Continents cruise in the slow lane.

Moving just millimeters at a time, it took the ancient supercontinent Pangea hundreds of millions of years to break apart into today’s landmasses. But a study published Tuesday shows that the journey wasn’t always a leisurely drive. When under extreme strain, the tectonic plates hit the throttle and accelerated to speeds 20 times faster than they were traveling before.

“It’s the equivalent of moving around as a pedestrian to moving around in a very fast BMW,” said Dietmar Muller, a geophysicist at the University of Sydney and an author of the paper, which appeared in Nature. “While the continental crust was still being stretched, all of a sudden there was this amazing acceleration, and we didn’t know why.”

After analyzing seismic data from across the world and building a model, Dr. Muller and his team discovered that plates move in two distinct phases: a slow phase and a fast one. During the slow phase, the continental crusts, which can be more than 20 miles thick, are stretched out little by little while remaining connected. But then suddenly, one or both of the continents step on the gas pedal.

“A critical point is reached when the connection between the two continents becomes so weak it can no longer resist the forces trying to pull it apart,” Dr. Müller said. “This acceleration is directly related to the thinning of the crust.”