Brief outline of Pennsylvania’s Geologic History
Kurt Friehauf
1 billion years ago – Precambrian Grenville Orogeny
“Orogeny” is the geologic term for a mountain-forming event. One billion years ago, the eastern edge of the North American tectonic plate[1] collided with another continent, causing great compressional stresses in the collision zone. The incredible stress and heat involved with this train-wreck-like collision caused buckling of the continental plate and so pushed mountains up in an event we call the Grenville Orogeny. Not only did the collision cause uplifting of the mountains, but the rocks themselves were transformed as they recrystallized in the high temperature / high pressure environment – a process geologists call “metamorphism.” Metamorphism changed common mudstones into slates and even schist, and changed many igneous rocks into gneiss. The mystery continent that collided with us stuck, forming a supercontinent.
