Plate tectonics explains the present and past features and movement of Earth's surface, from the deepest ocean trench to the tallest mountain.
The theory of plate tectonics was developed in the 1950s and 1970s as a modern update to continental drift. Scientist Alfred Wegener proposed in 1912 that Earth's continents had drifted across the globe over time. It is obvious now: plate tectonics explains how continents can move across the globe. Wegener didn't know how continents could move, but researchers do.
The theory of plate tectonics says that Earth's outer shell is made up of massive slabs of rock, known as "plates," which glide over its mantle, which is the rocky layer above Earth's core. The solid outer layer of Earth is known as the lithosphere, which includes the crust and the uppermost mantle.
The Encyclopedia Britannica states that it is 100 kilometers (60 miles) thick. Under the lithosphere lies the asthenosphere, a viscous layer kept malleable by the heat deep within the Earth. It lubricates the underside of Earth's tectonic plates, allowing the lithosphere to move.
Seismologist Nicholas van der Elst, of Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, believes plate tectonics is the "unifying theory of geology."
Van der Elst explained geologic features in his region before the theory of plate tectonics was developed. "Plate tectonics unified all these geological descriptions and said that geologic features should be described as if they were being driven by relative motions between tectonic plates."
How Plate Tectonics Works
Plate tectonics is driven by convection in the mantle. Colder mantle rock sinks, while hot material near the Earth's core rises. Van der Elst said it's like having a pot on the stove boiling.
At the same time, geologists imagine that the plates above the roiling mantle are like bumper cars that collide, stick together, and then rip apart. The boundaries between segments are known as plate boundaries by geologists. They're believed to encircle the Earth-like seams on a baseball.
There are three different ways in which plate boundaries meet, and each triggers a unique geological feature.
Convergent boundaries occur where plates collide with one another. Earth's crust crumbles and buckles where those plates meet. A 55 million-year-old convergent event created the Himalayan Mountains when India and Asia merged. As the mash-up continues, those mountains grow higher and higher.
In a study published in Earth-Science Reviews in 2020, geologists discovered the Swiss Alps were lifted faster than they were being eroded-and are thus growing every year. However, when the mountain's mass becomes too large to resist gravity, it will cease to grow.
According to the University of Hawai'i at Manoa, erosion also hinders growth by wearing mountains down, but because mountains can grow relatively rapidly, erosion usually loses out.
But converging plates do not always collide upward. Occasionally, an ocean plate (which is composed of denser rock than landmasses) collides with a continental plate, in which case it subducts beneath the other plate.
Afterward, it descends into the Earth's mantle, which is a layer below the crust, melts in the mantle's hot magma, and is spewed out in volcanic eruptions. Many spectacular volcanoes can be found along subduction zones, such as those that surround the Pacific Ocean on the "Ring of Fire."
When two oceanic plates meet, a deep trench is formed, such as the Mariana Trench in the North Pacific Ocean, which is believed to be the deepest point on Earth. These types of collisions can sometimes lead to the formation of underwater volcanoes.
As their name suggests, divergent boundaries are tectonic boundaries where plates "diverge" or are pushed apart. The motion creates huge troughs on land, such as the East Africa Rift. The same process results in mid-ocean ridges in the ocean.
Magma from Earth's mantle wells up at these ridges, forming new ocean crust and pushing the plates apart. Along this seam may rise underwater mountains and volcanoes, which may form islands. For example, the Mid-Atlantic Ridge runs directly through Iceland.
Transform boundaries exist when plates move sideways relative to one another. It's the slippage of plate boundaries that causes earthquakes. One of the world's most famous transform boundaries can be seen at California's San Andreas Fault, where the North American and Pacific tectonic plates grind against one another.
According to National Geographic, tectonic plates move one to two inches (3 to 5 centimeters) per year. That's about how fast your fingernails grow!
How many plates are there?
The Earth's tectonic or lithospheric plates are fractured into dozens of curved sections due to its spherical nature. (Imagine a cracked eggshell.) Each plate can range from a few hundred to thousands of kilometers in size, according to the U.S. Geological Service (USGS), and based on its size, is classified as "major," "minor," or "micro."
There are seven major tectonic plates according to the World Atlas: the North American, Pacific, Eurasian, African, Indo-Australian, South American, and Antarctic plates. A 2012 article in Nature claims that earthquakes over the past few decades are evidence that the Indo-Australian plate has cracked over the last 10 million years, creating a separate Indian Plate and Australian Plate, which will create eight major plates.
No matter whether that new boundary counts as a boundary or not, the Pacific Plate remains the largest of all tectonic plates. Located beneath the ocean, it measures 39,768,522 square miles (103,000,000 square kilometers).
Earth's minor plates include the Arabian Plate, Caribbean Plate, Cocos Plate, Nazca Plate, Philippine Plate, Scotia Plate, and more. Around the world, there are also many smaller plates.
When did Plate Tectonics start?
At subduction zones, oceanic crust is constantly recycled even though the Earth is estimated to be 4.54 billion years old. The oldest seafloor is approximately 200 million years old. On the northwest Pacific Ocean and the eastern Mediterranean Sea, the oldest ocean rocks are found. There are more fragments of continental crust found in Greenland, including large chunks that are 3.8 billion years old.
Geoscientists can reconstruct Earth's history by studying rocks and fossils left behind. A majority of scientists think that plate tectonics took over from planetoid evolution about 3 billion years ago, based on ancient magmas and minerals preserved in rocks of that age. Researchers have found evidence that plate tectonics may have existed for as long as 4 billion years, according to a 2020 article in Discover Magazine.
The origins of plate tectonics as we know them today are not known, but we do know we have a continental crust that was scraped off a down-going slab [in a subduction zone that is 3.8 billion years old]," Van der Elst said. A plate tectonic plate might have been operating in those times, but it may have appeared quite different from the way it does now."
Occasionally, the continents jostle around the Earth to form giant supercontinents or a single landmass. It is thought that one of the earliest big supercontinents, Rodinia, assembled about 1 billion years ago. Its breakup is related to a global glaciation known as Snowball Earth.
Approximately 300 million years ago, a supercontinent called Pangaea formed. As Pangaea separated, Africa, South America, North America, and Europe left behind a distinctive pattern of fossils and rocks for geologists to decipher. The pieces of the puzzle left behind by Pangaea, from fossils to shorelines along the Atlantic Ocean, provided the first indication that the Earth's continents moved.
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