The breakup of the Pangaea supercontinent.
Credit: U.S. Geological Survey
About 300 million years ago, Earth didn't have seven continents, but instead one massive supercontinent called Pangaea, which was surrounded by a single ocean called Panthalassa.
The explanation for Pangaea's formation ushered in the modern theory of plate tectonics, which posits that the Earth's outer shell is broken up into several plates that slide over Earth's rocky shell, the mantle.
Over the course of the planet's 3.5 billion-year history, several super continents have formed and broken up, a result of churning and circulation in the Earth's mantle, which makes up most of planet's volume. This breakup and formation of supercontinents has dramatically altered the planet's history.
"This is what's driven the entire evolution of the planet through time. This is the major backbeat of the planet," said Brendan Murphy, a geology professor at the St. Francis Xavier University, in Antigonish, Nova Scotia.
More than a century ago, the scientist Alfred Wegener proposed the notion of an ancient supercontinent, which he named Pangaea (sometimes spelled Pangea), after putting together several lines of evidence.
The first and most obvious was that the "continents fit together like a tongue and groove," something that was quite noticeable on any accurate map, Murphy said. Another telltale hint that Earth's continents were all one land mass comes from the geologic record. Coal deposits found in Pennsylvania have a similar composition to those spanning across Poland, Great Britain and Germany from the same time period. That indicates that North America and Europe must have once been a single landmass. And the orientation of magnetic minerals in geologic sediments reveals how Earth's magnetic poles migrated over geologic time, Murphy said.
In the fossil record, identical plants, such as the extinct seed fern Glossopteris, are found on now widely disparate continents. And mountain chains that now lie on different continents, such as the Appalachians in the United States and the Atlas Mountains in Morocco, were all part of the Central Pangaea Mountains, formed through the collision of the supercontinents Gondwana and Laurussia.
Pangaea formed through a gradual process spanning a few hundred million years. Beginning about 480 million years ago, a continent called Laurentia, which includes parts of North America, merged with several other micro-continents to form Euramerica. Euramerica eventually collided with Gondwana, another supercontinent that included Africa, Australia, South America and the Indian subcontinent.
About 200 million years ago, the supercontinent began to break up. Gondwana (what is now Africa, South America, Antarctica, India and Australia) first split from Laurasia (Eurasia and North America). Then about 150 million years ago, Gondwana broke up. India peeled off from Antarctica, and Africa and South America rifted, according to a 1970 article in the Journal of Geophysical Research. Around 60 million years ago, North America split off from Eurasia.
Life and climate
Having one massive landmass would have made for very different climactic cycles. For instance, the interior of the continent may have utterly dry, as it was locked behind massive mountain chains that blocked all moisture or rainfall, Murphy said.
But the coal deposits found in the United States and Europe reveal that parts of the ancient supercontinent near the equator must have been a lush, tropical rainforest, similar to the Amazonian jungle, Murphy said. (Coal forms when dead plants and animals sink into swampy water, where pressure and water transform the material into peat, then coal.)
"The coal deposits are essentially telling us that there was plentiful life on land," Murphy told Live Science.
Pangaea existed for 100 million years, and during that time period several animals flourished, including the Traversodontidae, a family of plant-eating animals that includes the ancestors of mammals.
During the Permian period, insects such as beetles and dragonflies flourished. But the existence of Pangaea overlapped with the worst mass extinction in history, the Permian-Triassic (P-TR) extinction event. Also called the Great Dying, it occurred around 252 million years ago and caused most species on Earth to go extinct. The early Triassic period saw the rise of archosaurs, a group of animals that eventually gave rise to crocodiles and birds, and a plethora of reptiles. And about 230 million years ago some of the earliest dinosaurs emerged on Pangaea, including theropods, largely carnivorous dinosaurs that mostly had air-filled bones and feathers similar to birds.
Cycle in history
The current configuration of continents is unlikely to be the last. Supercontinents have formed several times in Earth's history, only to be split off into new continents. Right now for instance, Australia is inching toward Asia, and the eastern portion of Africa is slowly peeling off from the rest of the continent.
Geologists have noticed that there is a quasi-regular cycle in which supercontinents form and break up every 300 to 400 million years, but exactly why is a mystery, Murphy said. But most scientists believe that the supercontinent cycle is largely driven by circulation dynamics in the mantle, according to a 2010 article in the Journal of Geodynamics.
Beyond that, the details get fuzzy. While the heat formed in the mantle likely comes from the radioactive decay of unstable elements, such as uranium, scientists don't agree on whether there are mini-pockets of heat flow within the mantle, or if the entire shell is one big heat conveyor belt, Murphy said.
- ClassZone: Animation of the breakup of Pangaea
- U.S. Geological Survey: This Dynamic Earth: The Story of Plate Tectonics
- Cornell University: Move the Continents! Interactive puzzle