What is plate tectonics? It is one of the many questions that you’ll tackle early in your geography/geology lessons. In layman’s terms, plate tectonics is a scientific theory that describes the movements of the earth’s outer shell over its subsequent layer.
Earth’s outer shell, known as the lithosphere, is rigid and believed to be about 100 km thick. It is composed of the crust (both oceanic and continental) and the uppermost layer of the mantle.
Below the lithosphere is the asthenosphere, a viscous and mostly malleable layer of the mantle that allows the solid layer on top to slide and glide. It is located between 80-200 km below the earth’s surface. The nature and mechanism of this movement is still an active field of research.
History of Plate Tectonic Theory
The theory of plate tectonics is a modern, much-refined version of Alfred Wegener’s famous continental drift hypothesis, which he presented in 1912. He theorized that all continents were once part of the single landmass (which he named Pangaea) before disintegrating and taking up their current shape. Wegener, however, failed to provide a plausible explanation of how the massive continents could move.
Animation of the continental drift in the past 250 million years | USGS
Researchers started to notice the resemblance between shapes of the continents on each side of the Atlantic Ocean for the first time in the 16th century. Several prominent geographers, in the 17th and 18th century, noted that the continents of Africa and South America seems to fit closely together.
Several theories were put forward to explain such phenomena, but none of them were credible enough. Wegener’s continental drift theory also faced criticism and was even outright rejected by several geoscientists.
It was only in the 1960s, after direct seismological evidence of seafloor spreading, that the scientific community accepted the plate tectonics (and eventually continental drift theory).
What Is a Tectonic Plate? And How Many Are There?
Major and some minor tectonic plates
A tectonic plate is a massive, irregularly shaped chunk of the lithosphere composed of the crust and the uppermost layer of the mantle. Geologists have identified several tectonic plates, which are subdivided into three basic categories; major, minor, and micro(plates).
There is a total of eight major tectonic plates including, Pacific, North American, South American, Eurasian, African, Antarctic, Australian, and Indian plate. Plates that have an area greater than 20 million km2 are classified as major plates. There are fifteen minor plates and numerous known microplates.
Tectonic plates interact with each other repeatedly, and the location where they do is called plate boundaries. Based on the nature of this interaction, plate boundaries can be classified into three types; divergent, convergent, and transform.
A divergent boundary is where two opposing lithospheric plates move away from each other, leaving a gap in their wake. This gap is filled by the magma that rises from within the earth’s mantle.
The best example of the divergent boundary is the mid-oceanic ridge, where tectonic plates gradually keep moving away from each other while the rising magma continuously creates a new crust.
A convergent boundary, on the other hand, is where one lithospheric plate subsides under the other. These regions are also known as subduction zones and experiences frequent earthquakes and volcanic eruptions.
The third type of plate boundary is a transform fault, where plates slide against each other horizontally. While most of the transform faults are under oceans, few are observed on land, such as California’s San Andreas Fault.
Other examples of transform boundary include Chaman Fault in Pakistan, North Anatolian Fault in Turkey, and Queen Charlotte Fault in the United States.
How Does It work?
So how the plate tectonics work? Or, more specifically, what makes the massive tectonic plates move around the planet? The answer is two-fold. First is some sort of mantle convection (it is still unclear), and the second is related to gravity.
Convection in the mantle
Mantle convection is a process in which heat from the earth’s interior is slowly transferred to the surface by convection currents. It guides plate tectonics on earth through pulling (sinking) and pushing (spreading).
The hot lava rises at the mid-oceanic ridges, and cold, relatively dense oceanic lithosphere sinks deep into the mantle at subduction zones. For a long time, this process is believed to be the leading force causing the tectonic plates to move.
Geoscientists, however, now believe that gravity plays a much important role in the plate tectonics than earlier thought. The new crust, which forms at mid-oceanic ridges, is considerably less dense than the asthenosphere. It gradually moves away from the divergent boundary and gets cooler (through conductive cooling) as well as denser. The higher density of oceanic lithosphere, compared to that of the asthenosphere, allows it to subside deep into the mantle at the subduction zones.
The mechanism, which allows the new crust to move away from the mid-oceanic ridges slowly, is known as gravitational sliding (commonly known as ridge push). As the new oceanic lithosphere is formed near the ridge, gravity forces it to slide down and push older materials to move further away from the ridge.
Read: Earth’s Mantle Is NOT ‘Magnetically Dead’ | New Observations
Tectonic Activity In the Past
The oldest piece of continental crust found on earth is about 4.02 billion years old (the age of earth itself is 4.54 billion years). However, since the oceanic lithosphere is continuously recycled, the earliest known seafloor is just about 340 million years old. It was discovered in the part of the eastern Mediterranean Sea.
Researchers believe that the tectonic activity first started on earth about 3-3.5 billion years ago, based on the ancient rocks and minerals recovered from around the globe. Continents have been here for most of the earth’s history; however, they are likely to have gone through several configurations before reaching the shape they are in today.
A significant amount of studies have been done to reconstruct the history of plate tectonics on earth. The continuous (albeit slow) movement of the tectonic plates allows continents to form and break over time. This includes the eventual formation (and breakup) of a supercontinent, a single mass of land that contains all the continents.
The first supercontinent was believed to be formed as early as 2 billion years ago and broke up around 1.5 years ago or so. It is named Columbia or Nuna.
Supercontinent Columbia (rendition) | Image Courtesy: Wikimedia Commons
The (possibly) next supercontinent, Rodinia, was formed 1 billion years ago and then ruptured sometime around 600 million years ago so. Pangaea, the most recent supercontinent, was established about 300 million years ago during the late Paleozoic era.
When Pangaea broke apart nearly 175 million years ago, it was split into two large sections; Proto-Laurasia and Proto-Gondwana, while both were separated by Tethys Ocean.
Laurasia became what we now know as Europe, Asia, and North America, while Gondwana became the rest of the world that includes the Indian Subcontinent, Africa, South America, Arabia, and Australia and Antarctica.
Read: Earth’s Backup? Can We Really Do It?
Their Role In Earth’s Climate
A number of studies conducted by astrobiologists and geologists have revealed that plate tectonics could be essential to maintain life on earth as it is. Without the recycling of its crust, we might not have a stable temperature on the surface. Without subduction and creation of new crust, earth’s oceans might have remained deprived of life-giving nutrients. A study conducted in 2015 even argues that plate tectonics is essential for the evolution of advanced species.