What are Earth's plate tectonics? All you need to know about the three major boundaries
- Alfred Wagner proposed a theory in 1912 that paved the way for plate tectonics. Wagner's concept of continental drift was used to explain what happened when the supercontinent of Pangaea broke up some 200 million years ago.
- There are thought to be 7 major and 8 minor tectonic plates, and they move relative to one another, creating plate boundaries.
- This movement is responsible for many different geological formations, including the East African Rift, the San Andreas Fault in California, and the Himalayan mountain range in Asia.
Have you ever wondered why the Earth has giant mountains? How do they form? And why some of them are volcanic? Why do earthquakes occur? And why do we have such large and deep oceans?
Well, the answer to these geological curiosities began with German meteorologist and geophysicist Alfred Wagner, who proposed a theory in 1912 to help explain why the continents of South America and Africa looked like they would fit together and why similar fossils were found on continents separated by oceans.
Wagner hypothesized that the modern continents had once all been part of a supercontinent he called Pangaea (meaning "all lands") and that, over millions of years, the continents had drifted apart. However, Wagner did not know what drove this continental drift, and his theory was slow to gain traction.
However, in the 1960s, instruments like seismometers and magnetometers allowed researchers to make observations that were consistent with Wagner's theory of continental drift and led to the emergence of the theory of plate tectonics to explain how the continents drifted over time.
What is the theory of plate tectonics?
In plate tectonics, the Earth's lithosphere, the outermost layer composed of the crust and upper mantle, is divided into sizable rocky plates. These plates rest on the asthenosphere, a semi-molten sheet of rock.
The asthenosphere is a thick and sticky (viscous) layer beneath Earth's lithosphere. It is made malleable by heat from deep within the Earth, allowing the lithosphere above to move. Essentially, it works as though the asthenosphere is lubricating the undersides of Earth's lithosphere.
Due to the convection of the asthenosphere and lithosphere, the plates move relative to one another at varying rates. This movement ranges from two to 15 centimeters (one to six inches) per year.
Many different geological formations, including the East African Rift, the San Andreas Fault in California, and the Himalayan mountain range in Asia, result from this interaction of tectonic plates.
How many tectonic plates are there on Earth?
There are generally considered to be seven "major" plates, based on how they are classified: African, Antarctic, Eurasian, North American, South American, Pacific, and Indo-Australian. There are also eight minor plates making the total of all its tectonic plates 15.
However, according to an article published in Nature in 2012, earthquakes in recent years indicate that the Indo-Australian plate has cracked over the last 10 million years. Scientists believe that this separation could increase the number of Earth's major plates to eight.
There are also a number of smaller plates, including the Arabian, Caribbean, Juan de Fuca, Cocos, Nazca, Philippine, and Scotia plates. Today, data sets from remote sensing satellites calibrated with ground station measurements are used to determine how the tectonic plates are currently moving.
What are the types of plate boundaries?
A tectonic plate boundary is the separation of two plates. The tectonic plates move gradually and constantly but in a variety of directions. Some are moving toward each other, while others are moving apart.
The type of plate boundary is determined by the relative motion of the plates where they meet. There are three types: convergent, divergent, or transform. Boundaries may also be a mixed type, defined by how the plates move relative to each other.
Earth scientists can tell the relative movement of tectonic plates by looking at data from geological structures called faults. The formation of these boundaries triggers unique geological features and various surface phenomena. Still, the full nature of these remains unclear.
Divergent plate boundary
This is the movement of two plates away from each other. Molten mantle rock erupts along the opening, forming a new crust. The earthquakes that occur along these zones, known as spreading centers, tend to be shallow and relatively small.
Divergent plate motion formed the Great Rift Valley in Africa, the Red Sea, and the Gulf of Aden.
Convergent plate boundary
This happens when two plates move in the same direction and collide. Whenever a continental plate collides with an oceanic plate, the oceanic Plate sinks beneath the thicker, more rigid continental Plate. This is known as subduction.
Deep ocean trenches, such as the Peru-Chile Trench that runs along the west coast of South America, can form due to subduction. The rocks dragged down beneath the continent start to melt. Occasionally molten rock rises to the surface and forms a row of volcanoes.
Once two oceanic plates collide, a deep trench may be formed. For example, the Puerto Rico trench formed by the North American Plate and the Caribbean Plate smashing into each other and is well known to be the deepest part of the Atlantic Ocean and the Caribbean Sea. Another, called the Mariana Trench in the North Pacific Ocean, is thought to contain the lowest point on Earth, the Challenger Deep, which is 36,070 feet below sea level.
These collisions can also result in underwater volcanoes. Additionally, nearly 80 percent of earthquakes happen at convergent boundaries, where plates are forced into one another.
Transform plate boundary
A fault along a plate boundary where the motion is primarily horizontal is referred to as a transform fault or transform boundary.
It happens when two lithospheric plates slide, or more precisely, crush past each other along transform faults. During this tectonic movement, plates are neither formed nor destroyed.
At a transform plate boundary, the two plates' relative motion is either sinistral (left side toward the observer) or dextral (right side toward the observer). But don't worry if this is difficult to grasp, as geologists conduct field studies for years before comprehending what this actually means.
Transform faults occur in the area of a spreading center. On continents, they tend to be wide zones of deformation. A transform fault can also be the site of powerful earthquakes. The San Andreas Fault in California is an example of a dextral motion transform boundary.
Plate boundary zones
Plate boundary zones form when the impacts of interactions are ambiguous. In this case, the boundaries, which typically occur along a broad belt, are not clearly defined and can exhibit a variety of movements in multiple episodes.
Driving source of plate tectonics
It is widely assumed that tectonic plates can move due to the relative density of the lithosphere and the relative weakness of the asthenosphere.
It was once thought that mantle convection alone is what drives plate movement. Hot material near the core of the Earth rises, while colder mantle rock sinks. Just like a pot boiling on a stove. As a result, the increased density of the oceanic lithosphere sinking in subduction zones is a powerful source of plate motion.
However, the process is now understood to be a lot more complex. Convection in the mantle, certainly plays a role, but does not tell the entire story. The current consensus, though still debated, models a dynamic system in which plates move as part of a gravity-driven convection system that pushes young hot plates away from spreading ridges and pulls old cold plates down into subduction zones.
Why do some volcanoes form far away from plate boundaries?
The plate tectonics theory had one pressing question: Most volcanoes form above subduction zones, but some form far away from these plate boundaries. How might this be accounted for?
John Tuzo Wilson, a Canadian geologist, finally answered this question in 1963. He proposed that fixed "hot spots" in the mantle create volcanic island chains like the Hawaiian Islands and areas of volcanic activity like Yellowstone.
Magma forces its way upward through the moving plate of the sea floor at those locations. One volcanic island or area after another is formed as the plate moves over the hot spot. Wilson's justification provided additional support for plate tectonics. The theory is now essentially accepted universally.
Could Earth's tectonics boil down to a Mars-like state?
There is no greater geological artist than Earth's plate tectonics. We have mountains and oceans, terrifying earthquakes, incandescent volcanic eruptions, and new land being born every second due to this ongoing activity.
However, nothing goes on forever. In fact, some scientists believe that although Earth will continue to have an atmosphere, mountain formation will eventually cease. Extreme erosion by wind and waves will reduce the mighty peaks to hilly plateaus, and much of the flattened continents will eventually be submerged. Speaking of wind erosion and no active plate tectonics, doesn't Mars come to mind?
While occasional earthquakes could still occur, truly catastrophic quakes greater than magnitude seven or so will be relegated to history. The mantle will eventually cool to the point where this planet-wide conveyor system will come to a halt.
At that point, assuming homo sapiens will still be around, you can bid adieu to the carbon cycle as well as the ongoing reshaping and shifting of landmasses, which have served as major forces in evolution over eons.
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