What Is A ‘tectonic Plate’ ‘plate Tectonics’ And ‘tectonic Activity’
The Earth consists of four concentric layers: inner core, outer core, mantle and crust. The crust and uppermost of solid mantle are known as lithosphere. Whereas asthenosphere is highly viscous, mechanically weak and semi-molten region of the upper mantle of the Earth. And, lithosphere floats over asthenosphere.
Figure showing the structure of the earth
A tectonic plate is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plates move horizontally over the asthenosphere as rigid units.
On the basis of size, a tectonic plate may be a major plate or a minor plate. For example, Pacific plate is a major plate whereas Nazca plate is a minor plate.
On the basis of nature, a plate may be referred to as continental plate or oceanic plate depending on which of the two occupy a large portion of the plate. For example, Pacific plate is mostly an oceanic plate whereas Eurasian plate may be called as a continental plate.
While a tectonic plate is a rigid lithospheric slab, plate tectonics is a collective term for evolution, nature and motion, deformation, the interaction of plate margins and resultant landforms.
The earth’s crust is continuously experiencing movements in horizontal as well as vertical direction resulting in breaking and bending of crustal rocks and this process of deformation is known as the tectonic activity.
Cross Section Of The Earth
The Earth is made up of four distinct layers:
- The inner core is in the centre and is the hottest part of the Earth. It is solid and made up of iron and nickel with temperatures of up to 5,500°C. With its immense heat energy, the inner core is like the engine room of the Earth.
- The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel. It is still extremely hot, with temperatures similar to the inner core.
- The mantle is the widest section of the Earth. It has a diameter of approximately 2,900 km. The mantle is made up of semi-molten rock called magma. In the upper parts of the mantle the rock is hard, but lower down the rock is soft and beginning to melt.
- The crust is the outer layer of the earth. It is a thin layer between 0-60 km thick. The crust is the solid rock layer upon which we live.
There are two different types of crust: continental crust, which carries land, and oceanic crust, which carries water.
The diagram below shows the structure of the earth. In geography, taking a slice through a structure to see inside is called a cross section.
Driving Forces Related To Gravity
Forces related to gravity are invoked as secondary phenomena within the framework of a more general driving mechanism such as the various forms of mantle dynamics described above. In modern views, gravity is invoked as the major driving force, through slab pull along subduction zones.
Gravitational sliding away from a spreading ridge: According to many authors, plate motion is driven by the higher elevation of plates at ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot mantle material, it gradually cools and thickens with age . Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with increased distance from the ridge axis.
Gravitational sliding away from mantle doming: According to older theories, one of the driving mechanisms of the plates is the existence of large scale asthenosphere/mantle domes which cause the gravitational sliding of lithosphere plates away from them . This gravitational sliding represents a secondary phenomenon of this basically vertically oriented mechanism. It finds its roots in the Undation Model of van Bemmelen. This can act on various scales, from the small scale of one island arc up to the larger scale of an entire ocean basin.
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The Major And Minor Plates
The Earths lithosphere is divided into seven major and some minor plates.
- Young Fold Mountain ridges, oceanic trenches, and/or transform faults surround the major plates. These include:
- The Antarctic plate
- The North American plate
- The South American plate
- The Pacific plate
- The Africa with the eastern Atlantic floor plate
- Eurasia and the adjacent oceanic plate
- Mountain Ridge: A mountain ridge refers to the chain of mountains or hills that form a continuous elevated crest for some distance.
- Ocean trenches are long, narrow depressions on the seafloor.
- These are the deepest parts of the ocean and some of the deepest natural spots on Earth.
- They are found in every ocean basin on the planet and the deepest ocean trenches ring the Pacific called the Ring of Fire”.
The Indian Plate:
Asthenosphere:
Types Of Plate Boundaries
A plate boundary is the border between two or more tectonic plates. There are different types of plate boundary, depending on the direction in which the plates are moving and whether the plates are continental or oceanic. The main types of plate boundary are: constructive, destructive, collisional and transform.
Constructive plate boundary
A constructive boundary is formed when two plates are moving apart, creating gaps in the Earths crust and thereby allowing magma to surface and form new crust. More than often, magma bursts through the crust, giving rise to volcanoes as the lava cools.
Many constructive plate boundaries are mid-oceanic ridges, eg. the fault line between the Eurasian and North American plates.
In the rare case that continental plates diverge, land is pulled apart and forms a rift valley. The most famous rift valley is East African rift system, where the African plate is splitting into the Somalian plate and the Nubian Plate. These plates are moving away from each other, so Africa will be divided into two land masses. Between them, a new oceanic curst will form as magma from the mantle will surface to fill any possible gap between the two plates.
Destructive plate boundary
A destructive plate boundary is formed when an oceanic plate and a continental plate collide. The oceanic plate is subducted under the continental plate , as it is more dense.
Also, subduction causes the seafloor and outermost crust to bend and form a steep, V-shaped depression.
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Is Japan Still Recovering From The 2011 Tsunami
TOKYO Ten years after a massive earthquake and tsunami devastated Japans northeastern coast, triggering meltdowns at the Fukushima nuclear power plant, much has been achieved in disaster-hit areas but they are still recovering. The magnitude 9.0 earthquake was one of the strongest temblors on record.
Who First Proposed The Idea Of Plate Tectonics
German meteorologist Alfred Wegener is often credited as the first to develop a theory of plate tectonics, in the form of continental drift. Bringing together a large mass of geologic and paleontological data, Wegener postulated that throughout most of geologic time there was only one continent, which he called Pangea, and the breakup of this continent heralded Earths current continental configuration as the continent-sized parts began to move away from one another. Wegener presented the idea of continental drift and some of the supporting evidence in a lecture in 1912, followed by his major published work, The Origin of Continents and Oceans .
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Island Arcs And Collision Of Oceanic Lithosphere
When a continent ruptures to form an ocean basin with axial rift, the two new plates move apart and create a new ocean. But eventually the plate motions may reverse, and the ocean basin may start to close. For this to happen, a plate must fracture and produce a subduction boundary. If the fracture occurs at a passive continental margin, oceanic crust will be subducted below continental crust, as shown in Figure 11.23. But what happens if the fracture occurs in the middle of a plate of oceanic lithosphere? This situation is shown in Figure 11.28.
As the subducted oceanic lithosphere plunges downward, oceanic crust is carried into the mantle. Since it originally came from the mantle and is of the same composition, it simply melts and disappears into the mantle rock.
However, the descent of the plate also carries a thin layer of ocean-floor sediment into the mantle. The heat and pressure of the mantle melt this sediment, and because it is less dense than the mantle rock, it begins to rise. The result is the formation of an island arca chain of volcanoes paralleling the subduction trench. We saw this process earlier in the formation of a volcanic arc on land, near the margin of a continent, in the collision of lithospheric crust with oceanic crust in Figure 11.23.
Is Tokyo Built On A Fault Line
Occasional quakes continue to plague the city, thanks to the fact that it sits on a fault line where two rocky plates meet: one holds Honshu Island, while another the Philippine Sea plate slides underneath it. But before now, seismologists had not accurately determined the depth of Tokyos fault line.
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How Plate Tectonics Works
The driving force behind plate tectonics is convection in the mantle. Hot material near the Earth’s core rises, and colder mantle rock sinks. “It’s kind of like a pot boiling on a stove,” Van der Elst said.
Meanwhile, geologists imagine the plates above this roiling mantle as bumper cars they repeatedly collide, stick together, then rip apart. Geologists refer to the places where segments meet and divide as plate boundaries. They’re thought to wrap around the Earth like seams on a baseball.
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There are three ways in which plate boundaries meet, and each one triggers a unique geological feature.
Convergent boundaries occur where plates collide into one another. Where those plates meet, Earth’s crust crumbles and buckles into mountain ranges. For example, India and Asia came together about 55 million years ago to create the Himalaya Mountains. As the mash-up continues, those mountains grow higher and higher. Geologists have discovered the Swiss Alps are being lifted faster than they are being lowered through erosionand are thus growing every year, according to a 2020 study in the journal Earth-Science Reviews. However, when a mountains mass becomes too large to resist gravity, it will cease to grow. Erosion also hinders growth by wearing mountains down, but because mountains can grow at a relatively fast rate, erosion typically doesnt win out, according to the University of Hawaii at Manoa.
How Do Plate Tectonics Function
The main impetus behind plate tectonics is convection in the mantle. Hot material close to the Earths center ascents, and colder mantle rock sinks. It is sort of like a pot bubbling on a stove. The convection drive plates tectonics through a mix of pushing and spreading separated at mid-sea edges and pulling and sinking descending at subduction zones, analysts think. Researchers proceed to study and discuss the components that move the plates.
Mid-sea edges are holes between tectonic plates that mantle the Earth-like creases on a baseball. Hot magma gushes at the edges, framing new sea crust and pushing the plates separated. Two tectonic plates meet at subduction zones, and one slides underneath the other go into the mantle, the layer underneath the crust. The cold, sinking plate hauls the crust behind it descending.
Numerous stupendous volcanoes are found along subduction zones, for example, the Ring of Fire that encompasses the Pacific Ocean.
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When Did Plate Tectonics Start
While the Earth is estimated to be 4.54 billion years old, oceanic crust is constantly recycled at subduction zones. That means the oldest seafloor is still only about 200 million years old. The oldest ocean rocks are found in the northwestern Pacific Ocean and the eastern Mediterranean Sea. Fragments of continental crust are much older, with large chunks at least 3.8 billion years old found in Greenland.
With clues left behind in rocks and fossils, geoscientists can reconstruct the past history of Earth’s continents. Most scientists think modern plate tectonics took over from earlier planetary development about 3 billion years ago, based on ancient magmas and minerals preserved in rocks from that period. However, researchers have found evidence that plate tectonics could have been active for as long as 4 billion years, according to a 2020 article in Discover Magazine.
“We don’t really know when plate tectonics as it looks today got started, but we do know that we have continental crust that was likely scraped off a down-going slab that is 3.8 billion years old,” Van der Elst said. “We could guess that means plate tectonics was operating, but it might have looked very different from today.”
As the continents jostle around the Earth, they occasionally come together to form giant supercontinents or a single landmass. One of the earliest big supercontinents, called Rodinia, assembled about 1 billion years ago. Its breakup is linked to a global glaciation called Snowball Earth.
The African Plate Is Moving Northwards Into The Eurasian Plate
Italy is sandwiched between the main huge African Plate ) and the large Eurasian Plate.
The Tectonic Plates have been moving for millions of years since a giant super-continent called Pangaea started breaking up 200-180 million years ago.
The tectonic plates are made up of cooler rigid rocks of the crust and upper mantle. This rigid layer is called the Lithosphere by scientists.
Below the lithosphere the lower mantle rocks are hot enough that they can flow.
Try to think of these two layers as toffee the rigid layer is like toffee in the fridge hard enough to break your teeth but if you stick somewhere warm for a few hours it becomes soft.
The centre of the Earth is very hot and this heat moves outwards to the surface one way that it does this is in giant convection currents in the softer mantle rocks.
These currents in the mantle pull the Tectonic Plates above them.
Where they meet we have different types of boundaries and these lead to different types of volcanoes, earthquakes and landforms.
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Continents Of The Past
Modern plate tectonic theory is only a half a century old. But as far back as the nineteenth century, German meteorologist and geophysicist, Alfred Wegener, suggested that our continents are fragments of a single supercontinent. As good navigational charts became available, geographers could see the close correspondence between the outlines of the eastern coast of South America and the western coastline of Africa. Wegener proposed the first full-scale scientific theory describing the breakup of a single supercontinent, which he named Pangaea, into multiple continents that moved apart in a process called continental drift . He suggested that Pangaea existed intact as early as about 300 million years ago, in the Carboniferous Period.
A storm of controversy followed Wegener’s proposal, but he had some loyal supporters. He presented several lines of hard scientific evidence for Pangaea, including the distribution patterns of fossils and present-day plant and animal species. But his explanation of the physical process that separated the continents was weak, and geologists soon showed that it was wrong.
The continents are moving today. Data from orbiting satellites shows that rates of separation or of convergence between two plates are on the order of 5 to 10 cm per year, or 50 to 100 km per million years. At that rate, global geography must have been very different in past geologic eras than it is today.
Geography Igcse: Plate Tectonics
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How Many Plates Are There
Because Earth is spherical, its tectonic or lithospheric plates are fractured into dozens of curved sections. Each plate ranges from a few hundred to thousands of kilometers in size, according to the U.S. Geological Service , and depending on its size, is categorized as “major,” “minor” or “micro.”
According to World Atlas, seven major plates exist: the North American, Pacific, Eurasian, African, Indo-Australian, South American and Antarctic tectonic plates. However, according to a 2012 article in Nature, earthquakes over the last few decades are evidence that the Indo-Australian plate has cracked over the last 10 million years, creating a which will increase the number of major plates to eight.
Whether that new divide counts as a boundary or not, the Pacific Plate is still the largest of all tectonic plates. It measures 39,768,522 square miles in size, and lies hidden beneath the ocean.
The list of Earths minor plates includes the Arabian Plate, Caribbean Plate, Cocos Plate, Nazca Plate, Philippine Plate, Scotia Plate, and more. There are also many smaller plates throughout the world.