How are you at puzzles? Did you know you are living on one? Earth’s crust-the thin, brittle surface of the planet is broken up into pieces like a giant jigsaw puzzle. There are about 20 or so of these pieces. The pieces are called tectonic plates. Every part of Earth’s surface, oceans and continents alike, rides on a tectonic plate.
The big thing about tectonic plates is that they move. That motion is slow, on average maybe an inch per year. But it adds up. If a plate moves just one inch each year, that’s about 15 miles every million years. And tectonic plates have been on the go for at least a billion years or more!
What causes the crust to break and shift so much? Earth’s crust varies from about 5 miles to 70 miles thick (it is much thicker under the continents than under the oceans). That may sound pretty thick, but it’s as thin and breakable as an eggshell compared to the deeper layers of the planet.
The layer directly beneath Earth’s thin crust is called the upper mantle. Geologists believe that the upper mantle has the consistency of very thick taffy or tar, and that it circulates in huge, slow currents. The currents tug at the pieces of crust above, causing those pieces, the tectonic plates, to constantly shift around.
On the Move
All this shifting around has really changed the face of Earth over time. In fact, once upon a time, most of Earth’s continents were clumped together into a couple of giant land masses. If you could travel back in time about 250 million years, you would find yourself on a very different planet Earth. Instead of the seven continents we recognize now, there were just two super-sized ones.
One was called Gondwanaland. This would later break apart into the continents of South America, Africa, Australia, Antarctica, plus the country of India. Just north of Gondwanaland was Laurasia. This would later pull apart to become North America, Europe, and Asia.
Together, these two continents are called Pangaea. Around them, there was just one big ocean called Panthalassa. Over millions of years, Gondwana and Laurasia moved and broke apart because of plate tectonics.
The action hasn’t stopped. Tectonic plates are still on the move. Their motion, and their interactions, account for a lot of what we experience and see on Earth’s crust.
Types of tectonic plate boundaries
As you might imagine, big things can happen where two tectonic plates in motion meet! Like bumper cars, plates interact in different ways as they move. They can pull away from each other, slide past each other, or collide. That makes for three broad types of boundaries between plates: divergent, transform, and convergent.
In divergent boundaries, plates move away from each other. When a divergent boundary occurs in an ocean, it creates a long volcanic fissure, or crack, called a mid-ocean ridge. As the plates pull apart, molten rock called magma wells up from the hot mantle through these cracks in the ocean floor. When the magma hits the ocean water, it cools and becomes a brand new part of the crust. That welling up also creates volcanoes, undersea and on land. Iceland sits right on top of a mid-ocean ridge in the Atlantic Ocean. That’s why Iceland is made of volcanoes, and why some of the newest rock on the planet can be found there.
When tectonic plates slide past each other, that is called a transform boundary. This is happening in California, where the North American Plate and the Pacific Plate are slowly moving past each other. That plate boundary, marked by faults including the famous San Andreas Fault, catches and drags and sometimes snaps. The sudden release cause earthquakes.
When two plates collide, it’s called a convergent plate boundary. Convergent boundaries can take several forms, because ocean plates are heavier than continental plates. If an ocean plate collides with a continental plate, the ocean plate slides underneath. That forms a subduction zone. As the ocean plate sinks, its heat rises up and forms volcanoes on the surface above. The volcanoes in the Cascade Mountains of Washington and Oregon formed this way as the Pacific Plate dives beneath the North America plate.
If both plates are made of continental crust, the plates can buckle like the bumper of two colliding cars. That can make for some really spectacular mountains! The collision between the Indian Plate and the Eurasian Plate began 50 million years ago. It created the Himalayas, the highest mountains in the world. The Indian Plate continues to move north, very slowly. As a result, the Himalayas continue to grow taller.
Shaping Our World
Earthquakes. Volcanoes. Deep rifts bubbling with hot magma. It’s no surprise that a lot of this action occurs along plate boundaries, as they pull apart, slide, and collide. Look at a map of Earth and you will see plenty of other places where plate tectonics has shaped the surface of the planet.
Have you ever noticed how South America and Africa seem to fit together like two pieces of a jigsaw puzzle? Or that there seems to be a line, or rift, that extends all the way from Israel to Africa? You guessed it: more evidence of tectonic plates on the go. Every day, over hundreds of millions of years, plate tectonics constantly shapes and re-shapes our world.
Written by Laura McCamy
Edited by Beth Geiger, MS Geology
Illustrated by Renee Barthelemy