4.4 Earth History and Plate Tectonics

That map is sort of familiar, but what is it?

Wegener’s persistent search for evidence that the continents had been joined paid off. Scientists who came after him developed an understanding of seafloor spreading, which was the mechanism for Wegener’s continental drift. Geologists know that Wegener was right because the movements of continents explain so much about the geological activity we see.

The existence of Wegener’s supercontinent Pangaea is completely accepted by geologists today. But did it all begin with Pangaea? Or were there other supercontinents that came before?

Plate Tectonics Theory

First, let’s review plate tectonics theory. Plate tectonics theory explains why:

Earth’s geography has changed over time and continues to change today.
some places are prone to earthquakes while others are not.
certain regions may have deadly, mild, or no volcanic eruptions.
mountain ranges are located where they are.
many ore deposits are located where they are.
living and fossil species are found where they are.

Plate tectonic motions affect Earth’s rock cycle, climate, and the evolution of life.

Supercontinent Cycle

Remember that Wegener used the similarity of the mountains on the west and east sides of the Atlantic as evidence for his continental drift hypothesis. Those mountains rose at the convergent plate boundaries where the continents were smashing together to create Pangaea. As Pangaea came together about 300 million years ago, the continents were separated by an ocean where the Atlantic is now. The proto-Atlantic ocean shrank as the Pacific Ocean grew.

The Appalachian mountains of eastern North America formed at a convergent plate boundary as Pangaea came together (Figure below). About 200 million years ago, the they were probably as high as the Himalayas, but they have been weathered and eroded significantly since the breakup of Pangaea.

The Appalachian Mountains in New Hampshire.

Pangaea has been breaking apart since about 250 million years ago. Divergent plate boundaries formed within the continents to cause them to rift apart. The continents are still moving apart, since the Pacific is shrinking as the Atlantic is growing. If the continents continue in their current directions, they will come together to create a supercontinent on the other side of the planet in around 200 million years.

If you go back before Pangaea there were earlier supercontinents, such as Rodinia, which existed 750 million to 1.1 billion years ago, and Columbia, at 1.5 to 1.8 billion years ago. This supercontinent cycle is responsible for most of the geologic features that we see and many more that are long gone (Figure below).

Scientists think that the creation and breakup of a supercontinent takes place about every 500 million years. The supercontinent before Pangaea was Rodinia. A new continent will form as the Pacific ocean disappears.

This animation shows the movement of continents over the past 600 million years, beginning with the breakup of Rodinia:http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_plate_reconstruction_blakey.html.

Summary

Pangaea came together as a set of continent-continent convergent plate boundaries.
Pangaea is still breaking up as the continents move apart. The Atlantic Ocean is getting bigger and the Pacific Ocean is getting smaller.
Pangaea was not the first supercontinent and it won’t be the last. The continents come together and break apart about every 500 million years in a process called the supercontinent (or Wilson) cycle.