Introduction to Plate Tectonics
The surface of the Earth is broken up into major and minor plates. These rigid plates ‘float’ like rafts on the deeper, molten layer of the mantle. Movements of molten rock known as convection currents cause the plates to move slowly.
As plate tectonics operate over time, the size, shape and location of the world’s continents and oceans slowly change. This movement has a major impact on the evolution of landforms on Earth’s land surfaces and ocean floors. While the plates have some flexibility, they generally act like rigid bodies and only deform at plate boundaries. Landforms are created and destroyed at the edges of the plates. There is a strong relationship between tectonic plates and the occurrence of volcanoes and earthquakes.
As plate tectonics operate over time, the size, shape and location of the world’s continents and oceans slowly change. This movement has a major impact on the evolution of landforms on Earth’s land surfaces and ocean floors. While the plates have some flexibility, they generally act like rigid bodies and only deform at plate boundaries. Landforms are created and destroyed at the edges of the plates. There is a strong relationship between tectonic plates and the occurrence of volcanoes and earthquakes.
Convection currents
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Convection Currents Planet Earth
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Plate Boundaries
The location where two plates meet is called a plate boundary. Plate boundaries are where events such as earthquakes and the creation of features such as mountains, volcanoes, mid-ocean ridges and oceanic trenches occur. The majority of the world’s active volcanoes occur along plate boundaries.
Global pattern of tectonic plates (USGS)
Tectonic Plates, including some minor plates (GeoWorld8, 2014: 89)
The Ring of Fire
The Pacific Ring of Fire is a 40,000km roughly horseshoe-shaped area where the boundary (or edges) of the Pacific Plate interact with a number of other plates. The Pacific Ring of Fire is the world’s most famous volcanically active region and where most earthquakes occur.
The Pacific Ring of Fire is a 40,000km roughly horseshoe-shaped area where the boundary (or edges) of the Pacific Plate interact with a number of other plates. The Pacific Ring of Fire is the world’s most famous volcanically active region and where most earthquakes occur.
- Approximately 90% of the world’s earthquakes occur along the Ring of Fire.
- There are 452 volcanoes in the Ring of Fire.
The 40,000km long Pacific Ring of Fire contains 75% of Earth’s active and dormant
volcanoes (GeoWorld8, 2014: 88)
Continental and Oceanic Plates
Some plates are large enough to consist of both continental and oceanic crustal portions (e.g. the African or South American plates) whilst the Pacific Plate is almost entirely oceanic.
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Continental plates
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Oceanic plates
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Plate Boundary Movements (Overview of all 3)
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Boundary movements are described as either:
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Plate Boundary Movements: Transforming Boundaries
Transform-plate boundaries grind horizontally past each other. Pressure builds up along the boundary and, as pressure releases, earthquakes occur. Transform boundaries neither create nor destroy crust; they just move them. The San Andreas Fault in California is an example of a transform boundary.
Example of a transforming plate boundary: The San Andreas Fault in California, USA
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Additional Reading
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Fake Post
A viral Facebook post (right) features a photo of a man peering into a rock chasm and claims that it is the San Andreas Fault in California. The post is from 20 July 2021 and apparently a similar post in May 2020 was shared more than 1,000 times. A fact checking website has stated that the photo is not California's San Andreas Fault, but a geological feature in Canyonlands National Park Utah called 'Black Crack, which is about 700km away from the famous fault. An internet search of Black Crack, Canyonlands National Park Utah corroborates this. |
(Viral 'San Andreas' image carries a major fault - Australian Associated Press (aap.com.au))
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Plate Boundary Movements: Diverging Boundaries
Diverging oceanic plates create new sea floor. As the plates pull apart, magma rises to fill the fissure (linear volcanic vent) and then cools to create new crust. As the process repeats, the ocean basin widens.
Diverging oceanic plates also produce mid-ocean ridges. These ridges are areas that are relatively high compared to the surrounding ocean floor. They are created by magma rising between the diverging plates. One of the best known divergent boundaries is the Mid-Atlantic Ridge, which is spreading apart at a rate of 2.5 cm per year or 25 km per million years.
Divergent boundaries also occur on land where they slowly tear plates apart and create rift valleys. Because continental plates are thicker than oceanic plates, the force of the plates pulling apart does not create a break, but rather an upward bulge. Lines of weakness, called faults, develop along either side of the bulge. If the faults fracture, the bulging area of the plate between the faults drops, creating a valley called a rift valley.
Northern Ethiopia is at the centre of a Y-shaped rift system where three tectonic plates meet. This is called a triple junction. The Arabian Plate is drifting away from the African Plate (also called the Nubian Plate) and enlarging the Red Sea and the Gulf of Aden. Further south, the African Plate itself is rifting along the East Africa Rift Valley to form two new plates: the Nubian and the Somalian plates.
Diverging oceanic plates also produce mid-ocean ridges. These ridges are areas that are relatively high compared to the surrounding ocean floor. They are created by magma rising between the diverging plates. One of the best known divergent boundaries is the Mid-Atlantic Ridge, which is spreading apart at a rate of 2.5 cm per year or 25 km per million years.
Divergent boundaries also occur on land where they slowly tear plates apart and create rift valleys. Because continental plates are thicker than oceanic plates, the force of the plates pulling apart does not create a break, but rather an upward bulge. Lines of weakness, called faults, develop along either side of the bulge. If the faults fracture, the bulging area of the plate between the faults drops, creating a valley called a rift valley.
Northern Ethiopia is at the centre of a Y-shaped rift system where three tectonic plates meet. This is called a triple junction. The Arabian Plate is drifting away from the African Plate (also called the Nubian Plate) and enlarging the Red Sea and the Gulf of Aden. Further south, the African Plate itself is rifting along the East Africa Rift Valley to form two new plates: the Nubian and the Somalian plates.
Example of a divergent plate boundary: East Africa Rift Valley
(The Geological Society (geolsoc.org.uk))
Predictions are made to see what will happen with the East Africa Rift Valley.
Below is a prediction by The Times of India of what it will look like in 10 million years.
Below is a prediction by The Times of India of what it will look like in 10 million years.
In the future the
Somalian and Arabian plates
will break away from the
Nubian Plate (GeoWorld8, 2014: 91)
(In 10 million years on, rift in Africa could give rise to a new continent, new peaks - Times of India (indiatimes.com))
Additional Reading
Example of a divergent plate boundary: The Mid Atlantic Ridge in Iceland
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Plate Boundary Movements: Converging Boundaries
Convergent or destructive boundaries occur where plates collide into each other and one plate is destroyed as it subducts under the other.
There are three types of convergent boundaries:
There are three types of convergent boundaries:
- oceanic
- continental
- oceanic–continental.
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Oceanic-continental converging boundaries
When an oceanic plate and a continental plate converge, the oceanic plate is subducted under the continental plate. The edge of the oceanic plate is pushed into the mantle, where it creates a deep trench in the floor of the ocean. The edge of the continental plate is pushed upwards, to create mountains. Often, volcanoes also form along converging boundaries. |
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Oceanic converging boundaries
When two oceanic plates converge, one is subducted under the other. Converging oceanic plates create undersea volcanoes. As the volcanoes continue to erupt, lava and volcanic debris build up until the volcanoes eventually rise above sea level.
Often, chains of volcanoes form along the boundary of converging oceanic plates. These are called island arcs. Anak Krakatau in Indonesia is part of an island arc formed at converging-plate boundaries.
When two oceanic plates converge, one is subducted under the other. Converging oceanic plates create undersea volcanoes. As the volcanoes continue to erupt, lava and volcanic debris build up until the volcanoes eventually rise above sea level.
Often, chains of volcanoes form along the boundary of converging oceanic plates. These are called island arcs. Anak Krakatau in Indonesia is part of an island arc formed at converging-plate boundaries.
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Continental converging boundaries
When two continental plates converge, one usually moves under or over the other. This is because both of the plates are relatively light. When two continental plates converge, it is called a continental collision. Both plates buckle and fold and the crust is pushed upwards (called uplift) or sideways, forming very large mountains and mountain ranges. This is how the Himalayas were formed at the boundary of the Indian plate and the Eurasian plate about 37 million years ago. Here the Earth’s crust is thrust upward faster than erosion wears it down. |
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Subduction Zones
Subduction zones are at the edges of tectonic plates. The more massive plate, normally a continental will force the other plate, an oceanic plate down beneath it. This is the subduction zone. When the other plate is forced down the process is called subduction. The plate enters into the magma and eventually it is completely melted. That is how the surface of the earth makes way for the crust created over time at other plate boundaries.
Subduction zones have key characteristics that help geologist and seismologist identify them. The first is mountain formation. Subduction zones always have mountain ranges caused by plate subduction. The next is volcanic activity as a plate is subducted the pressure and heat turns it into magma. These pockets of magma find paths to the surface and create volcanoes. The final sign is deep marine trenches. These are the best evidence of a subduction zone as they are visible evidence of the crease formed by subduction of a plate. The most famous is the Mariana Trench, which is the deepest location on Earth.
Subduction zones are at the edges of tectonic plates. The more massive plate, normally a continental will force the other plate, an oceanic plate down beneath it. This is the subduction zone. When the other plate is forced down the process is called subduction. The plate enters into the magma and eventually it is completely melted. That is how the surface of the earth makes way for the crust created over time at other plate boundaries.
Subduction zones have key characteristics that help geologist and seismologist identify them. The first is mountain formation. Subduction zones always have mountain ranges caused by plate subduction. The next is volcanic activity as a plate is subducted the pressure and heat turns it into magma. These pockets of magma find paths to the surface and create volcanoes. The final sign is deep marine trenches. These are the best evidence of a subduction zone as they are visible evidence of the crease formed by subduction of a plate. The most famous is the Mariana Trench, which is the deepest location on Earth.
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Mariana Trench
The Mariana Trench is located in the western Pacific Ocean. It is the deepest known depression on the earth's surface, having been measured by various means at 10,900–11,000 m below sea level. It is a crescent-shaped trough in the Earth's crust averaging about 2,550 km long and 69 km wide. The Trieste, a U.S. navy bathyscaphe (free-diving self-propelled deep-sea submersible) crewed by Swiss oceanographer Jacques Piccard and U.S. Navy Lieutenant Donald Walsh, reached its bottom in 1960. |
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Constructive and Destructive Boundaries
The different types of movements occurring along the plates can create and/or destroy crust:
More detailed information about how land is formed and destroyed is below.
- Constructive margins — new crustal material is continuously moving upwards and pushing outwards at oceanic boundaries. E.g. ridges and rises.
- Destructive margins — the ocean crust is destroyed and recycled into the mantle at ocean trenches. Here the oceanic crust subducts (slides downwards) and produces earthquakes. As material sinks and melts, the magma rises up through fissures and exits from volcanoes. E.g. ocean trenches.
More detailed information about how land is formed and destroyed is below.
Formation of New Land
Plate movement creates new land through processes associated with divergent boundaries, convergent boundaries, and volcanic activity. Here’s an overview of how these mechanisms contribute to the formation of new land:
Divergent Boundaries:
Convergent Boundaries:
Hotspots and Volcanic Activity:
These processes, driven by the dynamic nature of the Earth's lithosphere and mantle, continually reshape the planet's surface, creating new land and contributing to the ever-changing geography of the Earth.
Plate movement creates new land through processes associated with divergent boundaries, convergent boundaries, and volcanic activity. Here’s an overview of how these mechanisms contribute to the formation of new land:
Divergent Boundaries:
- Mid-Ocean Ridges: At divergent boundaries, tectonic plates move away from each other. This movement occurs primarily at mid-ocean ridges, where magma rises from the mantle to fill the gap created by the separating plates. As the magma cools, it solidifies to form new oceanic crust. Over time, this process can lead to the creation of new land as the oceanic crust accumulates and forms underwater mountain ranges.
- Continental Rifts: Divergent boundaries can also occur within continents, leading to rift valleys. As the continental crust stretches and thins, magma can rise to the surface, creating new land in the form of volcanic activity and rift valleys. An example is the East African Rift.
Convergent Boundaries:
- Island Arcs and Mountain Ranges: When oceanic plates converge, one plate is often forced beneath another in a process known as subduction. The subducting plate melts as it descends into the mantle, generating magma that can rise to the surface and form volcanic island arcs. Over time, these volcanic islands can merge and grow, creating new land. An example of this process is the formation of the Japanese archipelago.
- Continental Collision: When two continental plates converge, neither plate is easily subducted due to their buoyancy. Instead, the collision causes the crust to buckle and fold, leading to the uplift of mountain ranges. The Himalayas, formed by the collision of the Indian and Eurasian plates, are an example of new land created through continental collision.
Hotspots and Volcanic Activity:
- Hotspots: Volcanic activity not associated with plate boundaries can also create new land. Hotspots are areas where plumes of hot mantle material rise towards the surface, creating volcanic islands as the overlying tectonic plate moves over the stationary hotspot. The Hawaiian Islands are a classic example of land formed by a hotspot.
- Seafloor Volcanism: Volcanic eruptions on the ocean floor can create new land. When underwater volcanoes erupt, the lava that reaches the surface cools and solidifies, gradually building up to form volcanic islands. Over time, these islands can become significant landmasses.
These processes, driven by the dynamic nature of the Earth's lithosphere and mantle, continually reshape the planet's surface, creating new land and contributing to the ever-changing geography of the Earth.
Constructive and destructive tectonic plate boundaries (GeoWorld8, 2014: 88)
Destruction of Land
Plate movement can also lead to the destruction of land through processes associated with convergent boundaries, transform boundaries, and subduction. Here’s how these mechanisms contribute to the destruction of land:
Convergent Boundaries:
Transform Boundaries:
Volcanic Activity:
Rifting and Basin Formation:
Overall, the dynamic and interconnected processes of plate tectonics continuously build up and break down the Earth's surface, leading to the creation and destruction of land over geological timescales.
Plate movement can also lead to the destruction of land through processes associated with convergent boundaries, transform boundaries, and subduction. Here’s how these mechanisms contribute to the destruction of land:
Convergent Boundaries:
- Subduction Zones: At convergent boundaries where an oceanic plate converges with either a continental plate or another oceanic plate, the denser oceanic plate is forced beneath the lighter plate in a process known as subduction. As the subducting plate descends into the mantle, it melts and is gradually destroyed. This process consumes oceanic crust, effectively reducing the amount of land. The destruction of oceanic plates at subduction zones is a key part of the plate tectonic cycle. For example, the Pacific Plate is being subducted beneath the North American Plate along the Cascadia Subduction Zone.
- Mountain Erosion: Convergent boundaries that create mountain ranges through continental collision are also sites where land is destroyed over time through erosion. The uplifted mountains are subject to weathering and erosion by wind, water, and ice, which gradually wears down the mountains and transports sediment away, ultimately reducing the land's elevation and volume.
Transform Boundaries:
- Earthquakes: At transform boundaries, where tectonic plates slide past each other, the stress and friction can cause earthquakes. These seismic events can cause significant land destruction, including the collapse of landmasses, triggering landslides, and changing the landscape dramatically. The San Andreas Fault in California is a well-known example of a transform boundary where such destructive events occur.
Volcanic Activity:
- Volcanic Explosions: Volcanic eruptions, particularly those that are highly explosive, can lead to the destruction of land. These eruptions can blow apart entire sections of volcanic mountains, creating calderas or depressions. The 1980 eruption of Mount St. Helens in Washington State is an example where a significant portion of the volcano was destroyed, resulting in a massive landslide and the creation of a large crater.
- Collapse of Volcanic Islands: Volcanic islands can also be destroyed when they become unstable. The weight of the accumulating volcanic material can cause sections of the island to collapse into the ocean, either through gradual erosion or sudden landslides. This process can significantly reduce the size of the island or even cause it to disappear entirely.
Rifting and Basin Formation:
- Continental Rifts: In some cases, the rifting process that creates new land can also lead to the destruction of existing land. As continental rifting progresses, large sections of continental crust can be stretched, thinned, and eventually break apart, creating rift valleys that may later subside and fill with water to form new ocean basins. This process destroys the pre-existing continental landmass.
Overall, the dynamic and interconnected processes of plate tectonics continuously build up and break down the Earth's surface, leading to the creation and destruction of land over geological timescales.
Plate Tectonics Video
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Did You Know?
You can swim between continents (North America and Europe) at the Silfra fissure, Iceland. It is also some of the clearest waters in the world!
Read more here: Dive Between Two Continents in This Frigid Fissure in Iceland | Travel| Smithsonian Magazine
Read more here: Dive Between Two Continents in This Frigid Fissure in Iceland | Travel| Smithsonian Magazine
(https://www.insider.com/iceland-swim-between-touch-continents-2017-8)
