Plate Tectonics

The lithosphere, Earth's rigid outer shell, is cracked into plates that sit on the mantle below. The mantle is hot enough to flow slowly, and it does so in convection cells: hot rock rises, spreads sideways, cools, and sinks back down. Where mantle rock rises at a mid-ocean ridge, it drags the plates on either side apart and new crust forms in the gap. Where the flow turns downward, it pulls a plate down into a trench. This is why South America and Africa fit together: they were once joined and have been spreading apart from a ridge in the middle of the Atlantic for millions of years.

At a ridge, the new rock cools while Earth's magnetic field is doing its periodic flips, so the rock locks in alternating bands of magnetic orientation. These stripes are symmetric on both sides of the ridge, which is the direct evidence that the seafloor is spreading, and their width tells you how fast. Where an oceanic plate meets a continental one, the oceanic plate is denser, so it subducts, sliding underneath. That bending and sinking creates a deep trench, generates earthquakes at depth, and melts rock as it descends, feeding a chain of volcanoes on the continent above.

Because ocean floor is constantly made at ridges and destroyed at trenches, no patch of seafloor survives for long. That is why the oldest ocean crust is only around 200 million years old, while continental rock, which does not get subducted, can be over 3000 million years old. This cycle also connects to earthquakes and volcanism as direct surface consequences of plate motion.