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The Rock Cycle

Mount Cook Rock Avalanche 1991

Mount Cook Rock Avalanche 1991, Lloyd homer GNS Science

The rock cycle involves particles being continually eroded, transported, deposited, and cemented to form new rock that may later be uplifted. Then erosion starts the cycle again.

Compressional and tensional forces (squeezing and stretching) cause the earth’s crust to buckle and crack.

  • When compression forces the crust upward, mountain chains are formed often accompanied by severe folding, fracturing of the rocks and the intrusion of magma. The effect of mountain building (orogeny) varies from place to place, and locally the rocks may be only gently tilted.
  • Long-continued downward movement (downwarping) may occur to form extensive troughs or basins in which many thousands of metres of sediments can accumulate. The high temperature and pressure deep within the sedimentary pile can cause metamorphism. In contrast, slight downwarping allows the sea to extend over the land, producing shallow shelves close to the land and deeper basins further offshore.

Uplift and erosional episodes make it impossible for any one area to have a rock sequence representing the whole of geological time. Only when an area is submerged beneath the sea can there be continuous sedimentation for a prolonged time period, and even then there may be gaps (unconformities) from the removal of sediments by strong bottom currents or periods of non-deposition. When an area is above sea level, sporadic terrestrial deposits, such as river (fluvial) sediments, coal measures (swamps), and igneous rocks may be the only representatives of that geological time.