Brief outline of Pennsylvania’s Geologic History

Kurt Friehauf

210 million years ago – m. Jurassic rifting event begins

No empire is forever, however, and the mighty Pangea supercontinent ultimately began to break apart. The major tear formed in the general vicinity of the old continental margins. As the eastern block pulled away from North America to form Africa and Eurasia, a new ocean was born in between – the Atlantic Ocean. There was a bit of volcanic activity associate with the break-up. Black basaltic magma worked its way up the deep fractures in the Earth’s crust. Some of the magma solidified beneath the surface to form large slabs of coarse-grained basalt called diabase. The ridges of Gettysburg are made of this resistant rock. The Palisades of New Jersey and New York City also formed this way. Some of that magma oozed out onto the surface through long fissures to form the basalt flows of New Jersey and easternmost Pennsylvania.

Of interest to me personally are the iron deposits that formed during this period. Although they’re not nearly as large as the sedimentary iron deposits elsewhere in the world, these deposits appear to have formed by the circulation of deep salty groundwaters, stirred by magmatic intrusions.

Click here to move on to the next stage