THE FORMATION OF PANGAEA
The ABC's of Plate Tectonics
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The discovery of the one time existence of the supercontinent Pangaea was the crowning triumph of the theory of Plate Tectonics. Books, articles, maps, and diagrams abound describing Pangaea and documenting its ultimate breakup at the end of the Mesozoic era. But Pangaea was, geologically speaking, a rather short-lived phenomenon. Many learned articles have been written based on faulty assumptions of its antiquity and origin. Yet the evidence of the last round of orogenies leading to the formation of Pangaea remain visible for anyone who has the time and inclination to examine it.
What follows is one author's interpretation of those events during the Paleozoic Era that led up to the formation of this Mesozoic supercontinent.
The Initial Setting:
At the Dawn of the Paleozoic era, the land mass known as Gondwanaland was already a supercontinent in its own right, having formed relatively late in the Precambrian era. It sat in what is now the South Pacific, extending from perhaps the equator to as far south as the South Pole. It had by this time already scraped clean of all continental 'debris' a broad swatch through what we now know as the Pacific Ocean.
Straddling the Equator on the opposite side of the globe sat the small continent Laurentia, which later grew substantially to become North America. An unknown distance to the east, and also near the Equator, lay another small continent, Baltica, which now comprises a major portion of Europe. Shaped like a fat comma, Baltica's point lay along side and west of what is now the Iberian peninsula, with its bulbous end comprising Belarus and European Russia. An island arc bounded by a subduction zone ran along its western margin. (The northern margins of Baltica and Laurentia remain obscure; subsequent rifting and burial by sediments during the Pleistocene Ice Age have obliterated many traces.)
Various other continental masses, most now welded to form sections of Asia, were scattered in places as yet unknown. There is no definitive evidence that the Colorado Plateau, the Rocky Mountains, or Alaska were attached to the continent of Laurentia at this time. Circumstantial evidence suggests that they may not have been.
First Collision - the Acadian orogeny:
During the Ordovician period, around 500 million years ago, the continents of Laurentia and Baltica collided. First contact, in the Lower Ordovician, occurred when the southernmost of the islands off the southern peninsula of Baltica, west of modern Portugal, bumped into western New York state. This resulted in the Taconic orogeny, which formed the mountains of the same name west of the Hudson River. The New England States of the USA and most of the Maritime provinces of Canada were part of Baltica's island arc at this time.
The next impact occurred in the Lower Devonian in what is now the Maritime Provinces of Canada. The Acadian orogeny, as this first collision episode is know, continued through the Devonian and into the Mississippian. As the two continents ground into one another, they obliterated most of the ocean that had once separated them. (Many of the back arc basins behind the island arc on Baltica's western margin remain, as the North Sea, the Irish Sea, the Baltic sea, the Grand Banks off Newfoundland, and the Gulf of St. Lawrence.) When the orogeny was over, a range of mountains extended from the Adirondacks north through Nova Scotia, the British Isles, Norway and the eastern margin of Greenland, and possibly on to Spitsbergen.
Meanwhile, Gondwanaland was moving across the South Pole and northward up what is now the South Atlantic.
The Entrance of Gondwanaland:
The supercontinent of Gondwanaland included all of Africa, South America, India, Antarctica, Australia, New Guinea, and New Zealand, plus assorted pieces now attached to other continents. Along its northern or leading margin was a subduction zone, fronting an island arc that included several large islands, which now make up much of Central Europe, Italy, the Balkan Peninsula, Turkey, and probably the Middle East and Iran. (For a modern comparison, look at the islands of Indonesia.) All of the Eastern Seaboard and Gulf Coast of the United States and Mexico south of Long Island, east of the Appalachians and the Marathon Mountains of West Texas and south of the Ozark and Smoky Mountains, were also originally part of Gondwanaland, as were the major islands of the Caribbean.
The Pacific coast of what is now South America looked far different then. The Andes Mountains didn't exist yet. The Gulf of Mexico was probably a back-arc basin, open either to the north or west. (The existence of Central America at this time is doubtful. It seems likely that the Yucatan Peninsula was attached directly to Colombia or Venezuela.) The Amazon Basin, into which the Niger River discharged, was a large sea that opened into the Pacific.
Second Collision - the Appalachian orogeny:
The behemoth called Gondwanaland, comprising well over half of the world's land mass, was moving north. The combination of Laurentia and Baltica -- now known as Laurussia -- was in its way, and the Pennsylvanian and Permian periods were crunch time. The exact sequence of events remains unclear, but contact appears to have occurred in Late Mississippian in Oklahoma, in Early Pennsylvanian in the Appalachians, but not until Late Pennsylvanian in Alabama. (Remember that the coastlines of approaching continents don't match each other like adjacent pieces of a jigsaw puzzle. The 'bumps' tend to hit first.)
Timing of events in Europe are not known by this author, but the collision ultimately resulted in mountain ranges extending from the Marathon Mountains of West Texas, through the Ozarks and Ouachitas, the Great Smoky Mountains, the Appalachians, and the Cevennes of France, to the Carpathians of Eastern Europe. Any orogenies that may have occurred at this time in the Middle East are obscured by later events.
Third Collision - the Urals:
While this crunch was going on, but at a time difficult for the author to document precisely with the information at hand, another small continent, here called Angaria, collided with Baltica's eastern margin, uplifting the Urals and the mountains of Nova Zemlya. Angaria, named from the Angara shield at its core and the Angara river that cuts through it, comprised most of Siberia from the Ural Mountains to the Lena River. (A small wedge of land in modern Kazakhstan, identified as a separate plate by Russian geologists, remains unaccounted for.)
If all the land masses described above were truly joined by the Permian period, then Pangaea (from Greek, literally all of Earth) truly deserves its name. Only Eastern and Southeastern Asia, and (arguably) Western North America, including the subcontinent of Beringia (Alaska and Siberia east of the Lena River) were not included. However Gondwanaland was starting to break up even as it slammed into Laurussia; further evidence is needed to determine if Australia/Antarctica and India were still joined to the rest of the supercontinent at the time of impact.
The fact that most of the Earth's land masses were connected during the Mesozoic era does not mean that they actually comprised one continent in the sense in which we use that word in the modern world. Asia today is connected to Europe across the Ural Mountains, to Africa via the Suez, and technically, to North America via a shallowly submerged bridge of continental material that includes the Bering and Chukchi Seas. (This 'bridge' is a minimum of 1100 miles wide, and shallow enough that it stood high and dry during the last ice age.) North America is in turn connected to South America by the Isthmus of Panama.
So, in a very real sense, all the Earth's land masses except Australia and Antarctica are 'joined' today. If actual shorelines for Pangaea were known with any accuracy, we would undoubtedly be inclined to divide it into numerous 'continents' as well. Not all the back-arc basins along Baltica's west coast were collapsed during its collision with Laurentia; remnants remain today as the Gulf of Saint Lawrence, the Grand Banks off Newfoundland, the Irish, North, Barents, and possibly Baltic Seas. These 'seas' formed an interior seaway that virtually cut 'Europe' (i.e. eastern Pangaea) off from 'America' (i.e. western Pangaea). Parts of the Mediterranean Sea, the Black, Caspian, and Aral Seas, and perhaps the Persian Gulf represent possible remnants of similar basins along the north coast of Gondwanaland, separating 'Europe' from 'Africa' (i.e. southern Pangaea). The Gulf of Mexico as yet another a back-arc basin has been mentioned earlier.
For most of the Mesozoic era, roughly the time that Pangaea existed, Western North America was isolated from the rest of the continent by a waterway known as the Great Interior Seaway. Whether this was entirely made up of submerged continental crust - and thus was truly an 'interior' seaway, or whether an actual ocean basin separated the two land masses, remains conjectural, but the Laramide Orogeny of the later Mesozoic does look suspiciously like the result of a continental collision. It is my postulate that the Colorado Plateau, along with a northwest to southeast trending series of island arcs which now make up much of the Rocky Mountains and ranges westward, collided with Pangaea (Eastern North America) somewhere around the Late Jurassic period, causing the Laramide Orogeny. This mountain-building event occurred too far from any now apparent continental margins to be explained by known mechanisms of plate tectonics. Upper Jurassic deposits, such as the Entrada and Morrison formations, can be found on both sides of the orogeny zone in Colorado, but correlations between western and eastern slope formations earlier than these are problematical at best.
It is also unlikely that the various pieces that now make up eastern Asia (modern China, Tibet, and Mongolia), plus the subcontinent of Beringia (which includes Alaska from Anchorage to Mackensie Bay, the Chukchi and Bering Seas, and Russia east of the Lena River) were attached in their present positions to the Pangaean continent. Some of these pieces might have been attached to Pangaea elsewhere, and subsequently rifted away, only to collide again in a different position.
The opinions expressed in this article are those of its author, Donald L. Blanchard, and probably (in fact quite likely) do not coincide with those of any professional authority on plate tectonics or paleogeography.