Alfred Wegener, a German meteorologist, noticed that the coastlines of South America and Africa fit like puzzle pieces. Matching fossils, glacial deposits, and rock formations on opposite Atlantic shores convinced him: continents move.
His mechanism was wrong (he proposed continents plowing through ocean crust) and geophysicists derided him. He died in 1930 on a Greenland ice expedition, his theory still rejected.
Sonar surveys mapped a colossal mountain chain running down the middle of every ocean basin — the mid-ocean ridges. Then magnetometers pulled behind ships found something stranger: parallel stripes of normal and reversed magnetism, mirrored on either side of the ridge crest.
By the late 1960s, several lines of evidence converged into one unifying theory:
The lithosphere is broken into roughly fifteen plates. The seven major ones cover most of the surface; the minors sit at busy intersections.
Oceanic crust is thin (~7 km), dense, dark basalt — young (< 200 Ma).
Continental crust is thick (~35 km), light granite — ancient, with rocks up to 4 Ga old.
All the violence Earth offers above the weather — earthquakes, volcanoes, mountain belts, tsunamis — concentrates along these thin lines.
Oceanic crust is dense. When it meets continental crust, it bends and dives — typically at angles of 30 to 60 degrees — descending hundreds of kilometers into the mantle.
When two continental plates collide, neither will subduct — both are too buoyant. Instead the crust crumples and stacks, doubling its thickness and rising into mountains.
Most volcanism happens at plate boundaries. But Hawaii sits in the middle of the Pacific Plate. Yellowstone sits in the middle of North America. Why?
The leading explanation: mantle plumes — narrow columns of hot rock rising from deep in the mantle, perhaps from the core-mantle boundary 2,900 km down. The plate slides over the stationary plume, creating a chain of progressively older volcanoes.
Plates don't glide smoothly. They lock against each other, deform elastically, and then fail. The accumulated strain releases in seconds as a rupture propagates along the fault — that's an earthquake.
The San Andreas Fault is the textbook transform boundary, running ~1,200 km up California. The Pacific Plate slides northwest past the North American Plate at ~3-4 cm/yr. Locked sections store decades of strain, then release in M7+ events.
Plate motion runs in cycles. Roughly every 400-600 Myr the continents collect into a single supercontinent, then rift apart and disperse, only to reconvene on the far side of the globe.
If current motions continue, the continents will reassemble into a new supercontinent in roughly a quarter-billion years. Geologists have proposed several possible configurations:
A 2023 study suggests Pangaea Ultima would be hostile to mammals — a hot, dry interior with CO₂ levels driving surface temperatures past mammalian survival limits. Long after we're gone.