2.2.1 Weathering

Things to Know:

Weathering is the disintegration and decomposition of rocks in situ. 

There are several types:

            -Physical weathering: weathering where there is no change in the chemical composition of the rock, due to physical processes:

Freeze-thaw aka frost shattering – occurs where there are cracked rocks and temperatures fluctuate around freezing point, repeated freezing and thawing causes the cracks to widen.

Associated landform: (screes)

          

Exfoliation aka onion weathering – occurs in very warm climates where there are exposed, non-vegetated rocks. The outer layers heat up and cool down faster than the inner layers causing stresses in the rock; the outer layer peels off.

Associated landform: (Ayer’s rock, Australia)

   

Granular disintegration: Like exfoliation this occurs where there are extremes of temperature as in a desert. If the rock is made of different minerals of different colours. The darker minerals will expand & contract more than the lighter ones. This will form small angular pieces of rock like sand which will collect at the base of the rock.

Biological weathering – roots widen weaknesses in the rock until part of the rock detaches

Associated landform:

   

Chemical weathering: occurs in warm, moist climates

•       Lichens produce organic acids that dissolve rocks so that they can absorb the minerals.

•       Rotting vegetation also releases organic acids that will chemically react with rocks.

Limestone solution – aka carbonation

   carbon dioxide in air reacts with rainwater;

   forms carbonic acid/acid rain;

   reacts with calcium carbonate/limestone or chalk;

   pervious/seeps through cracks;

   forms calcium bicarbonate/calcium hydrogen carbonate;

   which is soluble in water;

   widens/deepen cracks etc.

Associated landform: dry valleys and limestone pavement

Oxidation:

•       Many minerals in rocks can be oxidised in the presence of water.

•       One on the most common minerals to oxidise is iron. It forms iron(III) oxide (Fe₂O₃) or rust. This gives the rock a reddish brown colour.

•       The rock is weakened & eventually crumbles away.