Rocks and Weathering Defintions Flashcards
Weathering
The break down of rocks in situ. It differs from erosion in that no movement or transportation of the broken-down material takes place.
Freeze-thaw
Freeze-thaw, also know as frost action, is a type of mechanical weathering due to freezing temperatures. Water gets into cracks of rocks, freezes and then expands by roughly 10%. This repeated action puts pressure on a rock, eventually causing it to shatter and break off.
Heating and cooling
Repeated heating and cooling of rocks can also cause rocks to be broken down and weathered away. Rocks can EXPAND RAPIDLY when hot and CONTRACT RAPIDLT when cold, causing the breaking apart of layers of rock.
For example, in hot desert environments when days are extremely hot and nights are extremely cold (high diurnal range), the changes in temperature causes shear stress on the outer layers of the rock. This causes the outer layer of the rock to peel off in a process called exfoliation.
Where does disintegration occur?
Occurs in areas with large diurnal differences eg deserts. Excessive heating and cooling results in exfoliation occurring.
Salt Crystallisation (salt weathering)
Salt weathering occurs due to SALT CRYSTAL GROWTH inside the cracks and pores in rocks.
When SALINE solutions (i.e. salt water) gets into cracks and evaporates, it leaves behind salt crystals that were in solution. As these salt crystals accumulate over time, pressure builds up and expands the gaps in the rocks. Eventually this causes the rocks to DISINTEGRATE.
Pressure release
Pressure release is a type of mechanical weathering caused when rocks that are under a great amount of pressure no longer have to bare a heavy load, causing EXPANSION and FRACUTRING. For example, when rocks forms beneath the earth’s surface under great pressure, and when the load above is eroded away, the pressure is removed, and the rock expands.
Unloading
Unloading is the reduction in pressure on rocks by the removal of overlying rocks.
Vegetation root action
When vegetation grows, such as plants and trees, their roots also grow and thus need more space.
The growing roots exert pressure onto the surrounding rock, eventually causing the rock to break apart.
Hydrolysis
Hydrolysis is a form of chemical weathering that involves the breaking down of rock when it reacts with water (hydro-). Rocks can break down when they are SOLUBLE in water, for example chalk can react and break down in water
Hydration
It is a form of chemical weathering that occurs when water is absorbed into a substance, causing a new substance to form. For example, anhydrite (CaSO4) reacts with water to form gypsum (CaSO4 . 2H20). Hydration causes rocks to expand in SIZE and VOLUME, which can make them susceptible to other forms of weathering.
Carbonation
Carbonation is when carbon dioxide dissolved in water (known as carbonic acid) reacts with rocks and breaks them down. Carbonic acid reacts with many rocks. For example, when carbonic acid seeps into the cracks of limestone, the rock dissolves.
