Pangaea

Pangaea was a supercontinent that existed between 270 and 200 million years ago. It contained most of the world's current continents in a single landmass. Supercontinents form because of tectonic forces within Earth that lead to the movement of continents and the expansion of oceans. Pangaea was the birthplace of the dinosaurs and other species that later colonized the entire planet.

The Pangaea Theory

In the early twentieth century, scientists determined that the continents of the world were not always in their present state. They were once united in a series of much larger landmasses known as supercontinents. Paleontologists and geologists have given the name Pangaea, derived from the Greek words meaning “all land,” to the supercontinent that existed during the Paleozoic and Mesozoic periods, between 270 and 200 million years ago, and that contained most of the world's current continents in a single landmass. German meteorologist Alfred Wegener was the first to suggest the existence of Pangaea in his 1915 book Die Entstehung der Kontinente und Ozeane (The Origin of the Continents and Oceans, 1922).

The Pangaea theory was controversial, but the scientific community now accepts it. It has been supported by evidence from many different branches of research. The Pangaea theory helps scientists understand the distribution of fossils and living species. When the continents were joined to form Pangaea, species could migrate throughout most of the globe. This ancient connection between continents is made clear by today's global distribution of species.

and the Formation of Supercontinents

The Pangaea hypothesis was part of Wegener's theory of continental drift, which held that the continents were in a state of constant motion and drifted across the ocean's surface in response to forces within Earth's core. The theory was based on evidence from early-twentieth-century paleontological and geological research.

One of the most prominent and compelling pieces of evidence for Pangaea is that the shapes of some continents—notably Africa, South America, and Australia—appear to be complementary, like puzzle pieces. Additionally, fossils of the same species and rocks of the same age and type are often found on continents separated by ocean basins. Before the arrival of the idea of continental drift, geologists and paleontologists could not explain the distribution of these fossil remains. Additional evidence came from closely examining the structure of intercontinental mountain passages and irregularities in Earth's magnetic polarity.

Wegener's continental drift hypothesis was the subject of intense debate until the 1950s, when new evidence derived from studies of the ocean floor bolstered the idea that the continents could move. The theory of seafloor spreading, proposed by meteorologist Harry Hess in 1962, holds that the ocean surface is in a constant cycle of recycling, as sediment is pulled into ocean trenches while new ocean sediment is released from trenches in other locations. Portions of Earth's floor are sucked into deep ocean trenches, creating a force called subduction, in which Earth's continents are pulled toward active trenches. The discovery of seafloor spreading, subduction, and related processes led to the science of plate tectonics, which studies the movement and formation of the continents.

According to plate tectonics, each continent is associated with a stable portion of Earth's lithosphere called a craton. As the sea floor moves in response to forces from Earth's core, the cratons are pushed or pulled on currents of moving earth, thereby shifting the position of the attached continents on Earth's surface. A supercontinent is a landmass that contains two or more of Earth's cratons in a single structure.

Pangaea was composed of all Earth's cratons, except those containing portions of what is now China, which existed during that time as a separate island continent. Geologists now believe that Earth's cratons also formed into a single supercontinent on other occasions. Some geologists have suggested that these periods of unification may be part of a supercontinent cycle, in which the continents form into a single unit and then split apart every 400 to 500 million years. While scientists have a great deal of evidence to support the Pangaea theory, evidence for earlier supercontinents remains debated among geologists.

The World Before Pangaea

Geologists believe that about 1.2 billion years ago, before the time of Pangaea, the Earth's continents were united in a single supercontinent known as Rodinia. Geological traces of this ancient union are still present in formations worldwide. Evidence suggests that life on Rodinia was confined to marine environments, while the terrestrial environment was barren. The dissolution of Rodinia, between 100 and 300 million years later, was a significant milestone in the evolution of life. As the environment changed in response to the geological forces that caused the breakup of the supercontinent, the environment was then able to support terrestrial life; it was at this time that cells colonized the land of each continent. Geologists are uncertain of what happened next, but eventually, the continents of Earth began to draw together again.

The initial formation of Pangaea began more than 500 million years ago, during the Ordovician period, when two supercontinents in the Northern Hemisphere, known as Baltica and Laurentia, collided to form a massive supercontinent known as Euramerica. By this time, Australia, Africa, India, Antarctica, and South America were already unified into a southern supercontinent known as Gondwanaland. Geologists believe that the collisions of cratons to form Euramerica and Gondwanaland caused the formation of some of today's mountain ranges in North America, Asia, and Africa.

Formation and Development

Near the end of the Permian period (290 to 248 million years ago), Gondwanaland and Euramerica collided to form Pangaea. This occurred through millions of years and involved prolonged periods of intense volcanic and seismic disturbances. Mountains and other geological features that formed in this collision attest to the force generated as the continents united. Pangaea was fully formed between 250 and 225 million years ago, though continued movement caused the development of seismic and volcanic zones within the supercontinent. Paleontologists have given the name Panthalassa, from the Greek terms meaning “all ocean,” to the single massive body of water surrounding most of Pangaea during the Permian and Triassic. To the east of Pangaea was a second, smaller ocean known as the Tethys.

The formation of Pangaea had a far-reaching effect on global conditions and organisms living both in the oceans and on Earth's surface. During this time in the late Permian, Earth experienced the largest mass extinction in its history, resulting in the loss of some 90 percent of species living on the planet. Various causes may have played a part in the extinction event, but paleontologists believe that the formation of Pangaea was an important contributing factor.

Because Earth's cratons were united in a single mass, there was less available land to support coastal shelf habitats, which support the majority of ocean life. This loss of habitat may have been one of the factors that led to high extinction rates for marine organisms. Part of the supercontinent covered one of Earth's poles, leading to glaciation and reduced the available habitat for some species. Land near the coasts was far lower than it is today because of rising ocean levels, which also may have contributed to a loss of habitat and habitat diversity. Additionally, because Pangaea was an exceptionally large continent, much of the inland area was far from the ocean. This inland zone developed into an arid region, a massive central desert that wreaked havoc on weather patterns.

The supercontinent existed for less than 100 million years, during which time life on Earth changed dramatically. The Permian-Triassic extinction left the planet with a variety of unoccupied habitats, and new species soon evolved to fill these empty niches. Among the most famous groups of animals to evolve on Pangaea were the dinosaurs, first appearing in the fossil record in sediment from the late Triassic period. The dinosaurs evolved from small reptilian ancestors that survived the worst of the Permian extinction. The dinosaurs possessed key evolutionary adaptations that allowed them to thrive in the late Triassic environment.

Because the continents were unified, early dinosaur species could spread across the globe. By the time Pangaea began to separate in the Jurassic period, dinosaurs had spread into many habitats around the globe. Dinosaurs were not the only new group of reptile descendants to colonize Pangaea; they were joined by the prehistoric marine and flying reptiles, filling the niches today occupied by marine mammals and birds. Like the dinosaurs, these groups also spread across the globe, leaving descendants in every continent. While the dinosaurs and other reptiles dominated the terrestrial environment of Pangaea, the ancestors of modern mammals also appeared at this time and spread across the supercontinent. Unlike the reptiles, however, mammals remained small throughout the Mesozoic period.

The Breakup of Pangaea

Pangaea began to break apart during the late Triassic period, possibly as much as 235 million years ago. Geologists have identified several distinct stages that occurred as the supercontinent separated. The separation began with the formation of rift zones, as tectonic pressures from Earth's interior caused the development of unstable fissures in its surface. Magma and superheated minerals were forced to the surface along these areas, frequently inundated by volcanic eruptions and earthquakes.

The southern and northern sections of Pangaea began to separate first, forming a central ocean called the Tethys between the two areas. This occurred as the eastern part of what is now North America separated from the western part of Africa. As the Tethys Ocean expanded, so did the diversity of life on Earth, especially sea life. The Tethys Ocean provided a variety of shallow oceanic habitats that supported large numbers of fish and prehistoric marine reptiles. The terrestrial portions of Earth also began to flourish as the ocean stabilized temperatures and created a vast forested zone inland from the coast.

In the middle of the Jurassic, the rift between Africa and North America continued to spread, detaching the southern portion of North America from the northern part of South America. This resulted in the formation of two supercontinents: Gondwana in the south and Laurasia in the north. For the dinosaurs and other animals living on Earth's surface, the split between the supercontinents led to divergent evolution, as unique species formed on each continent in response to different environmental conditions. This was the height of the age of dinosaurs, as southern and northern dinosaur faunas evolved into spectacular arrays of species.

The continents continued to pull farther apart, forming a new ocean, the Atlantic. Tectonic activity along the Atlantic Ocean trench widened the ocean and pushed the continents along their trajectories. By the middle of the Cretaceous, about 150 to 130 million years ago, the supercontinent of Gondwana had broken farther apart, forming Australia, India, South America, and Africa. These fissures led to the formation of the Indian Ocean and eventually closed most of what had originally been the Tethys Ocean. It was not until the Cenozoic period, about 65 million years ago, that Laurasia split into North America and Eurasia, giving rise to the Norwegian Sea, and eventually forming the current continental structure.

Researchers in the twenty-first century have posited that the continents will again join in a potential future supercontinent called Pangaea Ultima. This is expected to occur in 250 million years. The changes to Earth’s climate during the supercontinent period will likely make the planet inhabitable to many species. However, research into Pangaea Ultima also allows scientists to understand global climate change in the context of the twenty-first century. Research in the twenty-first century has only reinforced the belief in a supercontinent cycle that will likely repeat itself throughout Earth’s existence. In addition to the theory regarding Pangaea Ultima, an alternative theory regarding a supercontinent named Amasia is also being posited. These theories differ only in their predictions of where land masses will connect.

Principal Terms

continental drift: the hypothesis that continents shift position relative to one another rather than remaining fixed in a single location; this concept was first developed in the early nineteenth century

craton: a stable portion of the continental crust, as differentiated from the more malleable section vulnerable to transformation during continental movement

Gondwana: a supercontinent that emerged from the breakup of Pangaea in the Jurassic period and contained the continents that now constitute Africa, South America, Australia, and India

Laurasia: a northern supercontinent that broke away from Pangaea in the Jurassic period and contained the landmass that formed into most of Asia and North America

lithosphere: the outer shell or surface of Earth consisting of rocks, soil, and other sedimentary material and connected physically to Earth's molten inner core

Panthalassa: a single ocean that existed during the Permian and Triassic periods and that was surrounded by Pangaea

plate tectonics: a scientific theory used to explain shifting patterns in the distribution of land across Earth

subduction: the process by which portions of Earth's crust are sucked into the mantle, creating a vacuum that is largely responsible for the movement of the continents

supercontinent: a continental structure containing two or more cratons

Tethys Ocean: the ocean that formed between Gondwana and Laurasia during the initial breakup of Pangaea

Bibliography

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