomagnetism and sea-floor spreading. This discovery and the continuing accumulation of other evidence convinced most earth scientists that the theory of plate tectonics is valid. In 2007, scientists reported evidence that plate tectonics began at least 3.8 billion years ago. They found ancient oceanic crust in Greenland with layers of volcanic rock produced during sea-floor spreading.