Supercontinents
Supercontinents are vast landmasses that consist of multiple continents fused together, a concept rooted in the theory of continental drift. Scientists have discovered that the continents of Earth have shifted over geological time due to processes associated with plate tectonics, leading to periods of both unification into supercontinents and subsequent separation. The most famous supercontinent, Pangaea, existed during the late Paleozoic and early Mesozoic eras, approximately 320 million years ago, before breaking apart into two smaller supercontinents: Laurasia and Gondwana. This division significantly influenced the evolution and distribution of species, as different dinosaur fossils found across regions indicate distinct evolutionary paths shaped by continental separation.
The mechanisms behind these movements include subduction and seafloor spreading, which are processes that drive the movement of tectonic plates. Current research suggests that the shift of continents is not only crucial for geological formations but also plays a vital role in maintaining Earth's magnetic field, which protects life from solar radiation. The interplay between tectonics and life indicates that these geological processes are fundamental to the conditions that allow complex life to thrive on Earth. Understanding supercontinents and their cycles provides insight into both our planet's past and the underlying processes that continue to shape its future.
Supercontinents
Introduction
Utilizing a variety of evidence collected over centuries of research, scientists determined that the continents of the world were not always in their present state, but were once united in a series of much larger landmasses, known as supercontinents. Scientists now understand that the continents of the earth shift according to the influence of constant, gradual forces, resulting in alternating periods of unification and division. These discoveries helped scientists to better understand the distribution of modern life, as well as the evolution of species.
The theory of continental drift, proposed in 1915, led to the realization that the continents move across the surface of the earth. Scientists gradually discovered the mechanisms responsible for these changes, culminating in the development of plate tectonics, the branch of geology that studies the movement of the enormous plates that make up the earth's crust throughout time.
The best-known supercontinents are those that existed during the time of the dinosaurs, the Mesozoic (251–66 million years ago). During this time, the supercontinent of Pangaea gradually split into northern and southern supercontinents, called Laurasia and Gondwana, respectively. These supercontinents continued to split, giving rise to the modern continents. As paleontologists discover new dinosaur fossils, they provide clues to the structure of the earth during the time of the dinosaurs. The sciences of plate tectonics and paleontology now work together to provide a more detailed perception of the earth's prehistoric past.
Key Terms
Continental Drift: Hypothesis that continents shift position relative to one another rather than remaining fixed in a single location, first developed in the early 1800s.
Craton: The stable portion of the continental crust, as differentiated from the more malleable section vulnerable to transformation during continental movement.
Gondwana: Southern supercontinent that emerged from the breakup of Pangaea in the Jurassic period; contained the land that later became part of Antarctica, South America, Australia, Africa, and India-Pakistan.
Laurasia: Northern supercontinent that broke away from Pangaea in the Jurassic period; contained the landmass that later became most of Asia and North America.
Lithosphere: The outer shell or surface of the planet, consisting of rocks covered by soil and other sedimentary material, which is connected physically to the molten inner core of the earth.
Paleogeography (or Palaeogeography): Branch of science concerned with the geographic distribution of the continents before the formation of their current configuration.
Pangaea: Supercontinent that existed during the Paleozoic and the early Mesozoic; its breakup gave rise to the supercontinents of Gondwana and Laurasia in the Jurassic period.
Plate Tectonics: The geological processes by which lithospheric plates move, thus changing the configuration of the continents and ocean basins.
Subduction: Process by which portions of the earth's crust (plates) are drawn down and melted into the mantle, as a result of the collision of oceanic and continental plates.
Key Players
Alfred Wegener: In 1915, meteorologist Alfred Wegener published Origin of the Continents and the Oceans (Die Entstehung der Kontinente und Ozeane), in which he proposed the theory of continental drift, an attempt to address evidence building since the mid-1800s that suggested the continents were once united. Wegener also proposed the name “Pangaea” for his theoretical supercontinent. Though Wegener's theory was later replaced by the theory of plate tectonics, many of Wegener's ideas became integral concepts in geological science.
Alexander du Toit: South African geologist Alexander du Toit was responsible for much of the early research used to support Wegener's ideas concerning continental drift and the supercontinents. In his 1937 book Our Wandering Continents, du Toit provided a variety of supporting evidence for Wegener's theories and proposed the name “Laurasia” for the prehistoric supercontinent that broke from Pangaea and contained the landmass that would later become North America, Europe, and most of Asia.
Harry Hess: In his 1962 report History of Ocean Basins, meteorologist Harry Hess established the theory of seafloor spreading, which provided a scientific explanation for continental movement and is often cited as the origin of plate tectonics. Hess based his theory on measurements indicating that sediment on the ocean floor was millions of years younger than that found on land. From this, Hess postulated that newly formed lava continuously replenishes the floor and leads to the movement and creation of continents.
Christopher Scotese: Building on an earlier proposal in 1974 by Canadian geophysicist John “Jock” Tuzo Wilson, who introduced the theory that supercontinents form in regular cycles, Scotese coined the newest potential supercontinent Pangea Proxima, meaning the next Pangea. He surmised that, since the oceanic crust of the Atlantic Ocean is slowly sliding beneath the continental crust, a ring-shaped landmass would cluster around the remains of an inner sea.
History
Continental Drift: In the 1850s, geologists and paleontologists were debating a growing body of evidence suggesting that the continents were once united. For one thing, paleontologists noticed fossils of the same or related species, as well as similar geologic sediments in formations found on different continents. From this, paleontologists surmised that these formations were somehow related, though separated by great distances.
Most importantly, maps of continental boundaries, which had been increasing in accuracy since the 1500s, indicated that the continents had complementary shapes, seemingly fitting together like pieces of a puzzle. Before geologists had determined the mechanism responsible for these changes, many believed that Africa and South America were once united; Austrian geologist Eduard Suess proposed the name “Gondwanaland” for this theoretical supercontinent.
In 1915, meteorologist Alfred Wegener published his theory of continental drift, in which he proposed that the continents were always in a state of movement. He suggested the name Pangaea (“all the land” in Greek) for his theoretical supercontinent containing all of the world's continents in a single landmass. South African geologist Alexander du Toit was one of Wegener's biggest supporters and was responsible for naming the two continents that resulted from the breakup of Pangaea: Laurasia in the north and Gondwana (a modified form of Suess's suggested name) in the south.
Plate Tectonics: The next major leap in understanding continental movement came in the 1960s, when geologist Harry Hess proposed the theory of seafloor spreading. Hess suggested that the earth's surface is covered by a series of crustal plates, which move as sediment falls into deep ocean trenches, drawing the continents into place (a process called “subduction”). The movement of plates is also fueled by radioactive energy from the earth's core, creating convection currents throughout the globe. Hess's theory formed the core of a new branch of geology called “plate tectonics.”
In the 1920s, geologists developed the concept of a craton, which is the stable, relatively unchanging portion of the earth's crust contained within each continent. Geologists named the various cratons associated with the continents and defined a “supercontinent” as any landmass containing at least two cratons. Geologists believe that Pangaea was the last supercontinent to contain all of the world's cratons in a single structure. Since the 1920s, geologists have found evidence for the existence of many other supercontinents in the distant past. Geologists believe that the first theorized supercontinent, now called Vaalbara, formed more than 3 billion years ago and was the first supercontinent to contain all of the earth's cratons.
Breakup of Pangaea: Paleontologists believe that Pangaea was formed during the Carboniferous at least 320 million years ago and began to split apart during the Late Triassic (235–201 million years ago). The earliest evidence of dinosaurs and pre-dinosaur relatives has been found in sediment from the Triassic (251–201 million years ago). Early dinosaurs apparently spread across the globe, achieving worldwide distribution by the time that Pangaea broke apart.
Geologists now believe that Laurasia and Gondwana began to split apart around 230 million years ago and were well separated in Jurassic time. Jurassic dinosaurs in Laurasia evolved in different ways from their relatives in Gondwana. The separation of the continents helps to explain why dinosaur fossils from excavation sites in Africa, South America, and Australia (formerly Gondwana) differ markedly from species uncovered in North America, Europe, and Asia (formerly Laurasia). Laurasia and Gondwana continued to split apart under the influence of continental movement, eventually reaching the current continental configuration between 60 and 30 million years ago.
Current Research and Implications
The Origin of Plate Tectonics: Scientists are uncertain about how the process of continental drift began. In 2009, geologist Vicki Hansen, from the University of Minnesota, suggested that a massive meteor impact, occurring as early as 2 to 2.5 billion years ago, may have initiated the process by damaging the cooling earth to such a degree that it formed rifts to the earth's core. These rifts became focal points in the cycles that underlie plate tectonics. Hansen points to numerous craters found on Mars and the Moon as evidence that massive meteor collisions were common occurrences in the early history of the solar system.
Tectonics and Life: Many scientists now believe that plate tectonics is one of the keys to the evolution of life on earth. The shift of continental plates is essential for the maintenance of the earth's magnetic field, which shields the planet from solar radiation that would otherwise prevent complex life-forms from evolving. The magnetic field also plays a role in maintaining a climate conducive to the evolution of life.
The movement of the continents also creates a constant cycle of mineral replacement, in which the sediment of both the seafloor and the terrestrial surface are renewed with minerals produced in the earth's core. Some paleontologists now believe that without this replenishment, the essential minerals and chemicals needed to fuel the chain of life would be exhausted.
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