Pressure release, a mechanical weathering process, creates domed formations.
Mechanical weathering occurs when atmospheric or geological forces break rocks into small pieces that remain essentially unchanged and largely unmoved from the starting location. It is distinguished from erosion, which abrades mechanically but also transports material at the same time (through wind or water action). These processes include frost wedging, salt crystal growth, thermal action, hydraulic pressure and pressure release.
Freeze-Thaw Cycle: Frost Wedging
In areas where the temperature often fluctuates between freezing and thawing, the water trapped within rock pores or between small fractures expands 9 percent when ice crystals are formed. Over time that expansion breaks off small fragments from the surface of rocks. Larger formations can be broken into blocks along existing fracture lines. Water seeps into the cracks, freezes and expands, pushing the rocks apart a bit further. This allows in more water, which pushes even further in a repeated cycle.
Salt Crystals and Evaporation
In a process similar to ice crystals, salt dissolved in water is carried into crevices in rocks generally along sea costs or deserts. Given enough heat, the water evaporates and salt crystals grow, which can then exert force against the walls of the crevice, eventually cracking off pieces or splitting entire formations. This is most active in hot desert regions where the evaporation can take place quickly.
Thermal Weathering
Thermal stress again most often occurs in desert environments where there is a large difference between day and night temperatures. Repeated heating and cooling expands and contracts the different minerals within a rock at different rates and fracturing occurs.
Pressure Release or Exfoliation
Igneous rocks like granites and basalts are often pushed close to the surface in large magma chambers which then cool and form very hard rock. Pressure is maintained on the formation by the overlying material that may be other rock formations or gravels, clays or soils. When erosion or plate tectonic shifting removes enough of the overburden, most often by glacial action and then the melting of the glacier itself, the pressure is released. The formation expands and fractures parallel to the outer surface and large sheets of rock can fall away. This is what causes some large round boulders and also dome mountains.
Hydraulic Pressure
Though not as noticeable because there is so much erosion also taking place, hydraulic pressure plays a role in the breakdown of coastal rocks. When a high speed wave comes ashore and pushes into crevices, it pushes the air in front of it, compressing it to very high pressures. When the wave retreats suddenly the pressure is released, almost like an explosion, which can shatter surface rock and widen cracks. The key to all of these types of weathering is that a mechanical force is applied rather than abrasive action so common in water or wind erosion.
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