The Earth's surface is made of large slabs called plates. They move faster than your fingernails grow, and life on Earth might not exist without them.
The ingredients for life to exist on our planet are liquid water, oxygen, and … plate tectonics?
What is plate tectonics?
Looking at a map of the world, the coastlines of certain continents appear to complement each other. If we could just nudge them a little closer, they look like they could lock together like puzzle pieces. This isn’t just a trick of the eye! Geologists now think that North America, Africa, South America, and Europe were once one continuous landmass or one big continent called Pangaea.
Pangaea eventually split into pieces sometime around 230 million years ago. Those pieces spread out to form the continents we know today. Geophysicist Alfred Wegener first proposed this theory of continental drift in the early 1900s, but at the time his idea didn’t gain much traction—people thought it was too strange. After all, the ground beneath our feet is rock solid and steady, right?
Evidence began to grow to support the idea of continental drift. Fossils from the same plants and animals were found on both sides of Earth's oceans, suggesting that those distant coastlines were once connected. Similarly, the age and types of rock matched up.
Then in the 1950s, geologists discovered the mid-Atlantic ridge, a mountain range that runs along the ocean floor. The ridge is the largest geological feature on the planet and has been classified as a World Heritage Site. Most of the ridge sits far below the depths of the ocean. But in a few spots—including Norway, the UK, and Brazil—the mountains pop up above the surface of the water to form island chains.
When studying the seafloor around the ridge, geologists learned that the farther from the ridge they looked, the older the seafloor. In 1962, geologist Harry Hess proposed that this age differential arose due to seafloor spreading. At the ridge, rock was being forced up from the Earth’s interior forming new a seafloor, which effectively pushed the older layers out of the way.
The theory of plate tectonics brings all of this evidence together. The Earth’s core is far hotter than it is at the surface. In fact, at roughly 6000 degrees Celsius, the center of our planet is about as hot as the surface of the Sun. All of this energy pushes outward from the core and puts pressure on the layers of rock above like the mantle that sits just below the crust or outer layer.
When more interior layers of the Earth’s mantle heat up, they become less dense and rise closer to the Earth’s surface. There they can cool off and move back down again in a process called convection. (Yes, that’s the same kind of convection that happens with the air in your oven.)
At the outermost surface of the Earth sits the lithosphere, a layer of rock that is far more rigid than the mantle. But the lithosphere isn’t solid—it has cracks, which break it up into plates. So all of this motion in the mantle moves around the plates that make up the Earth’s lithosphere, pulling the continents along with them.
The plates move at a speed of around two inches per year, which is actually faster than the growth of your fingernails (about 1.6 inches or 41 millimeters per year).
The plates move at a speed of around two inches per year, which is actually faster than the growth of your fingernails (about 1.6 inches or 41 millimeters per year). We know this because scientists at NASA track the plates' movements down to millimeter precision using satellite laser imaging.
So plate tectonics describes the constant movement and change in the Earth’s surface. Spots like the mid-Atlantic ridge mark the boundary between plates. Earthquakes occur when the pressure builds up and these plates collide or slide along each other. Volcanic eruptions are related to tectonic shifting. Sometime around half a billion years ago, a plate collision began forming the Himalayas.
Do we need plate tectonics?
But all of this movement goes beyond producing the occasional earthquake or volcanic eruption—it drives constant change at the Earth’s surface. Growing research suggests that this renewal might be a necessary ingredient for life to exist.
The movement of plate tectonics drives constant change at the Earth’s surface. Growing research suggests that this renewal might be a necessary ingredient for life to exist.
Plate tectonics affect the carbon cycle, help regulate the atmosphere, and thus ultimately contribute to keeping our planet's surface at a comfortable not-too-hot and not-too-cold temperature.
The Sun didn't always put out as much heat as it does now. Around 2 billion years ago our planet would have been much colder and eruptions from volcanoes—thanks to plate tectonics—may have added enough carbon dioxide into the air to trap enough heat to raise the temperature at the surface. Now that the Sun, and thus the Earth, has heated up, rainfall can transfer carbon dioxide from the atmosphere to the oceans. There, it sinks into the mantle, thanks to tectonic plates that attempt to slide underneath other plates, a process called subduction.
Plate tectonics also stir up the chemistry of the ocean floor, giving rise to the diverse forms of ocean life, including those we count on for oxygen production in the atmosphere.
Do other planets show signs of plate tectonics?
As our planet ages, it will cool down, bringing all of this motion to a halt.
Some planetary scientists think this cooling is what happened to Mars. Our planetary neighbor may have been set on the same evolutionary path as Earth, but it may have also cooled off faster because it's smaller. There's an outside chance the Red Planet once had moving plates as well—it does have the largest volcano in our solar system, Olympus Mons—but its surface has been inactive for more than 4 billion years.
Mercury also shows some step-like features in its terrain that may indicate past tectonic activity.
But we don’t yet know how Earth’s plates start moving and when, and without these details of their origin story, we can’t be sure how likely other planets are to host plate tectonics. A planet’s ability to support plate tectonics depends in large part on the elements that make up that planet. In particular, planets with more silicon and sodium are less likely to be able to sustain tectonic activity like what we see here on Earth.
Scientists have estimated there could be as many as 40 billion Earth-like exoplanets out there in our galaxy. That means a possible 13 billion planets hosting plate tectonics.
By looking at the compositions of stars, and translating those stellar elemental abundances to the presumed abundances of their orbiting planets, one recent study found that only one-third of extrasolar planets could host plate tectonics, at least long enough for complex life to evolve.
One third may still be a lot. Based on a statistical analysis of the Earth-like exoplanets found by Kepler, NASA scientists have estimated there could be as many as 40 billion Earth-like exoplanets out there in our galaxy. That means a possible 13 billion planets hosting plate tectonics. We don’t yet have the ability to know for sure. So for now our planet Earth is still a rare gem.