Iapetus, Pangea, Appalachia, and The Atlantic
By: Scott H. Nyhof, P.G.
The present-day Appalachian Mountains stretch from central Alabama, along the Atlantic Coast, and into southern Quebec. A continuation of the Appalachian Mountains extends across the present-day Atlantic, across the British Isles and into Scandinavia. For an ever-so brief introduction into the formation of a mountain range that used to rival the modern-day Alps in Europe or the Rocky Mountains, we look back into the Cambrian, about 1 billion years ago, when there was a large land mass and one ocean (Panthalassa).
Geologist’s graphic interpretation of opening and closing of Iapetus, a “proto-Atlantic”, that resulted in the formation of the Appalachian Mountains when it closed.
With tectonic forces jostling more than 1 billion years ago, the Grenville Orogeny resulted in the formation of several ancient mountain ranges, of which the Adirondacks still remain. The compression that resulted in the mountain building eventually relaxed and reversed as a prolonged period of “rifting”, where two large land masses pull away from one another, much like the Eastern Horn of Africa is doing today. This rifting occurred during Cambrian time, about 500 million years ago (Ma) and resulted in the formation of Iapetus, a “proto-Atlantic” that would serve as a basin for the deposition of the sedimentary rocks we see in today’s Appalachian Mountains.
The Laws of Superposition and Uniformitarianism require simply that if a mountain belt made of sedimentary rocks, as Appalachia is, the sediments must be deposited and lithified first. With the closure of Iapetus in Devonian time (400 Ma), the Appalachian Mountains rose up along the eastern edge of North America, while the Atlas Mountains formed in modern-day Morocco. By Pennsylvanian time (300 Ma) Iapetus was closed, with Africa once again welded to North America. Pangea was formed.
Relative difference between Iapetus (green) and Atlantic (yellow & white) basins
The configuration held through Permian time (225 Ma). Later, in Triassic time (200 Ma) the modern-day Atlantic formed (blue arrow, below) as Laurasia (North America and Eurasia) separated from Gondwana (South America, Africa, Antarctica, and Australia). As noted on the figure above, Iapetus does not appear to have closed along the same suture that the Atlantic opened along. Only along the eastern seaboard of the United States are ancient scenes displayed in the rocks of such a story. Rocks exposed along the Blue Ridge Parkway represent ocean floor sediments from Iapetus. Coal seams mined from Pennsylvania and West Virginia formed from plants that grew during Pennsylvanian time. The opening of the Atlantic also left modern-day Newfoundland, which is a “slice” of oceanographic floor, “welded” onto North America; such a remnant is referred to as an “ophiolite”.
