Tuesday, September 14, 2010

Rock Cycle

The moon is dead. With no atmosphere and a cold, solidified interior, most features on the surface are several billion years old. The earth is very different and little survives the aeons intact. Even great mountain ranges arise and are eroded over a few hundred million years. The rocks on the Earth’s surface are perpetually being recycled. Magma rises from deep inside the Earth. Some is trapped underground and hardens into intrusive igneous rock. Some erupts onto the surface as extrusive igneous rock. Some erupts onto the surface as extrusive igneous rock. Pressure and heat from below cook or metamorphose the rocks and colliding continents push them up to the surface. Wind rain and ice erode the rock and with the help of gravity carry away the sediment. Rivers deposit it along their flood plains or at the bottom of lakes and seas, where it builds up in layers and hardens under pressure into sedimentary rocks. These sink into the ground and are metamorphosed by head and pressure, or folded and uplifted again by more tectonic activity, continuing the cycle.

The rock cycle is powered from above and below. Heat from within the earth ultimately derived from radioactive decay and the slow solidification of the inner core causes the upwelling of intrusive and extrusive rocks and produces the process of metamorphism of rocks. It also drives the drifting continents, uplifting mountain ranges. The energy of the Sun heats and expands rocks at the surface and ultimately drives the wind, waves and precipitation that cause erosion. Gravity causes landslides which contribute to the circulations of rocks.