“GSA Today”  (2006) v. 16, no. 2, p. 4-11

In “Tectonic inheritance at a continental margin,” William A. Thomas discusses the theory of plate tectonics, which includes the cyclic assembly of supercontinents (when all continental crust comes together to make one giant continent) followed by breaking apart of the supercontinent to form scattered continents and ocean basins as exist today. 

The rocks now in eastern North America record two complete cycles each of assembly and breakup of supercontinents: (1) the assembly of the supercontinent Rodinia approximately one billion years ago; (2) the breakup of Rodinia and opening of the Iapetus Ocean (ancient counterpart of the Atlantic Ocean) along the eastern margin of the Laurentian continent (ancient counterpart of the North American continent) approximately 540 million years ago; (3) the closing of the ocean and assembly of the supercontinent Pangaea approximately 270 million years ago; and (4) the breakup of Pangaea and opening of the Atlantic Ocean, beginning approximately 230 million years ago. The assembly of Pangaea included collisions between continents, which produced the Appalachian Mountains and distorted the record of the earlier events, requiring careful geologic work to decipher the inheritance of geologic structures from one cycle to the next.  Nevertheless, the record in the rocks tell the story of two supercontinents and two ocean basins where the Atlantic Ocean now is, as the planet continues to change.

William A. Thomas is the James S. Hudnall Professor of Geology in the Department of Earth and Environmental Sciences in the College of Arts and Sciences at the University of Kentucky.  He received bachelor’s and master’s degrees in geology from the University of Kentucky, and his doctoral degree in geology from the Virginia Polytechnic Institute. He was president of The Geological Society of America in 2004-2006, and the article, “Tectonic inheritance at a continental margin,” is published from his 2005 Presidential address.