Prelude to the Paleozoic Late Proterozoic Collisions and the Grenville Orogeny

The oldest rocks in New York, exposed in the Adirondack Mountains and the Hudson Highlands, are somewhat more than 1 billion years old. These are crystalline rocks that were metamorphosed or altered from older rocks by enormous heat and pressure (Figure 4.2). Some were originally igneous rocks, formed from the cooling and crystallization of magmas, and others are sedimentary deposits, such as quartz sandstones, limestones, and shales. These were transformed (metamorphosed) by recrystallization and under intense heat and pressure; for example, sandstones were altered to tough metaquart/ites, limestones to marbles, and shales to mica-rich schists. By looking at the types of minerals formed within these rocks by metamor-phism, geologists can be certain that the rocks now exposed in the Adirondacks were once buried up to 25 km within Earth during a great orogenic or mountain-building episode — about a billion years ago. This event, the Grenville Orogeny, apparently resulted when the (present) eastern edge of ancestral North America was overridden by another continent, perhaps the northwestern side ot" present-day South America. This enormous collision helped to weld together a supercontinent known as Protopangea or Rodinia (Figures 4.3 and 4.4). For nearly half a billion years, this supercontinent held together, and the massive

0 0

Post a comment