Spotting a Supercontinent: How Pangea Was Discovered
Earth of the present day is made up of six or seven continents and four or five oceans, depending on whom you ask. But this wasnât always the case. Through the course of geologic time, the continents âdriftâ about on tectonic platesâlarge parts of Earthâs crust that float on a heated plastic layer of mantle and periodically crash into one another and break apart. Every so often (that is, every several hundred million years or so), the conditions are such that most or all of the continents come together to form a single larger landmass called a supercontinent. Notable supercontinents of the past include Laurasia, Gondwana (or Gondwanaland), andâthe mother of all supercontinentsâPangea, which lasted from the early Permian Period (roughly 299 million years ago) into the early Jurassic Period (roughly 200 million years ago).
But how do we know that Pangea actually existed? After all, human beings evolved only a few hundred thousand years ago, so no one was around to witness this geomorphological monstrosity. How did scientists âdiscoverâ Pangea and other supercontinents of the past? Nowadays, they can study the geologic record and use radioactive dating, seismic surveys, and other technologies to construct maps of how the world looked at various points in Earthâs history. Pangeaâs existence was first proposed in 1912, however, well before the invention of these tools and the development of the modern theory of plate tectonics.
German meteorologist Alfred Wegener first presented the concept of Pangea (meaning âall landsâ) along with the first comprehensive theory of continental drift, the idea that Earthâs continents slowly move relative to one another, at a conference in 1912 and later in his book The Origin of Continents and Oceans (1915). Like a handful of other scientists who came before him, such as 19th-century German naturalist Alexander von Humboldt, Wegener became impressed with the similarity in the coastlines of eastern South America and western Africa and wondered whether those lands had once been joined together. Sometime around the year 1910 he began to consider whether all of Earthâs present-day continents had once formed a single large mass, or supercontinent, long ago, and had subsequently broken apart. Wegenerâs presentation ran counter to the dominant paradigm of the time, which suggested that large portions of continents foundered and sank beneath the oceans over time.
Wegener pointed out that the outline, the geomorphology (rocks and landforms), and the climate belts of eastern South America were similar to those of the southwestern coast of Africa. He also argued that fossils of certain plants and animals appeared on both of these continentsâand that while they were alive these organisms couldnât have traversed the width of the South Atlantic that currently separates the two continents. So, logic suggested that South America and Africa had once been part of the same landmass. Wegener concluded that South America and Africa (as well as others) had been connected to one another, possibly through land bridges, some 250 million years ago. He also believed that Pangea had lasted through most of Earthâs history. Wegener relied on the work of Austrian geologist Eduard Suess, who (although he was a big proponent of the existence of sinking continents) first developed the concept of Gondwanalandâa supercontinent lasting from 600 million to 180 million years ago and made up of present-day Africa, South America, Australia, India, and Antarctica. Suess spotted rock formations in India that compared well in terms of age and composition with similar formations across various Southern Hemisphere continents. Wegener used Seussâs work to support his own continental drift hypothesis and considered Gondwanaland to be the southern half of Pangea.
Despite having this geological and paleontological evidence, Wegenerâs theory of continental drift was not accepted by the scientific community, because his explanation of the driving forces behind continental movement (which he said stemmed from the pulling force that created Earthâs equatorial bulge or the gravitational pull of the moon) were refuted. Wegener died in 1930, well before many of his ideas regarding Pangea and continental drift were vindicated. Other scientists, however, such as South African geologist Alexander Du Toit, continued to collect evidence in support of continental drift. Du Toit proposed the idea of Laurasiaâan ancient supercontinent in the Northern Hemisphere that included North America, Europe, and Asia (except peninsular India)âin his book Our Wandering Continents (1937).
Developments in rock and mineral dating, sonar, and geophysics ultimately vindicated Wegener. The rock formations of eastern North America, Western Europe, and northwestern Africa were later found to have a common origin, and they overlapped in time with the presence of Gondwanaland. Together, these discoveries supported the existence of Pangea. In addition, evidence supporting continental drift mounted during the 20th century, and scientists described a mechanism that seemed to explain continental movement by the 1960s, which was folded into the modern theory of plate tectonics. This mechanism was the process of mantle convectionâwhere heated mantle from Earthâs interior rises to the surface to drive apart tectonic plates in opposite directions. Although so-called spreading centers (linear boundaries between diverging plates on the ocean floor characterized by rising magma) have been shown to exist, an explanation of how mantle convection actually works remains elusive to this day.
Modern geology has shown that Pangea did actually exist. In contrast to Wegenerâs thinking, however, geologists note that other Pangea-like supercontinents likely preceded Pangea, including Rodinia (circa 1 billion years ago) and Pannotia (circa 600 million years ago). Today, Earthâs tectonic plates continue to move, and their motions are slowly bringing the continents together once again. Within the next 250 million years, Africa and the Americas will merge with Eurasia to form a supercontinent that approaches Pangean proportions. Such an episodic assembly of the worldâs landmasses has been called the supercontinent cycle or, in honor of Wegener, the Wegenerian cycle.
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