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Plate Tectonics
The theory of plate tectonics provides an explanation for the past, present, and future arrangements
of the continents. This theory also explains the mechanism behind geologic phenomena, such as
earthquakes, volcanic activity, and mountain building.
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 movement 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 and
events on Earth, especially those associated with plate boundaries.
Sea-floor 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 midocean 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 builtup 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.
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