5.1 Weathering

2 types of weathering Mechanical Weathering Occurs when physical forces break rock into smaller & smaller pieces without changing the rock’s mineral composition Chemical Weathering The transformation of rock into 1 or more new compounds

Mechanical Weathering Breaking a rock into smaller pieces increases the total surface area more SA is exposed to chemical weathering Unloading Exfoliation (layers of onion) Causes the outer layers of the rock to expand more than the rock below Igneous rock may be exposed by uplift & erosion of overlying rocks. Pressure exerted on the igneous rock is reduced. Frost Wedging Water freezes and expands Frozen water in rocks expand cracks Freeze – thaw cycles = rock breaks into pieces

Chemical Weathering Water Most important factor in CW Absorbs gases in the atmosphere & ground Chemical Weathering of Granite Can change the properties of rocks Chemical Weathering of Silicate Minerals Make up most of Earth’s crust Na, Ca, K, Mg they contain dissolve & are carried away by groundwater Spheroidal Weathering Form of CW Corners & edges become more rounded by CW

Rate of Weathering Differential Weathering Different parts of rock mass often weather at different rates Factors that affect the rate of chemical weathering: Rock Characteristics Climate Temperature, Moisture High Temps. & high moisture are best for chemical weathering Mechanical Weathering By breaking rock into small pieces MW accelerates CW by increasing the SA