THE STATE AS A PLATE TECTONICS LABORATORY
Understanding New York State’s historic development requires an appreciation of how its geology was shaped by plate tectonics. Plate tectonics theory sees the earth as a dynamic planet, with the distribution of oceans, continents, and mountains continually defined by processes that originate deep in the planet. Plate tectonics is based on the recognition that the earth’s surface is broken into huge plates, which may have continents on them. Plates move relative to each other as a consequence of a slow convective movement of deep, hot rock with a plastic consistency. Pairs of convection cells rise under the middle of the Atlantic (and elsewhere under the oceans) and diverge east and west about 60 miles (100 km) below the seafloor. The seafloor is stretched, broken, and liquid rock is injected into cracks along submarine mountains that mark the boundary between plates. The distance New York State and Europe increased 2 inches (5 cm) per year from convection currents beneath the floor of the Atlantic Ocean. Spreading centers (rifts) also develop under continents and break them apart.
If plates move away from each other, they must collide on their far sides with more distant plates. Major mountain chains result from continent-continent collisions when the lithosphere (the brittle crust of the earth and underlying rock) of two continents is greatly thickened and deformed during collision. The subcontinent that contains modern Indian has traveled from the southern hemisphere to collide with South Asia; that collision is now uplifting the Himalayas. Western North America shows a different type of collision, with the continental lithosphere overriding the higher-density oceanic lithosphere of the Pacific plate. The Pacific plate is forced under the American plate. A belt of very deep ocean (a trench) appears above the collision (subduction) zone. Explosive volcanoes (the Pacific “ring of fire”) develop near the edge of the overriding plate as the subducting plate melts, and low-density liquid rock rises to the surface.
A result of seafloor spreading and plate movement is the ultimate coalescence of all continents in to a supercontinent. Supercontinents appear with a period of about 600 million years through geologic time and then fragment into many small continents. The geologic history of New York State includes two intervals when this region was part of a supercontinent (first Rodinia, then much late Pangaea) and two period of continent fragmentation.