What is the Theory of Plate Tectonics?

Our familiar continents didn’t always exist!

From Pangea and Gondwana to Africa and Australia—how and why does the world keep changing?

 

If you have ever looked at a world map really carefully, you may have noticed that Africa and South America look like pieces of a jigsaw puzzle. The continents seem to fit together. Why is this the case?

Table of Contents

What drives plate tectonics?

When we stand on the Earth, it’s difficult to imagine that the continents and mountains are just a thin layer on the Earth’s surface. All land is just a part of the uppermost layer of the Earth, known as the crust. If the Earth is an apple, the crust is just as thin as the apple’s skin. Below this skin exists a layer known as the asthenosphere. The asthenosphere is about 10% liquid magma, the rest being heated rock with the consistency of Play-Doh. which is heated up by the hot, solid inner core.

The crust and the asthenosphere are the two outermost layers of the Earth
The crust and the asthenosphere are the two outermost layers of the Earth, Credit: Wikimedia/Kelvinsong

If you’ve ever heated a pot of water from above, you might notice something funny. The top of the pot of water gets very hot, but the bottom stays relatively cool. This is due to a phenomenon known as convection. Warm water is ever-so-slightly less dense than cool water, and so it rises up, while the cool water sinks down.

The movement of warm and cold water molecules in a pot of heated water
The movement of warm and cold water molecules in a pot of heated water, Credit: 123rf.com/normaals

The mantle is a liquid, and it has convection currents too! The core is like a stove, slowly heating the mantle. Imagine covering a pot of milk with hot cocoa powder. As you heat up the milk, it bubbles up into the layer of cocoa powder and pushes it around. The crust is not one continuous layer, and is instead broken up into smaller sections known as tectonic plates. The movement of the mantle through convection currents pushes these tectonic plates around. There are two kinds of plates: oceanic and continental. Oceanic plates are made of heavy, dense rocks, and continental plates are much lighter.

Some of the main plates that make up the Earth’s crust
Some of the main plates that make up the Earth’s crust, Credit: Wikimedia/Eric Gaba (Sting - fr:Sting)

It took a long time for humans to figure out the theory of plate tectonics! The first person to propose the theory was Alfred Wegener, in 1912. He found similar fossils on the coasts of Africa and South America, and noticed that the continents’ coastlines fitted into each other like puzzle pieces. Wegener was also the first to propose the existence of Pangea, a supercontinent which had all the landmasses we know today put together. Unfortunately, the scientists of his time did not believe the theory, and it was not well accepted.

Alfred Wegener’s sketch of Pangea
Alfred Wegener’s sketch of Pangea, Credit: Wikimedia/Von Alfred Wegener erstellte Karte

How do plate tectonics cause earthquakes?

Plates usually move incredibly slowly—slower even than your fingernails grow. Some move up to four inches (ten centimeters) a year, but most are much slower than this. Even though they move so slowly, it’s important to remember that they are massive! A huge amount of pressure can build up, and then suddenly release. When plates move parallelly, they generate friction and pressure, which releases in the form of an earthquake. Earthquakes can occur at non-continental plate boundaries too, but those are less common.

Two tectonic plates moving parallel to each other
Two tectonic plates moving parallel to each other, Credit: Wikimedia/domdomegg

One example is the famous San Andreas fault in California, between the Pacific plate and North American plate. It has been the cause of many earthquakes, though seismologists have gotten better at predicting when earthquakes can take place.

How do mountains form?

There are two main kinds of plate movements that can cause geological formations. The first is a Convergent plate boundary, where plates move towards each other. The second is a Divergent plate boundary, where plates move apart. Mountains form at convergent boundaries between continental plates. Continental plates are the same weight, so when they push against each other, neither gives! Instead, both rise. The most spectacular example is the Himalayas, the tallest mountain range in the world. The Himalayas are caused by the Indian plate and the Eurasian plate pushing against each other. The Himalayas formed 30 million years ago, and they are still forming. Each year, the mountains grow taller and taller as the Eurasian and Indian plates continue their journey towards each other.

Mountains forming at a continental–continental plate boundary
Mountains forming at a continental–continental plate boundary, Credit: Wikimedia/domdomegg

Where do volcanoes form?

Volcanoes can form in four main ways, but the three most common ways involve an oceanic plate. Oceanic plates are very dense, so at a convergent boundary, a continental plate coming into contact with an oceanic plate ends up getting pushed down. When one plate gets pushed beneath another, magma bubbles to the surface, forming a volcano! These can form both between oceanic–oceanic plate boundaries and oceanic–continental plate boundaries.

A volcano forming due to one plate being pushed beneath another
A volcano forming due to one plate being pushed beneath another, Credit: Wikimedia/domdomegg

Volcanoes can also form at Divergent boundaries. When plates move away from each other, the mantle has space to spread upwards. The bubbling magma forms new volcanoes.

 

Plate tectonics can explain so much of the world around us, from mountains to volcanoes and earthquakes! Are any geological features in your region formed due to the movement of tectonic plates?

Glossary

Asthenosphere: The hot liquid mantle beneath the crust

 

Convection: The movement of hot molecules through a liquid medium when being heated

 

Convergent Plate Boundary: A boundary where two plates move towards each other

 

Divergent Plate Boundary: A boundary where two plates move away from each other

 

Seismologist: A scientist that studies the movement of the tectonic plates

 

Tectonic Plate: Sections of the Earth’s crust, that can be continental or oceanic

Flesch Kincaid Grade Level: 8.1

 

Flesch Kincaid Reading Ease: 57

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  • Yamini Srikanth

    Yamini's (he/they) interests lie in environmental education, science communication and trying to build a better world. When not languishing in front of his laptop, they can be found outside, poking at any insect, bird or plant. They love making science accessible, especially to those who aren't encouraged to pursue it. Yamini hopes that the young women who read Smore love learning from their articles and get just a little bit more excited about science!