From the deepest ocean trench to the highest mountain, plate tectonics explains the characteristics and movement of the Earth’s surface in the present and the past.
Developed from the 1950s to the 1970s, the theory of plate tectonics is the modern update of continental drift, an idea first proposed by scientist Alfred Wegener in 1912 which stated that the continents of Earth had âDriftedâ across the planet over time. Wegener didn’t have an explanation for how continents could move around the planet, but researchers do now: plate tectonics.
Plate tectonics is the theory that the Earth’s outer shell is divided into large plates of solid rock, called “plates,” which slide over the Earth’s mantle, the inner layer of rock above the Earth’s core. The solid outer layer of the Earth, which includes the crust and the upper mantle, is called the lithosphere. It is 100 km (60 miles) thick, according to the Encyclopedia Britannica. Below the lithosphere is the asthenosphere – a viscous layer held malleable by heat deep within the Earth. It lubricates the underside of Earth’s tectonic plates, allowing the lithosphere to move.
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Nicholas van der Elst, a seismologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York, views plate tectonics as the “unifying theory of geology.”
âBefore plate tectonics, people had to find explanations of the geological features of their region that were unique to that particular region,â Van der Elst said. “Plate tectonics unified all of these descriptions and said you should be able to describe all of the geological features as if they were driven by the relative motion of these tectonic plates.”
How plate tectonics works
The driving force behind plate tectonics is mantle convection. Hot matter near the Earth’s core rises and the rock in the cooler mantle sinks. âIt’s a bit like a boiling pot on a stove,â Van der Elst said.
Meanwhile, geologists imagine the plates above this agitated mantle like bumper cars; they repeatedly collide, stick together, and then tear apart. Geologists call plate boundaries the places where segments meet and divide. They are believed to wrap around the Earth like seams on a baseball.
Related: The Alps are always growing faster than they are eroding
There are three ways the plate boundaries meet, and each triggers a unique geological feature.
Converging boundaries occur when the plates collide with each other. Where these plates meet, the earth’s crust crumbles and deforms into mountain ranges. For example, India and Asia came together about 55 million years ago to create the Himalayan Mountains. As the mash-up continues, these mountains get higher and higher. Geologists have found that the Swiss Alps are uplifted faster than they are lowered by erosion – and therefore increasing every year, according to a 2020 study in the journal Earth Sciences Opinion. However, when a mountain’s mass becomes too large to withstand gravity, it will stop growing. Erosion also hampers growth by wearing down mountains, but since mountains can grow at a relatively rapid rate, erosion usually does not win out, according to the University of Hawaii at Manoa.
But the converging plates do not always collide upwards. Sometimes an oceanic plate (which is made up of rocks denser than landmasses) collides with a continental plate, in which case it “subducts” or plunges under the other plate. It then descends into the Earth’s mantle, the layer under the crust, melts into the hot magma of the mantle and is vomited in a volcanic eruption. Many spectacular volcanoes are found along subduction zones, such as the “Ring of Fire” which surrounds the Pacific Ocean.
When two oceanic plates converge, a deep trench forms, such as the Mariana Trench in the North Pacific Ocean, considered the deepest point on Earth. These types of collisions can also lead to underwater volcanoes.
As the name suggests, divergent boundaries are tectonic boundaries where the plates “diverge” or are separated. This movement creates giant hollows on earth, like the East African Rift. In the ocean, this same process creates mid-ocean ridges. Hot magma from the Earth’s mantle gushes out from these ridges, forming new oceanic crust and spreading the plates apart. Underwater mountains and volcanoes can rise along this vein, in some cases forming islands. For example, the Mid-Atlantic Ridge crosses Iceland directly.
The last type of plate boundary, transform boundaries, exist when plates move sideways with respect to each other. It is the sliding and sliding movement of the boundaries of the plates that sets off many earthquakes. The San Andreas Fault in California, where the tectonic plates of North America and the Pacific rub against each other with predominantly horizontal movement, is a famous example of a transformational frontier.
Tectonic plates move at a speed of 3 to 5 centimeters (1 to 2 inches) per year, according to National Geographic. It’s about as fast as your fingernails grow!
How many plates are there?
Because the Earth is spherical, its tectonic or lithospheric plates are fractured into dozens of curved sections. (Imagine it as a cracked eggshell.) Each plate measures from a few hundred to thousands of miles, depending on the United States Geological Survey (USGS), 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 past decades are evidence that the Indo-Australian Plate has cracked over the past 10 million years, creating a Separate Indian and Australian plate which will bring the number of main plates to eight.
Whether or not this new division counts as a boundary, the Pacific Plate is still the largest of all tectonic plates. It measures 39,768,522 square miles (103,000,000 square kilometers) and hides beneath the ocean.
The list of minor Earth plaques includes the Arabian plaque, the Caribbean plaque, the Cocos plaque, the Nazca plaque, the Philippine plaque, the Scotia plaque and more. There are also many small plates around the world.
When did plate tectonics start?
While the earth is esteemed 4.54 billion years, the oceanic crust is constantly recycled at the level of the subduction zones. This means that the oldest seabed is still only about 200 million years old. The oldest oceanic rocks are found in the northwest Pacific Ocean and the eastern Mediterranean Sea. The continental crust fragments are much older, with large pieces of at least 3.8 billion years old found in Greenland.
With clues left in rocks and fossils, geoscientists can piece together the past history of Earth’s continents. Most scientists believe that modern plate tectonics took over from earlier planetary development around 3 billion years ago, based on ancient magmas and minerals preserved in rocks from this period. However, researchers found evidence that plate tectonics could have been active for as long as 4 billion years, according to a 2020 article in Discover the magazine.
“We don’t really know when the plate tectonics as it appears today began, but we do know that we have a continental crust that was probably scraped from a descending slab. [a tectonic plate in a subduction zone] which is 3.8 billion years old, “Van der Elst said.” We could guess that means plate tectonics worked, but it could have been very different today. “
As continents jostle around Earth, they sometimes come together to form giant supercontinents or a single landmass. One of the first large supercontinents, called Rodinia, came together about 1 billion years ago. Its rupture is linked to a global glaciation called Snowball Earth.
A newer supercontinent called Pangea formed around 300 million years ago. Africa, South America, North America and Europe have intertwined closely, leaving geologists with a characteristic pattern of fossils and rocks to decipher once Pangea parted. The puzzle pieces Pangea left behind, from fossils to corresponding shores along the Atlantic Ocean, provided the first clues that Earth’s continents are moving.