TECTONIC PLATES: THEORY, TYPES, MOVEMENT, CONSEQUENCES - GEOGRAPHY - 2025

The tectonic or lithospheric plates are the blocks or fragments into which the lithosphere is divided, which move dragged by the earth's mantle. These plates have been formed from the mantle and reintegrated into it in a constant process since the last 3 billion years.

From the theories of Wegener (continental drift) and Hess (expansion of the ocean floor) the theory of plate tectonics was consolidated. This theory postulates the existence of two basic types of tectonic plates, oceanic and continental.

Main tectonic plates. Source: USGS - Spanish version Daroca90 / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

The lithosphere has several dozen tectonic plates of varying magnitude and 8 of the largest are: the Eurasian, African, Australian, North American, South American, Nazca, Pacific and Antarctic. These plates move thanks to the dynamics of the mantle and the lithosphere, by the convection currents generated by the thermal flux.

The tension of the mantle flow drags the rigid crust, which cracks and separates forming the plates. When the oceanic plates separate, magma (molten basalt) rises to the surface and a new ocean floor forms.

Plate theory

Origin of plate theory

The theory initially arises with the proposals of Alfred Wegener in 1915 on continental drift. Wegener postulated that all the continents were united and then fragmented, separating and colliding.

Wegener derived his conclusions by studying the geology and contours of the continents, as well as data on the distribution of fossils of fauna and flora. For example, when comparing the eastern edge of South America with the western edge of Africa, it is noted that they fit together like two pieces of a puzzle.

Later, in 1960 Harry Hess proposed the theory of the expansion of the ocean floor, providing an explanation to the mechanism of plate tectonics. Later, the theory was strengthened with the work of John Tuzo Wilson on the expansion of the ocean floor and the proposals of Jason Morgan in 1963 on the existence of the mantle feathers.

As evidence accumulated on the composition and dynamics of the Earth's crust and mantle, the theory of plate tectonics was consolidated.

Shaping of the Earth

The Earth originated as part of the solar system in a process of condensation of rotating cosmic dust subjected to gravitational attraction. This mass of dust was subjected to high temperatures and as it cooled, its density and gravity increased.

This process gave it its current rounded shape, bulging in the Equator and flattened at the poles (oblate spheroid).

Layers

The gravitational attraction determined that the densest materials were towards the center and the least dense towards the outside. The cooling of this geoid from the outside in, determined a structure in differentiated concentric layers.

The outer layer hardened as it cooled 4.4 billion years ago, forming a relatively thin crust (5-70 km) composed of silicates called crust. The density of the continental crust is less than the density of the oceanic crust.

Layers of the Earth. Source: Vectorized and translated from the English version by Jeremy Kemp. Based on elements of an illustration by USGS. http://pubs.usgs.gov/publications/text/inside.html / Public domain

Under the crust there is a viscous layer of about 2,855 km called the mantle and finally an incandescent core formed mainly of iron. This core, approximately 3,481 km in diameter, is divided into two layers, the inner core of solid iron and nickel and the outer liquid core.

Mechanical Properties of Layers and Forces Driving Plate Tectonics

From the point of view of plate tectonic mechanics, the most relevant layers are the crust and the mantle.

The crust is rigid although with some plasticity and together with the upper layer of the mantle it forms the lithosphere. It is divided into fragments or plates of various sizes, called tectonic plates.

The asthenosphere

The mantle in turn is made up of two different layers, the upper and the lower mantle. The upper mantle is less viscous but fluid, while the lower (subjected to higher pressure and temperature) is more viscous.

The upper layer of the mantle is called the asthenosphere and plays an important role by being directly in contact with the lithosphere. The asthenosphere causes the movement of the tectonic plates, that is, continental drift, and produces a new ocean floor in the ridges.

On the other hand, it generates the hot spots or areas of magma accumulation under the crust due to the feathers of the mantle. These are vertical channels of magma that reach from the asthenosphere to the crust.

Process factors and forces

The density of the materials that make up the planet and the force of gravity determined the arrangement in the layers. The increasing pressure and temperature inside the Earth define the mechanical properties of these layers, that is, their rigidity or fluidity.

On the other hand, the forces that promote the movement of materials inside the Earth are thermal flux and gravity. Specifically, convection heat transfer is key to understanding plate tectonic motion.

Convection is manifested by the circulation of the mantle matter, where the warmer lower layers rise and displace the cooler upper layers, which descend. The layers that rise lose heat, while those that descend increase their temperature, thus driving the cycle.

The ocean ridges

In certain areas of the deep ocean, there are volcanic mountain ranges that are areas where the rupture of plates occurred. These fractures are produced by the stresses generated by the movement of the lithosphere pushed by the asthenosphere.

The flow of the viscous mantle stresses the rigid crust and separates the tectonic plates. In these areas, called oceanic ridges, the molten basalt rises due to internal pressures and emerges through the crust forming a new ocean floor.

Types of tectonic plates

Tectonic plates are basically of two types, oceanic and continental, thus generating three possibilities of convergent boundaries between plates. These are convergence of continental plate against one oceanic, one oceanic against another oceanic, and one continental against another continental.

Ocean plates

They are formed by oceanic crust (denser than continental crust) and are made up of iron and magnesium silicates (mafic rocks). The crust of these plates is thinner (7 km on average) compared to the continental crust and is always covered by marine waters.

Continental plates

The continental crust is made up of sodium, potassium and aluminum silicates (felsic rocks), being of lower density than the oceanic crust. It is a plate with thicker crust, reaching up to 70 km thick in mountain ranges.

It is really a mixed plate, in which although the continental crust predominates, there are also oceanic portions.

Tectonic plates of the world

Plate tectonic boundary map. Daroca90 / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Traditionally 7 large tectonic plates are recognized, which are the Eurasian, African, Australian, North American, South American, Pacific and Antarctic. Likewise, there are intermediate plates such as the Nazca, the Philippines, the Coco and the Caribbean and other very small ones.

Some of small size are those of Anatolia and the Aegean and only in the western Pacific more than 20 small tectonic plates are located.

- Main boards

• African Plate
• Antarctic Plate
• Arabic plate
• Coconuts Plate
• Juan de Fuca plaque
• Nazca plate
• Caribbean plate
• Pacific Plate
• Eurasian plate
• Philippine Plate
• Indo-Australian plate
• North American plate
• Scotia Badge
• South American Plate
• Australian plate

Some of the most important are described below:

Eurasian plate

This tectonic plate includes Europe, almost all of Asia, part of the North Atlantic Ocean and the Arctic. Asia excludes Hindustan, Southeast Asia, and Far East Siberia, Mongolia, and China.

It is a mainly continental tectonic plate with divergent limits on the Atlantic ridge to the west. While to the south it presents a convergent limit with the African, Arabian and Indian plates, and to the east with various minor continental plates.

African Plate

This covers the eastern Atlantic and almost the entire African continent, except for its eastern strip, which corresponds to the Arabian and Somali plates. The limits of this plate are divergent throughout its perimeter, except in its contact with the Eurasian plate which is convergent.

Australian plate

The Australian tectonic plate includes Australia, New Zealand, and parts of the southwestern Pacific. The Australian plate shows divergent limits to the south and west, while to the north and east its limits are convergent.

North American plate

It includes the entire North American subcontinent up to the Yucatan Peninsula, Greenland, part of Iceland, areas of the western North Atlantic and the Arctic. The limits of this plate are divergent from the Atlantic ridge to the east and convergent in the Pacific.

On the Pacific coast, it interacts with two small plates with transforming boundaries (Coco and Juan de Fuca).

South American Plate

It includes the subcontinent of the same name and has divergent limits from the Atlantic ridge. While on the western side it shows convergent limits with the Nazca plate, to the southwest with Antarctica and to the north it interacts with the Caribbean plate.

Pacific Plate

It is an oceanic plate with divergent boundaries from the Pacific ridge separating it from the Nazca plate. On the other hand, to the north and west it has convergent limits with the North American, Eurasian, Philippine and Australian plates.

Antarctic Plate

This tectonic plate includes the entire Antarctic continental shelf and the ocean of the same name, with divergent limits on its perimeter.

Nazca plate

It consists of an oceanic plate that subducts into the western coast of the South American plate (convergence). While it diverges to the north with the Coco plate and to the south with Antarctica.

On the other hand, to the west it diverges from the Pacific plate from its ridge and its collision with the South American plate gave rise to the Andes mountain range.

- Secondary plates

• Amuria plate
• Apulian or Adriatic Plate
• Bird Head Plate or Doberai
• Arabic Plate
• Plate of Altiplano
• Anatolian plate
• Burma plate
• North Bismarck Plate
• South Bismarck Plate
• Chiloé plate
• Futuna Plate
• Gorda's Plate
• Juan Fernández plaque
• Kermadec plate
• Manus Plate
• Maoke Plate
• Nubian plate
• Okhotsk plate
• Okinawa plate
• Panama plate
• Easter plate
• Sandwich Plate
• Shetland plate
• Timor Plate
• Tonga plate
• Probe Plate
• Plaque of the Carolinas
• Mariana plate
• Plate of the New Hebrides
• Northern Andes Plate

Plate tectonic movements

Tectonic plates or delimited fragments of the lithosphere move transported by the movement of the asthenosphere. Convection currents cause the viscous material of the mantle to move, forming circulation cells.

- The "conveyor belt"

The material of the mantle of the upper layer (asthenosphere) descends at a lower temperature, pushing the hot material below. This hotter material is less dense and rises, displacing matter and causing it to move horizontally, until it cools down and descends again.

Movement in the lithosphere. Source: USGS / Public domain

This stream of viscous flow from the mantle, drags the tectonic plates formed of solid material (lithosphere).

New ocean floor

When tectonic plates move, magma (molten basalt) from within the mantle emerges at the separation points. This emergent basalt creates a new ocean floor, pushing the old substrate horizontally and the crust expands.

Subduction

As the ocean floor expands, it collides with the continental masses. As this bottom is denser than the continental shelf, it sinks below it (subduction), so it melts and becomes part of the mantle again.

In this way, the material follows the cycle driven by convection and tectonic plates drift across the planet's surface.

- Continental drift

The movement of the mantle caused by convection and that of the tectonic plates of the lithosphere, causes continental drift. This is the relative displacement of the continents relative to each other.

Since the origin of tectonic plates about 3 billion years ago, they have merged and divided at various times. The last great confluence of most of the continental masses occurred 300 million years ago with the formation of the supercontinent Pangea.

Then, as the movements continued, Pangea fragmented again forming the current continents, which continue to move.

Types of boundaries between plates

Tectonic plates are in contact with each other constituting three basic types of limits depending on their relative motion. When two plates collide with each other, it is referred to as a convergent or destructive boundary, be it orthogonal (colliding head-on) or oblique.

On the other hand, when the plates move away from each other, it is called a divergent or constructive limit, which is the case of oceanic ridges. An example of a divergent boundary is the separation of the South American and African plates from the ridge of the Atlantic Ocean.

Whereas when two plates rub sideways moving in opposite directions along a transform fault, it is called a transform boundary. In California, a case of transforming boundary occurs between the North American and Pacific plates, forming the San Andrés fault.

The rise of the Himalayas is caused by the collision of the Indian plate with the Eurasian plate which is an orthogonal convergent boundary. In this case, it is the convergence of two continental plates, so obduction occurs (integration of the two continental masses raising the relief).

Direction of movement

Due to the earth's rotational motion, tectonic plates move by rotating around an imaginary axis. This movement implies that two colliding plates vary their angle, going from a fully convergent (orthogonal) limit to an oblique one.

Then, they will move laterally in opposite directions (transforming limit) and finally they will assume a divergent movement, separating.

Movement speed

The directions of motion that are described are perceived in periods of millions of years because the scale of continental drift is measured in millimeters per year. That is why on a human scale it is not easy to perceive the idea of ​​the displacement of tectonic plates.

For example, the African plate collides with the Eurasian plate forming the Betic mountain range on the Iberian Peninsula, at a rate of 5 mm / year. While the maximum speed recorded is the displacement generated in the eastern Pacific ridge, which is 15 mm / year.

Consequences of movement

The movement of the tectonic plates releases the energy from the interior of the planet at the limits of the plates mechanically (earthquakes) and thermally (volcanism). In turn, the displacements, shocks and frictions shape the land and ocean relief.

- Volcanic activity

The thermal flux of the mantle and its circulation by convection pushes the molten magma or basalt towards the surface causing volcanic eruptions. These in turn cause catastrophes by expelling lava, gases and particles that pollute the environment.

Volcanic island arches and continental volcanic arches

The convergence of two oceanic plates can produce chains of volcanoes that, when emerging, originate arches of islands. At the convergence of an oceanic plate with a continental one, continental volcanic arcs are formed, such as the trans-Mexican volcanic belt.

- Seismic activity

The collision of tectonic plates and especially the transformation limits, cause seismic movements or earthquakes. Some of them reach great magnitude and negatively affect human beings, destroying infrastructure and causing the death of people.

San Andrés fault (United States). Source:

Among the consequences of these phenomena are tidal waves or tsunamis, when the seismic movement occurs in the ocean.

- Relief of the Earth

The movement and interaction of the tectonic plates with each other, models the land relief and the ocean floor. The great continental mountain ranges, such as the Andes and the Appalachians, are the product of the convergence of tectonic plates when subduction occurs and those of the Himalayas by obduction.

In turn, due to isostatic or gravitational equilibrium, when one area rises, another is formed as a depression or plain. Diastrophic processes, such as faults, folding and others, are caused by the movements of the tectonic plates.

- Weather

The distribution of the continental masses affects the regime of marine currents and the world climate. Large continental masses due to the convergence of plates form drier continental interiors, in turn affecting the water cycle.

In the same way, the mountainous elevations produced by the subduction and obduction processes affect the wind regime and the distribution of rainfall.

References

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