Tukuk-OSEarth’s rigid outer shell is broken like an eggshell into a dozen big and several small plates, each carrying a continent or an ocean floor — or both. Beneath, the hot mantle creeps in convection at fingernail speed, a few centimetres a year, and the plates ride it. Africa’s east is splitting (the East African Rift, future new ocean), the Atlantic is widening about 2.5 cm a year at its mid-ocean ridge, and the Pacific is shrinking because its edges are being consumed at trenches faster than new floor forms.
Where plates meet is where geology happens. They can slide past each other (California’s San Andreas), collide and crumple upward (the Himalaya, still rising centimetres a year as India drives into Asia), or one can dive beneath the other (subduction — the Andes, Japan, and most of the world’s explosive volcanoes). The famous “Ring of Fire” around the Pacific is simply the longest chain of these subduction seams, and about 80–90% of the world’s large earthquakes occur along it — the planet’s most honest map label.
The idea is younger than you would expect: continents proposed to have drifted as recently as 1912 (Alfred Wegener, widely mocked for lacking a mechanism), and the seafloor-spreading proof arrived only in the 1960s. Everything that fits now — matching coastlines, identical fossils on separated continents, the age stripes of ocean rock recording magnetic reversals — clicked into place in a single decade. Plate tectonics is biology’s evolution moment for Earth science: late, then sudden.