Evidence of Plate Tectonics

Plate Tectonics

Overview

Scientific evidence suggests that all seven of the continents on Earth today used to be connected, forming a single landmass called Pangaea. The breakup of the supercontinent Pangaea and the movement of continents away from their placements within Pangaea was labeled continental drift.

The theory that explains these movements is called plate tectonics. The term plate refers to large rigid blocks of the Earth's lithosphere (crust plus uppermost mantle), which move and interact with one another. Tectonics comes from the Greek root "to build".

This animated image shows the break-up of supercontinent Pangaea to form the present continental configuration.

Image courtesy of USGS.

According to the theory of plate tectonics, the Earth's lithosphere is divided into a dozen or more large and small plates. These plates are in constant motion because they are floating on a slowly flowing part of the upper mantle called the asthenosphere. Plates consist of continental crust, oceanic crust, or a combination of both. So when the plates move, the continents and ocean floor move as well.

Earth's Tectonic Plates

The rigid lithospheric plates move constantly, but slowly (centimeters per year) on top of the asthenosphere. As the plates move around, they push into each other, move away from each other, or slide past each other along their boundaries. There are many possible forces driving these motions, including push/pull forces in the plates and convection deep within the Earth.

Map of Earth's Tectonic Plates

Image courtesy of USGS.

Plate tectonics is a relatively new scientific theory. Earlier ideas about the movements of continents and Earth's crust such as continental drift and sea-floor spreading couldn't explain how the movements occurred. Plate tectonics explains these movements as well as many other features of and events on Earth, especially those associated with plate boundaries.

Plate Boundaries

There are three major types of plate boundaries.

• Convergent plate boundaries exist where two tectonic plates move toward each other. If two continental plates push together, they buckle and fold over millions of years to form mountain ranges.
  
  A boundary where an oceanic plate moves toward a continental plate is called a subduction zone. In a subduction zone, the oceanic plate plunges (subducts) underneath the continental plate, forming a deep-ocean trench on the seafloor. As the oceanic plate continues to plunge downward into Earth's interior, hot molten rock (magma) rises up to form a chain of volcanoes on the continental plate. This chain of volcanoes is called a continental volcanic arc.
  
  A subduction zone will also form where two oceanic plates move toward each other. As one oceanic plate subducts beneath another, rising magma forms volcanic island arcs on the upper, overriding oceanic plate.

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• Divergent plate boundaries exist where two tectonic plates move away from each other. Where two oceanic plates pull apart, magma rises and erupts as lava at the surface. The lava quickly cools and hardens to form new crust. However, the newly formed crust is still much hotter than older crust farther away from the plate boundary. Because the new crust is hotter, it is less dense. Because of its lower density, the crust along the plate boundary forms a mid-ocean ridge, which is 2 to 3 km higher than the surrounding ocean floor. In many areas of the world, mid-ocean ridges have narrow valleys along their center-line where the new crust is forming. These valleys are called rift valleys.
  
  Rift valleys can also form on land. When part of a continental plate is stretched and thinned, a long valley called a continental rift forms, and pieces of the plate begin to pull away from each other along the rift. An example of a continental rift is the East African Rift.

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• Transform plate boundaries occur where two plates slide alongside each other without significant vertical movements or major volcanic activity. Like other boundaries, though, earthquakes are associated with transform plate movements.

Seafloor Spreading

Plate tectonics is a relatively new scientific concept, combining the earlier theories of continental drift and seafloor spreading. Seafloor spreading is the movement of the Earth's crust away from the mid-ocean ridges.

During the process of seafloor spreading, hot rock rises up from the mantle and forces its way to the surface to form new sections of oceanic crust at a mid-ocean ridge. The new crust pushes the older crust away from the mid-ocean ridge, causing the seafloor to spread. This causes the ocean basin to widen and the continents to move away from each other.

Mountain Formation

Plates move very slowly from a human's perspective—at rates of centimeters per year. Over time, however, these plate movements cause great changes to the Earth. For example, where continental plates collide, the crust tends to buckle and be pushed upward to form folded mountain ranges.

Volcano Formation

At some tectonic plate boundaries, an oceanic plate plunges beneath another plate and sinks into the Earth's interior. As it sinks, it releases water, which rises into the overriding plate. This causes parts of the overriding plate to melt and form magma. The magma rises up, squeezing through widening cracks. Sometimes the magma reaches the surface and erupts as lava and ash. These erupting materials can build up over time to form volcanoes. Most volcanoes form in this manner near tectonic plate boundaries, but they form in other areas as well.

Earthquakes

Earthquakes occur along faults. Faults are cracks in bodies of rock along which the rock moves. Some of the different ways in which rocks may move along a fault are shown below.

Rocks do not constantly move along a fault, though. Most of the time, the rocks on either side of the fault are locked together by friction. Tectonic forces gradually build up in the rocks. When the forces have built up enough to overcome the friction, the rocks suddenly slip past each other, releasing built-up energy as an earthquake. Tectonic plate boundaries are made up of large, interconnecting faults, which are the sites of most earthquakes.

Earthquakes & Volcanoes at Plate Boundaries

Although earthquakes and volcanoes occur in many different settings, they are especially common along tectonic plate boundaries. At plate boundaries, rocks grind against each other, releasing energy as earthquake waves. As mentioned above, volcanic activity is common at convergent plate boundaries. Volcanoes along convergent boundaries at the edge of the Pacific Plate form a prominent pattern, called the "Ring of Fire".

Image courtesy of USGS.

The world map above shows the locations of tectonic plate boundaries (black lines) and active volcanoes (red dots). The pattern of active volcanoes along the edge of the Pacific Plate is called the "Ring of Fire."

Volcanic activity is also common at divergent boundaries. For example, at the Mid-Atlantic Ridge, lava regularly erupts and hardens to form new seafloor as the North American and African Plates pull away from each other. Depending on eruption rates, lava flows may build on top of each other to form tall seafloor volcanoes in this setting. Some of these volcanoes build up above sea level, while others remain completely underwater. Another example of volcanic activity at a divergent boundary can be seen at the boundary between the African and Arabian Plates. As shown in the image below, several active volcanoes have formed where these plates pull away from each other.

Image courtesy of USGS.

The map above shows the locations of active volcanoes (red triangles) along the tectonic plate boundary between the African Plate and the Arabian Plate. Also, a new divergent plate boundary is forming along the East African Rift Zone (dotted lines). In the East African Rift Zone, active volcanoes have formed where the Nubian and Somalian sections of the African Plate are pulling away from each other.

Fracture Zones

Fracture zones are cracks or faults on the ocean floor that horizontally offset mid-ocean ridges. Fracture zones typically run perpendicular to mid-ocean ridges.

The diagram below shows two tectonic plates moving apart. One plate is dark blue, and the other is light blue. Several fracture zones are shown at places where parts of the same tectonic plate are moving in the same direction but at different speeds.

Fracture zones are located at places that were once active underwater faults—places where two different tectonic plates slid past each other in opposite directions. Over time, the plates moved away from each other and left the fracture zones behind as evidence. The locations and properties of fracture zones strongly support the idea that the Earth's tectonic plates have been in continual motion.

The locations of several fracture zones off the western coast of the United States are shown below.

Image courtesy of USGS

Plate Tectonics and Distribution of Organisms

The breaking apart of the supercontinent, Pangaea, not only changed the distribution of continents on Earth, it also changed the distribution of living species on Earth. Species and populations that were once concentrated relatively closely together on the supercontinent gradually drifted apart as the continents drifted apart.

The diagram below shows the locations of the fossils of certain species on present-day continents. When the continents are pieced together to resemble the supercontinent, as shown, one can see that the geographic distributions of the fossils also fit together, like pieces of a puzzle.

Image courtesy of USGS.

This example illustrates how continental drift impacts the global distribution of organisms over time.