How did we discover that continents move?

After World War II, scientists took magnetic instruments built to hunt enemy submarines and towed them over the ocean floor. They expected nothing. Instead, they found that the seafloor was striped like a zebra. This is called magnetic striping. Magnetic striping means the rock of the ocean floor alternates, band after band, between two magnetic directions, in rows running parallel to the mid-ocean ridge. Why would rock have a magnetic direction at all? Seafloor basalt is full of a mineral called magnetite. Magnetite grains act like tiny compass needles. While magma is liquid, they swing to line up with the Earth's magnetic field, and when the magma cools into solid rock, they lock in place. So every piece of basalt remembers which way the field pointed the moment it hardened. Now the odd part. Some rock points the way a compass does today, and geologists call that normal polarity. Other rock points the opposite way, with its compass needle aimed south. That's reversed polarity. Since the rock records the field when it cooled, the only explanation is that the Earth's magnetic field has flipped back and forth through time. In 1963, two young British geologists, Frederick Vine and Drummond Matthews, put the pieces together. Magma rises at the ridge and hardens, recording the field as it does. Then the new crust moves outward, and fresh magma fills the gap and records the field again. Every time the field flips, a new stripe starts. The stripes match on both sides of the ridge because the floor spreads in both directions. Lawrence Morley, in Canada, reached the same idea. But a clever idea still needs dates. For that, scientists used an isotopic clock. An isotopic clock is a radioactive element that decays at a steady rate, so measuring what's left tells you a rock's age. The one that cracked this case was potassium-argon dating. Radioactive potassium-40 turns into argon-40, and half of it is gone after 1,310 million years. Allan Cox, Richard Doell, and Brent Dalrymple used potassium-argon dating on volcanic rocks on the continents, and they built a timeline of every magnetic reversal over the last 4 million years. Then in 1966, Vine and Matthews, and Morley working separately, laid that timeline beside the stripes on the seafloor. They matched. The clincher came in 1968. The research ship Glomar Challenger spent a year criss-crossing the Mid-Atlantic Ridge between South America and Africa, drilling core samples from the ocean floor. When the cores were dated, the rock at the ridge crest was brand new. And the farther the drill sat from the ridge, the older the rock got. That's Hess's conveyor belt, confirmed rock by rock. Hess lived until 1969, long enough to see his idea accepted. Wegener never did, but his moving continents turned out to be right all along. So the ocean floor works as a tape recorder. Magnetite is the recording head, and the magnetic stripes are the tape. Potassium-argon dating is how we read the tape's timestamps. And the oldest pieces of that tape, in places, are about 180 million years old.