Mountain Building Part IV: the formation of Pangea and the Appalachian Mountains
Figure 1.16: When ancestral Africa collided with North America, the Appalachian Mountains were formed. Figure by J. Houghton.
Today's Appalachian Mountain chain formed 470 million years ago at the time of the Taconic mountain-building event, with the initial squeeze of the margin of North America. The Acadian mountain-building, 380 million years ago, crunched the crust of North America a bit more. Finally, approximately 250 million years ago, the Alleghanian mountain-building event occurred as ancestral Africa collided with North America to create the central and southern Appalachians during the Permian (Figure 1.16, Figure 1.17). The Acadian orogeny helped to shape the northern Appalachian Mountains, but the Alleghanian orogeny gave the final squeeze to the margin of the continent to form today's Appalachian Mountain chain, extending from Alabama to Maine and beyond into Canada. From the time of the Acadian mountain-building event until the Triassic, the Appalachians were continuous with the Caledonide Mountains of northwestern Europe and Greenland.
Why are the Appalachians still here?
Although the Appalachian Mountains were formed over 250 million years ago, they are still around today. The forces of erosion and weathering have worn down the Appala- chians over time; periodic uplift of the range, however, has prevented them from completely eroding away.
The Appalachian Mountains that we see today, however, are merely the worn down remnants of the Appalachians created millions of years ago. At one time the Appalachians were probably as tall as the modern Himalayas, but today the Appalachians are the rounded, weathered and aged peaks of a more mature mountain range that has seen millions of years of erosion and uplift.
Figure 1.17: Late Pennsylvanian: 306 million years ago.
The direct cause of the creation of the Appalachian Mountains was the merging of all continents into the supercontinent Pangea as the Iapetus Ocean closed 290 million years ago. Baltica and North America had merged to form effectively creating the ancestral northern Appalachians. In the meantime, through the Mississippian and Pennsylvanian periods, ancestral Africa (already joined to other continents as Gondwana) drifted closer to North America and Baltica. The Iapetus Ocean narrowed as the oceanic crust was subducted under the North American continental crust. When ancestral Africa finally collided with North America during the Permian, the continental crusts crumpled together to create a tall range of mountains. Sediments from the proto-Atlantic ocean basin and the continental shelf and slope of North America, were pushed upwards and squeezed along with the crust.
Evidence for Pangea
How do we know that Pangea existed 250 million years ago? Fossil evidence and mountain belts provide some of the clues. The Permian-age fossil plant, Glossopteris, had seeds too heavy to be blown across an ocean. Yet Glossopteris fossils are found in South America, Africa, Australia, India and Antarctica! The mountain belts along the margins of North America, Africa and Europe line up as well and have similar rock types, indication that the continents at one time were joined as Pangea. The discovery of Glossopteris and the evidence in the rocks helped geologists to formulate the theory of Continental Drift, which, when the processes of continental movement were later discovered, was reformulated under the modern theory of Plate Tectonics.
Figure 1.18: Alleghanian Mountain Building:
- Africa collides with North America.
- central/southern Appalachians form.
- Pangea assembled, one supercontinent on Earth.
