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