Continental Collision

Continental Plates

How did rocks that formed on sea floors and islands become the mountains and valleys of today? The ocean that formed during the continental breakup about 540 million years ago continued to expand. During that time bacteria, algae, and many species of invertebrates flourished in the oceans, but there were no plants or animals on land. Then, about 470 million years ago, the motion of the crustal plates changed, and the continents began to move toward each other. As the continental plates moved closer together, fragments of oceanic crust, island arc volcanoes (such as the islands of Japan and the Aleutian Islands), and other continental masses collided with the eastern margin of ancestral North America. By this time, plants had appeared on land, followed by scorpions, insects, and amphibians. The ocean continued to shrink until, about 270 million years ago, the continents that were ancestral to North America and Africa collided. Huge masses of rocks were pushed westward along the margin of North America and piled up to form the mountains that we now know as the Appalachians.

As blocks of continental crust rode across one another, some rocks became so hot that they melted. Where the temperature is high but below the melting point of the rocks, solid rock flows occur and create metamorphic rocks. The collision of continental plates is also expressed in the rocks by folds (bends) and faults (breaks). Earthquakes happen because of slippage along a fault. Faults act as channels for migration of fluids and were a key factor in localizing gold in certain zones. The collision of continents hundreds of millions of years ago also set the stage for the patterns of human settlement, travel, and transportation routes in the region.