Circa 1915 – Fossils Point to a Supercontinent
Mesosaurus was a small reptile that swam in shallow coastal waters about 275 million years ago. It couldn't have crossed an ocean, yet Mesosaurus fossils are found in both South America and Africa. The two continents must have once been separated by a much narrower sea than the present-day Atlantic.

	Mesosaurus, South Africa		Mesosaurus, Brazil

Circa 1915 – Fossils Point to a Supercontinent

Similarly, fossils of the land plant Glossopteris, all roughly 300 million years old, are found in Africa, South America, Australia, India, and Antarctica. From dispersed fossils like these, German meteorologist Alfred Wegener deduced that a single large land mass had broken up to form today's continents.

	Glossopteris, Australia		Glossopteris, South Africa

1960’s – Strange Stripes Discovered
on the Ocean Floor

The square area with black stripes was mapped in 1966 by analyzing the magnetic properties of sea-floor rocks. The black stripes record reversals of the Earth’s magnetic field. These parallel the mid-Atlantic Ridge and are symmetical on either side of it.

The magnetic "stripes" formed as the two plates slowly spread apart, and Earth’s north and south magnetic poles periodically reversed themselves.

1980s – Telescopes Track Plate Motion

Ten years of telescope-based measurements from Earth to distant quasars confirmed that the North American and Eurasian Plates are moving away from each other at a rate of 1.7 cm (0.7 in) per year. Geological studies of ocean-floor rocks suggest the plates have moved at this same average speed for tens of millions of years.

Two telescopes — one in Massachusetts, the other in Germany — measured their distance to a pulsing quasar at the same time. This measurement was repeated 803 times over 10 years. Each point on the graph indicates how far apart the two stations were at that time. As you can see, they slowly moved apart.