In the late 1500s, Flemish cartographer Abraham Ortelius noted that the continents on his map looked as though they fit together like a puzzle. He deduced that the continents may have been connected at one time, and had somehow moved. Observations such as this led to the initial concept of continental drift, a theory that the continents were once connected and have been slowly moving over time.

Pangaea

Alfred Wegener, a German meteorologist, scientist, and polar researcher, expanded on the theory of continental drift in 1912 by pointing out the similarities between rocks, geologic structures, and fossils across the Atlantic Ocean. He also noted that the shape of the coastline on the western side of Africa matched South America’s eastern coastline.

Based on his observations, Wegener proposed that a supercontinent composed of all of the current continents once existed. He called this supercontinent Pangaea, Greek for “all lands.” He argued that forces within Earth broke Pangaea into two continents, Laurasia and Gondwanaland. Over time, these two land masses divided even further into the continents we see today.

Despite evidence supporting continental drift, the idea of massive continents moving created heated debates. How did these land masses move apart?

Convection

In the early 1930s, British geologist Arthur Holmes thought about mechanisms inside Earth that could move continents on the surface. Holmes speculated that deep below the surface was a layer of molten rock. This molten rock was heated by the center of the planet. Through the process of convection, this heated rock would rise toward the surface. Near the surface it would cool and sink back down, producing convection currents. Holmes proposed that these convection currents were strong enough to move entire continents!

Unfortunately, the technology at the time limited Holmes' ability to investigate his theory. To investigate further, scientists needed to develop a way to look deep into Earth.