Lava cools and keeps a compass reading inside it. Much later, fresh lava on the same block of land records a different tilt. The rock has moved across the face of Earth.
Tiny magnetic grains line up with Earth's field as lava cools. Their tilt helps show latitude: how far north or south the rock sat. It cannot tell us east or west. You get a band on a map, with room for error, rather than a neat coastline.
In the Pilbara of Western Australia, old lava flows record a block shifting at least about as fast as fingernails grow. The chapter gives a minimum of 2.5 centimeters a year, about an inch. Slow enough to miss by eye. Fast enough to move a continent over deep time.
The land itself had to grow too. Water helps some buried rock melt; lighter melt can rise and add to a continent. Scientists still argue about how this worked on early Earth and when plates first began to move.
Old continents keep clues that the ocean floor often loses as it sinks and gets remade. But we cannot draw the whole vanished map. What survives is a compass mark, sealed in a cooled flow.
