Monday, December 13, 2010

Rock Folding and The life cycle of a mountain range

Rocks that are deeply buried have nowhere to go if they fault, so instead they form folds. They can be broad, gentle folds those under southeast England, where the top of the fold or anticline has eroded away leaving the North and South Downs exposed and the London basin full of sediments. Such gentle folds are the comparatively minor knock on effects of the formation of the Alps. There, the collision of Africa with Europe compressed the sediments so much that folds piled northwards one on top of another in great over folds, or nappies: a vertical cliff can expose a repeating sequence of layers.

The life cycle of a mountain range

In a wide sedimentary basin, deposits accumulate layer by layer, sinking under their own weight and hardening as they are compressed these sediments laden troughs which are known as geosynclines are the potential birth place of mountain ranges of they occur between two colliding continental plates. Colliding continents begin to uplift the sediment, deforming it by folding produces symmetric anticlines and synclines. Continuing pressure may cause uneven folding and therefore asymmetric anticlines and synclines which eventually produce a recumbent fold the anticline is now in effect above the syncline and the rock layers on one side of the anticline are inverted. Further pressure may break the inverted layer, resulting in an over thrust fold. A nappe is formed when this layer disappears due to stretching and fracturing as uplift and folding continues.

Tall mountain ranges are produced by large scale faulting, the intrusion of magma domes and extrusive volcanic activity, but most importantly by large scale folding. As soon as mountains are formed weathering processes break up the rock surface and water and ice erode incisions into the mountainsides. Landslides, glaciers and rivers carry material away. The mature landscape stabilizes as rocky peaks become gently  rounded hills, rivers widen and slow, and vegetation stabilizes the soil.