Plate tectonics 101 for the layman (includes females in its traditional usage) / laywoman (for ultra feminists who do not want to be listed under the term layman)

Recently, I saw on FB, Shak Lim Ho’s fantastic photos of China’s beautiful sights. One is that of an enormous sedimentary rock, maybe about 60 ft high, which protrude slanting out of the ground in district of Shennongjia, Province of Hubie, China. It is very beautiful and spectacular so I could not help but make a comment which included the geological basis for such an outcrop of rock and mentioned about the massive earthquakes that might have happened there in the distant past to have the original horizontal rock that size break and tilt that much.

To my surprise, one of Shak’s friends commented on my comment and mentioned about the San Fransisco earthquakes and asked about the small earthquakes there.

I am not a geologist, so I admitted it and left the question unanswered at first.

However, I as have been working and living together with geologists_ both petroleum and mineral_for nearly 20 years and have learned from them some basic geological facts and I have also read about some geological articles including the plate tectonics. So I know a little of the basics of the cause of earthquakes and volcanoes which can be explained by plate tectonics and I cannot just leave the question unanswered.

So I tried to explain about it as much as I can, without the heavy geological jargon which a geologist would use. I thought that as a layman (non geologist) I can explain as clearly to another to the extent I understand because difficult geological theories will not be included in my explaination (I still do not understand even the basics of Einstein’s theories however much it is explained because all the articles I read explain in terms beyond my comprehension).

This led me to write a note in FB to explain in more detail, the topic of plate tectonics for non-geologists, and when I began to post it in my blogsite and looking for diagrams to post to explain the theories involved, I came across newer facts I had not known earlier and this led me to revise my original note in FB.

The heading of this blog was “Plate tectonics 101” at first, but I changed it to “Plate tectonics 101 for the layman” as I do not want geologists to dwell on it. Later, I realized that some feminists consider many aspects of living in the male dominated world unappealing and might object to it. So I have changed it again, after several tries, to the final “Plate tectonics 101 for the layman (includes females in its traditional usage) / laywoman (for ultra feminists who do not want to be listed under the term layman)”. I hope they will be satisfied, and myself too after posting it as I cannot change it again then. I even have doubts now whether the original “Plate tectonics 101” might have been better.

I also hope that geologists will forgive me for writing about a topic which I am not properly trained, and will comment and point out any mistakes that might be included in it so that it will be of benefit for others and myself, and yet, anyway, ……, to continue ……or begin the explaination,…..(my prelude has been too much I am afraid) …..

Although the world seems to be solid, continuous and stationaly, it is actually moving relative to one another. The whole earth mass once was in continuity and it is called Pangaea which existed about 250 million years ago

The world is made up of large plates of solidified larva that sort of float on the still fluid magma deep inside the earth and move relative to the adjacent ones. At the edge of the adjacent plates which move horizontally in opposite directions, different directional forces move the adjacent plates but as they are locked in contact and cannot move freely, great tension developed at their contact edges which, when it exceeds the resistance that is holding the adjacent margins of the plates together, slip a little and then lock up again at another point until the pressure builds up to cause it to slip again. Earthquakes occur when the slip happens and when the resistance that hold the 2 plates is great, large tension will be required before it slips and if it do, the slip will be greater and the magnitude of the earthquake will be so too.

The Pacific ring of fire, the eastern margin which includes the west coast of the USA is an example. That is also why Indonesia has frequent earthquakes and tsunamis, and also many volcanoes which result from seepage of magma at the edge weak point there and also on the west coast of S. America.

You will find the following sites interesting:

The famous San Andreas Fault on the West Coast of the USA is an example of the edge between 2 tectonic plates

Sometimes the plates diverge rather than slip past each other and that is why the Americas and the Europe-Africa coasts which once was in continuity, is now separated by the Atlantic ocean, in whose bed in the middle is the “mid Atlantic ridge” …which is the boundary between the North American plate and the Eurasion plate which are moving away from each other and where new sea bed continuously form. see:

Sometimes the plates meet head on and converge on each other and one plate might go under the other:
subduction of the Nazca Plate beneath the South American Plate to form the Andes.
subduction of the northern part of the Pacific Plate and the NW North American Plate that is forming the Aleutian Islands.

Sometimes both the plates press on each other and at the point where they meet, the earth is piled up:

Collision between the Eurasian Plate and the Indian Plate that is forming the Himalayas.
Collision of the Eurasian Plate and the African Plate formed the Pontic Mountains in Turkey.

also see:

Those who are interested can read more below, which are actually only parts of the original articles I have reproduced from the resources available to me, which I find interesting and have emphasized with bold and underscore, the important facts in the interesting topic. To read more, please go to the original sites mentioned.


From Wikipedia, the free encyclopedia

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For other uses, see Pangaea (disambiguation).

Pangaea, Pangæa, or Pangea (pronounced /pænˈdʒiːə/, pan-JEE-ə[1], from Ancient Greek πᾶν pan “entire”, and Γαῖα Gaia “Earth”, Latinized as Gæa) was the supercontinent that existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration.[2]

The name was coined in the scientific discussion of Alfred Wegener‘s theory of the Continental drift. In his book “The Origin of Continents and Oceans” (Die Entstehung der Kontinente und Ozeane) he postulated that all the continents had at one time formed a single supercontinent which he called the “Urkontinent”, before later breaking up and drifting to their present locations. The term Pangaea appeared in 1928 during a symposium to discuss Alfred Wegener’s theory.[3]

The single enormous ocean which surrounded Pangaea was accordingly named Panthalassa.


The breaking up and formation of supercontinents appears to be cyclical through Earth’s 4.6 billion year history. There may have been several others before Pangaea. The next-to-last one, Pannotia, formed about 600 million years ago (Ma) during the Proterozoic eon, and lasted until 540 Ma. Before Pannotia, there was Rodinia, which lasted from about 1.1 billion years ago (Ga) until about 750 million years ago. Rodinia formed by the accretion and assembly of fragments produced by breakup of an older supercontinent, called Columbia or Nuna that was assembled in the period 2.0-1.8 Ga [4][5]. The exact configuration and geodynamic history of Rodinia are not nearly as well understood as Pannotia and Pangaea. When Rodinia broke up, it split into three pieces: the supercontinent of Proto-Laurasia and the supercontinent of Proto-Gondwana, and the smaller Congo craton. Proto-Laurasia and Proto-Gondwanaland were separated by the Proto-Tethys Ocean. Soon thereafter Proto-Laurasia itself split apart to form the continents of Laurentia, Siberia and Baltica. The rifting also spawned two new oceans, the Iapetus Ocean and Paleoasian Ocean. Baltica was situated east of Laurentia, and Siberia northeast of Laurentia.

Around 600 Ma, most of these masses came back together to form the relatively short-lived supercontinent of Pannotia, which included large amounts of land near the poles and only a relatively small strip near the equator connecting the polar masses.

Only 60 million years after its formation, about 540 Ma, near the beginning of the Cambrian epoch, Pannotia in turn broke up, giving rise to the continents of Laurentia, Baltica, and the southern supercontinent of Gondwana.

In the Cambrian period the independent continent of Laurentia, which would become North America, sat on the equator, with three bordering oceans: the Panthalassic Ocean to the north and west, the Iapetus Ocean to the south and the Khanty Ocean to the east. In the Earliest Ordovician, around 480 Ma, the microcontinent of Avalonia, a landmass that would become the northeastern United States, Nova Scotia and England, broke free from Gondwana and began its journey to Laurentia.[6]

Baltica, Laurentia, and Avalonia all came together by the end of the Ordovician to form a minor supercontinent called Euramerica or Laurussia, closing the Iapetus Ocean. The collision also resulted in the formation of the northern Appalachians. Siberia sat near Euramerica, with the Khanty Ocean between the two continents. While all this was happening, Gondwana drifted slowly towards the South Pole. This was the first step of the formation of Pangaea.[7]

The second step in the formation of Pangaea was the collision of Gondwana with Euramerica. By Silurian time, 440 Ma, Baltica had already collided with Laurentia to form Euramerica. Avalonia had not collided with Laurentia yet, and a seaway between them, a remnant of the Iapetus Ocean, was still shrinking as Avalonia slowly inched towards Laurentia.

Meanwhile, southern Europe fragmented from Gondwana and started to head towards Euramerica across the newly formed Rheic Ocean and collided with southern Baltica in the Devonian, though this microcontinent was an underwater plate. The Iapetus Ocean’s sister ocean, the Khanty Ocean, was also shrinking as an island arc from Siberia collided with eastern Baltica (now part of Euramerica). Behind this island arc was a new ocean, the Ural Ocean.

By late Silurian time, North and South China rifted away from Gondwana and started to head northward across the shrinking Proto-Tethys Ocean, and on its southern end the new Paleo-Tethys Ocean was opening. In the Devonian Period, Gondwana itself headed towards Euramerica, which caused the Rheic Ocean to shrink.

In the Early Carboniferous, northwest Africa had touched the southeastern coast of Euramerica, creating the southern portion of the Appalachian Mountains, and the Meseta Mountains. South America moved northward to southern Euramerica, while the eastern portion of Gondwana (India, Antarctica and Australia) headed towards the South Pole from the equator.

North China and South China were on independent continents. The Kazakhstania microcontinent had collided with Siberia (Siberia had been a separate continent for millions of years since the deformation of the supercontinent Pannotia) in the Middle Carboniferous.

Western Kazakhstania collided with Baltica in the Late Carboniferous, closing the Ural Ocean between them, and the western Proto-Tethys in them (Uralian orogeny), causing the formation of the Ural Mountains, and the formation of the supercontinent of Laurasia. This was the last step of the formation of Pangaea.

Meanwhile, South America had collided with southern Laurentia, closing the Rheic Ocean, and forming the southernmost part of the Appalachians and Ouachita Mountains. By this time, Gondwana was positioned near the South Pole, and glaciers were forming in Antarctica, India, Australia, southern Africa and South America. The North China block collided with Siberia by Late Carboniferous time, completely closing the Proto-Tethys Ocean.

By Early Permian time, the Cimmerian plate rifted away from Gondwana and headed towards Laurasia, with a new ocean forming in its southern end, the Tethys Ocean, and the closure of the Paleo-Tethys Ocean. Most of the landmasses were all in one. By the Triassic Period, Pangaea rotated a little, in a southwest direction. The Cimmerian plate was still travelling across the shrinking Paleo-Tethys, until the Middle Jurassic time. The Paleo-Tethys had closed from west to east, creating the Cimmerian Orogeny. Pangaea looked like a C, with an ocean inside the C, the new Tethys Ocean. Pangaea had rifted by the Middle Jurassic, and its deformation is explained below.

Evidence of existence

Fossil evidence for Pangaea includes the presence of similar and identical species on continents that are now great distances apart. For example, fossils of the therapsid Lystrosaurus have been found in South Africa, India and Australia, alongside members of the Glossopteris flora, whose distribution would have ranged from the polar circle to the equator if the continents had been in their present position; similarly, the freshwater reptile Mesosaurus has only been found in localized regions of the coasts of Brazil and West Africa.[8]

Additional evidence for Pangaea is found in the geology of adjacent continents, including matching geological trends between the eastern coast of South America and the western coast of Africa.

The polar ice cap of the Carboniferous Period covered the southern end of Pangaea. Glacial deposits, specifically till, of the same age and structure are found on many separate continents which would have been together in the continent of Pangaea.[9]

Paleomagnetic study of apparent polar wandering paths also support the theory of a super-continent. Geologists can determine the movement of continental plates by examining the orientation of magnetic minerals in rocks; when rocks are formed, they take on the magnetic properties of the Earth and indicate in which direction the poles lie relative to the rock. Since the magnetic poles drift about the rotational pole with a period of only a few thousand years, measurements from numerous lavas spanning several thousand years are averaged to give an apparent mean polar position. Samples of sedimentary rock and intrusive igneous rock have magnetic orientations that typically are an average of these “secular variations” in the orientation of Magnetic North because their magnetic fields are not formed in an instant, as is the case in a cooling lava. Magnetic differences between sample groups whose age varies by millions of years is due to a combination of true polar wander and the drifting of continents. The true polar wander component is identical for all samples, and can be removed. This leaves geologists with the portion of this motion that shows continental drift, and can be used to help reconstruct earlier continental positions.[10]

The continuity of mountain chains also provide evidence for Pangea. One example of this is the Appalachian Mountains chain which extends from the northeastern United States to the Caledonides of Ireland, Britain, Greenland, and Scandinavia.[11]

Rifting and break-up

There were three major phases in the break-up of Pangaea. The first phase began in the EarlyMiddle Jurassic (about 175 Ma), when Pangaea began to rift from the Tethys Ocean in the east and the Pacific in the west, ultimately giving rise to the supercontinents Laurasia and Gondwana. The rifting that took place between North America and Africa produced multiple failed rifts. One rift resulted in a new ocean, the North Atlantic Ocean[12].

The Atlantic Ocean did not open uniformly; rifting began in the north-central Atlantic. The South Atlantic did not open until the Cretaceous. Laurasia started to rotate clockwise and moved northward with North America to the north, and Eurasia to the south. The clockwise motion of Laurasia also led to the closing of the Tethys Ocean. Meanwhile, on the other side of Africa, new rifts were also forming along the adjacent margins of east Africa, Antarctica and Madagascar that would lead to the formation of the southwestern Indian Ocean that would also open up in the Cretaceous.

The second major phase in the break-up of Pangaea began in the Early Cretaceous (150–140 Ma), when the minor supercontinent of Gondwana separated into multiple continents (Africa, South America, India, Antarctica, and Australia). About 200 Ma, the continent of Cimmeria, as mentioned above (see “Formation of Pangaea“), collided with Eurasia. However, a subduction zone was forming, as soon as Cimmeria collided.[12]

This subduction zone was called the Tethyan Trench. This trench might have subducted what is called the Tethyan mid-ocean ridge, a ridge responsible for the Tethys Ocean’s expansion. It probably caused Africa, India and Australia to move northward. In the Early Cretaceous, Atlantica, today’s South America and Africa, finally separated from eastern Gondwana (Antarctica, India and Australia), causing the opening of a “South Indian Ocean”. In the Middle Cretaceous, Gondwana fragmented to open up the South Atlantic Ocean as South America started to move westward away from Africa. The South Atlantic did not develop uniformly; rather, it rifted from south to north.

Also, at the same time, Madagascar and India began to separate from Antarctica and moved northward, opening up the Indian Ocean. Madagascar and India separated from each other 100–90 Ma in the Late Cretaceous. India continued to move northward toward Eurasia at 15 centimeters (6 in) per year (a plate tectonic record), closing the Tethys Ocean, while Madagascar stopped and became locked to the African Plate. New Zealand, New Caledonia and the rest of Zealandia began to separate from Australia, moving eastward towards the Pacific and opening the Coral Sea and Tasman Sea.

The third major and final phase of the break-up of Pangaea occurred in the early Cenozoic (Paleocene to Oligocene). Laurasia split when North America/Greenland (also called Laurentia) broke free from Eurasia, opening the Norwegian Sea about 60–55 Ma. The Atlantic and Indian Oceans continued to expand, closing the Tethys Ocean.

Meanwhile, Australia split from Antarctica and moved rapidly northward, just as India did more than 40 million years earlier, and is currently on a collision course with eastern Asia. Both Australia and India are currently moving in a northeastern direction at 5–6 centimeters (2–3 in) per year. Antarctica has been near or at the South Pole since the formation of Pangaea about 280 Ma. India started to collide with Asia beginning about 35 Ma, forming the Himalayan orogeny, and also finally closing the Tethys Seaway; this collision continues today. The African Plate started to change directions, from west to northwest toward Europe, and South America began to move in a northward direction, separating it from Antarctica and allowing complete oceanic circulation around Antarctica for the first time, causing a rapid cooling of the continent and allowing glaciers to form. Other major events took place during the Cenozoic, including the opening of the Gulf of California, the uplift of the Alps, and the opening of the Sea of Japan. The break-up of Pangaea continues today in the Great Rift Valley

Plate Tectonics – Pangaea Continent Maps

Plate tectonics is the study of the lithosphere, the outer portion of the earth consisting of the crust and part of the upper mantle. The lithosphere is divided into about a dozen large plates which move and interact with one another to create earthquakes, mountain ranges, volcanic activity, ocean trenches and many other features. Continents and ocean basis are moved and changed in shape as a result of these plate movements.

The sequence of maps below show how a large supercontinent, known as Pangaea was fragmented into several pieces, each being part of a mobile plate of the lithosphere. These pieces were to become Earth’s current continents. The time sequence show through the maps traces the paths of the continents to their current positions..

In the early 1900′s Alfred Wegener proposed the idea of Continental Drift. His ideas centered around continents moving across the face of the earth. The idea was not quite correct – compared to the plate tectonics theory of today – but his thinking was on the proper track. In addition, a variant spelling of Pangaea isPangea“. It appears in some textbooks and glossaries, however, Pangaea is the current preferred spelling.

The theory of plate tectonics has done for geology what Charles Darwin’s theory of evolution did for biology. It provides geology with a comprehensive theory that explains “how the Earth works.” The theory was formulated in the 1960s and 1970s as new information was obtained about the nature of the ocean floor, Earth’s ancient magnetism, the distribution of volcanoes and earthquakes, the flow of heat from Earth’s interior, and the worldwide distribution of plant and animal fossils.

The theory states that Earth’s outermost layer, the lithosphere, is broken into 7 large, rigid pieces called plates: the African, North American, South American, Eurasian, Australian, Antarctic, and Pacific plates. Several minor plates also exist, including the Arabian, Nazca, and Philippines plates.

The plates are all moving in different directions and at different speeds (from 2 cm to 10 cm per year–about the speed at which your fingernails grow) in relationship to each other. The plates are moving around like cars in a demolition derby, which means they sometimes crash together, pull apart, or sideswipe each other. The place where the two plates meet is called a plate boundary. Boundaries have different names depending on how the two plates are moving in relationship to each other

Convergent Boundaries

Places where plates crash or crunch together are called convergent boundaries. Plates only move a few centimeters each year, so collisions are very slow and last millions of years. Even though plate collisions take a long time, lots of interesting things happen. For example, in the drawing above, an oceanic plate has crashed into a continental plate. Looking at this drawing of two plates colliding is like looking at a single frame in a slow-motion movie of two cars crashing into each other. Just as the front ends of cars fold and bend in a collision, so do the “front ends” of colliding plates. The edge of the continental plate in the drawing has folded into a huge mountain range, while the edge of the oceanic plate has bent downward and dug deep into the Earth. A trench has formed at the bend. All that folding and bending makes rock in both plates break and slip, causing earthquakes. As the edge of the oceanic plate digs into Earth’s hot interior, some of the rock in it melts. The melted rock rises up through the continental plate, causing more earthquakes on its way up, and forming volcanic eruptions where it finally reaches the surface. An example of this type of collision is found on the west coast of South America where the oceanic Nazca Plate is crashing into the continent of South America. The crash formed the Andes Mountains, the long string of volcanoes along the mountain crest, and the deep trench off the coast in the Pacific Ocean.

Are They Dangerous Places to Live?
Mountains, earthquakes, and volcanoes form where plates collide. Millions of people live in and visit the beautiful mountain ranges being built by plate collisions. For example, the Rockies in North America, the Alps in Europe, the Pontic Mountains in Turkey, the Zagros Mountains in Iran, and the Himalayas in central Asia were formed by plate collisions. Each year, thousands of people are killed by earthquakes and volcanic eruptions in those mountains. Occasionally, big eruptions or earthquakes kill large numbers of people. In 1883 an eruption of Krakatau volcano in Indonesia killed 37,000 people. In 1983 an eruption-caused mudslide on Nevada del Ruiz in Columbia killed 25,000 people. In 1976, an earthquake in Tangshan, China killed an astounding 750,000 people.

On the other hand, earthquakes and volcanoes occurring in areas where few people live harm no one. If we choose to live near convergent plate boundaries, we can build buildings that can resist earthquakes, and we can evacuate areas around volcanoes when they threaten to erupt. Yes, convergent boundaries are dangerous places to live, but with preparation and watchfulness, the danger can be lessened somewhat.

Divergent Boundaries

Places where plates are coming apart are called divergent boundaries. As shown in the drawing above, when Earth’s brittle surface layer (the lithosphere) is pulled apart, it typically breaks along parallel faults that tilt slightly outward from each other. As the plates separate along the boundary, the block between the faults cracks and drops down into the soft, plastic interior (the asthenosphere). The sinking of the block forms a central valley called a rift. Magma (liquid rock) seeps upward to fill the cracks. In this way, new crust is formed along the boundary. Earthquakes occur along the faults, and volcanoes form where the magma reaches the surface.

Where a divergent boundary crosses the land, the rift valleys which form are typically 30 to 50 kilometers wide. Examples include the East Africa rift in Kenya and Ethiopia, and the Rio Grande rift in New Mexico. Where a divergent boundary crosses the ocean floor, the rift valley is much narrower, only a kilometer or less across, and it runs along the top of a midoceanic ridge. Oceanic ridges rise a kilometer or so above the ocean floor and form a global network tens of thousands of miles long. Examples include the Mid-Atlantic ridge and the East Pacific Rise.

Plate separation is a slow process. For example, divergence along the Mid Atlantic ridge causes the Atlantic Ocean to widen at only about 2 centimeters per year

Transform Boundaries

Places where plates slide past each other are called transform boundaries. Since the plates on either side of a transform boundary are merely sliding past each other and not tearing or crunching each other, transform boundaries lack the spectacular features found at convergent and divergent boundaries. Instead, transform boundaries are marked in some places by linear valleys along the boundary where rock has been ground up by the sliding. In other places, transform boundaries are marked by features like stream beds that have been split in half and the two halves have moved in opposite directions.

Perhaps the most famous transform boundary in the world is the San Andreas fault, shown in the drawing above. The slice of California to the west of the fault is slowly moving north relative to the rest of California. Since motion along the fault is sideways and not vertical, Los Angeles will not crack off and fall into the ocean as popularly thought, but it will simply creep towards San Francisco at about 6 centimeters per year. In about ten million years, the two cities will be side by side!

Although transform boundaries are not marked by spectacular surface features, their sliding motion causes lots of earthquakes. The strongest and most famous earthquake along the San Andreas fault hit San Francisco in 1906. Many buildings were shaken to pieces by the quake, and much of the rest of the city was destroyed by the fires that followed. More than 600 people died as a result of the quake and fires. Recent large quakes along the San Andreas include the Imperial Valley quake in 1940 and the Loma Prieta quake in 1989

How Plate Tectonics Works

Way back in 1912 a scientist by the name of Alfred Wegener came up with a crazy idea. He noticed that all of the continents seemed to fit together like the pieces of a giant puzzle. He thought, “Maybe they were once all joined together in a single, giant landmass that broke up and drifted apart over time?”. He decided to give this supercontinent a name and called it Pangea, meaning, “all lands”. At the time he presented his idea to the scientific community it came to be known as continental drift theory. Wegener was unable to find solid evidence to support his theory, so the other scientists laughed him off as a crackpot. One of his suggestions for the cause of continental drift was that centrifugal force from the rotation of the earth caused the continents to slide into each other and move around on the surface. They all calculated that there wasn’t enough force generated by the earth’s rotation to cause shifting of the crust and nobody took him seriously. They were all convinced the earth was rock-solid and immovable.

But then in 1929, along came a scientist named Arthur Holmes who didn’t think that Wegener’s theory of continental drift was too farfetched. “Now wait just a minute. Maybe he’s got something here”, he told them. He mentioned one of Wegener’s other theories about the source of continental drift; the idea that the molten mantle beneath the earth’s crust experiences thermal convection and that the movement of these convection currents in the mantle could cause an upwelling beneath the crust, forcing it to break apart and move. Now, that sounded like a semi-reasonable explanation for the movement of the earth’s crust. As a matter of fact, if you looked closely at this idea it explained a lot of things, not just the continental puzzle idea. It also explained how mountain ranges were formed – by continents crashing into each other and ‘rumpling up rock’. Still, the other scientists just nodded and said, ‘Yeah. Fine. Whatever’. And the theory was neatly tucked away and ignored.

Scientists are trained to be skeptical. They were all waiting for a preponderance of evidence that backed up this harebrained theory.

Over the next thirty years a lot of new and surprise discoveries were made as new technologies were developed for exploring the ocean floor . The discovery of volcanic activity on the ocean floor in the middle of the Antlantic that turned out to be part of a long, unbroken mountain chain of undersea volcanoes was the most ground-breaking discovery that supported the theory of continental drift. Scientists developed instruments for measuring earthquake activity around the world and began plotting the locations of earthquakes. They all got together and started drawing a new map of the world that showed volcanic and seismic (earthquake) activity was concentrated along certain areas that looked like the margins of huge crustal plates. During the 1960s several scientists published papers that reviewed the preponderance of evidence that had been gathered for the theory of continental drift and it soon came to be known as the theory of plate tectonics.

Plate tectonics

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The tectonic plates of the world were mapped in the second half of the 20th century.

Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός “pertaining to building”) (Little, Fowler & Coulson 1990)[1] is a scientific theory which describes the large scale motions of Earth‘s lithosphere. The theory builds on the older concepts of continental drift, developed during the first decades of the 20th century (one of the most famous advocates was Alfred Wegener), and was accepted by the majority of the Geoscientific community when the concepts of seafloor spreading were developed in the late 1950s and early 1960s. The lithosphere is broken up into what are called “tectonic plates”. In the case of the Earth, there are currently seven to eight major (depending on how they are defined) and many minor plates. The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: convergent, or collisional boundaries; divergent boundaries, also called spreading centers; and conservative transform boundaries. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The lateral relative movement of the plates varies, though it is typically 0–100 mm annually (Read & Watson 1975)[2].

The tectonic plates are composed of two types of lithosphere: thicker continental and thin oceanic. The upper part is called the crust, again of two types (continental and oceanic). This means that a plate can be of one type, or of both types. One of the main points the theory proposes is that the amount of surface of the (continental and oceanic) plates that disappear in the mantle along the convergent boundaries by subduction is more or less in equilibrium with the new (oceanic) crust that is formed along the divergent margins by seafloor spreading. This is also referred to as the “conveyor belt” principle. In this way, the total surface of the Globe remains the same. This is in contrast with earlier theories advocated before the Plate Tectonics “paradigm, as it is sometimes called, became the main scientific model, theories that proposed gradual shrinking (contraction) or gradual expansion of the Globe, and that still exist in science as alternative models.

Regarding the driving mechanism of the plates various models co-exist: Tectonic plates are able to move because the Earth’s lithosphere has a higher strength and lower density than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Their movement is thought to be driven by a combination of the motion of seafloor away from the spreading ridge (due to variations in topography and density of the crust that result in differences in gravitational forces) and drag, downward suction, at the subduction zones. A different explanation lies in different forces generated by the rotation of the Globe and tidal forces of the Sun and the Moon. The relative importance of each of these factors is unclear.

Key principles

The outer layers of the Earth are divided into lithosphere and asthenosphere. This is based on differences in mechanical properties and in the method for the transfer of heat. Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. This division should not be confused with the chemical subdivision of these same layers into the mantle (comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times, depending on its temperature and pressure.

The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like (visco-elastic solid) asthenosphere. Plate motions range up to a typical 10–40 mm/a (Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/a (Nazca Plate; about as fast as hair grows) (Zhen Shao 1997[3]; Hancock, Skinner & Dineley 2000[4]). The driving mechanism behind this movement is described below in a separate section.

Tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal material: oceanic crust (in older texts called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium). Average oceanic lithosphere is typically 100 km thick (Turcotte & Schubert 2002)[5]; its thickness is a function of its age: as time passes, it conductively cools and becomes thicker. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed. For a typical distance oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km thick at mid-ocean ridges to greater than 100 km at subduction zones; for shorter or longer distances, the subduction zone (and therefore also the mean) thickness becomes smaller or larger, respectively (Turcotte & Schubert 2002)[6]. Continental lithosphere is typically ~200 km thick, though this also varies considerably between basins, mountain ranges, and stable cratonic interiors of continents. The two types of crust also differ in thickness, with continental crust being considerably thicker than oceanic (35 km vs. 6 km) (Turcotte & Schubert 2002)[7].

The location where two plates meet is called a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, and oceanic trenches. The majority of the world’s active volcanoes occur along plate boundaries, with the Pacific Plate’s Ring of Fire being most active and most widely known. These boundaries are discussed in further detail below.

As explained above, tectonic plates can include continental crust or oceanic crust, and many plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers, and continental crust is formed through arc volcanism and accretion of terranes through tectonic processes; though some of these terranes may contain ophiolite sequences, which are pieces of oceanic crust, these are considered part of the continent when they exit the standard cycle of formation and spreading centers and subduction beneath continents. Oceanic crust is also denser than continental crust owing to their different compositions. Oceanic crust is denser because it has less silicon and more heavier elements (“mafic“) than continental crust (“felsic“) (Schmidt & Harbert 1998)[8]. As a result of this density stratification, oceanic crust generally lies below sea level (for example most of the Pacific Plate), while the continental crust buoyantly projects above sea level (see the page isostasy for explanation of this principle).

Types of plate boundaries

Basically, three types of plate boundaries exist (Meissner 2002, p. 100), with a fourth, mixed type, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are:[9][10]

  1. Transform boundaries (Conservative) occur where plates slide or, perhaps more accurately, grind past each other along transform faults. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer). The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion.
  2. Divergent boundaries (Constructive) occur where two plates slide apart from each other. Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa’s Great Rift Valley) are both examples of divergent boundaries.
  3. Convergent boundaries (Destructive) (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones. The subducting slab contains many hydrous minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism. Examples of this are the Andes mountain range in South America and the Japanese island arc.
  4. Plate boundary zones occur where the effects of the interactions are unclear and the broad belt boundaries are not well defined.

slanting sedimentary rock, district of Shennongjia, Hubie province, China

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