Plate Tectonics: Moving Plates and the Evidence

0:00South America and Africa face each other across the Atlantic. Are they moving?

0:06Here is a slice from A to B. The rigid outer layer, the lithosphere, is the crust and top of the mantle.

0:14It is broken into seven or eight large tectonic plates and many small ones.

0:20They move a few centimetres a year, about as fast as fingernails grow.

0:25A common myth says plates float on a sea of melted rock.

0:30In fact, the mantle is almost all solid rock, hot enough to flow very slowly.

0:37Hot rock rises and cooler rock sinks in slow convection currents that help move the plates.

0:44Here, where the plates move apart, rising rock makes new sea floor at a mid-ocean ridge.

0:51Where an old, cold plate sinks into the mantle, it pulls the rest of the plate along.

0:57How do we know they move? Here are three kinds of evidence.

1:02First, slide South America back, and its coast fits Africa like a puzzle piece.

1:09Second, fossils of Mesosaurus, a small reptile that could not cross an ocean, are found in both.

1:16Rocks of the same type and age line up too.

1:20Third, rock cooling at the ridge records the direction of Earth's magnetic field.

1:26The field has flipped many times, so the sea floor has stripes, normal and reversed.

1:32Sea floor spreads both ways from the ridge, so the two sides mirror each other.

1:38Plates move a few centimetres a year, and coasts, fossils, rocks and stripes all show it.