mechanical weathering

Mechanical/physical weathering is the mechanical/physical breakdown of rocks into smaller particles. If the process continues indefinitely, it has the ability to grind the rock down into sand. It is most likely to occur in regions with little or no vegetation, such as mountains or deserts. The mineral composition of a rock does not change during mechanical/physical weathering (i.e. there is no chemical change).

• Freeze-thaw action/frost Shattering/frost wedging - This is the most common form of mechanical/physical weathering. It is most prominent in cold and upland areas where temperatures fluctuate above and below freezing point, 0°C. It affects already rather weakened rocks such as those with fractures and joints. It occurs when water enters cracks in the rock in warm or daytime temperatures; when the temperature cools or night falls, this water freezes and expands by up to 9%, exerting great pressure on the rock. The next day, when temperatures rise again, the ice melts, relieving the pressure. The rock then contracts. When this process of freezing and thawing (hence the name) is repeated over a long period of time, it widens the cracks, weakening them even further. Eventually, the cracks give way, with pieces of rock breaking off. These angular particles then accumulate at the bottom of the slope and are known as scree/talus e.g. Wicklow Mountains and the Burren.

Exfoliation/onion weathering/onion peeling - This is most prominent in areas where there is a large diurnal temperature range (i.e. a massive difference), often up to 50°C, between daytime and nighttime temperatures. It occurs when the rock absorbs heat and energy from the sun during the day when temperatures are high due to the absence of cloud cover, causing it to expand ever so slightly. At night when temperatures drop, all heat and energy is lost to the atmosphere, causing the rock to cool and contract. When this process of expansion and contraction is repeated over a long period of time, it weakens the rocks outer layer to be exfoliated like skin or peeled like an onion. Precipitation ca hasten this process eg Sahara Desert