2.2 Landforms and landscape processes

2.2.1: Weathering

Weathering Process

Definition:

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 the 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

  
  

  

  Exfoliation Process

  
  

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 and contract more than the lighter ones.

• This forms small angular pieces of rock like sand, which collect at the base of the rock.

  

  
  

Biological Weathering

• Roots widen weaknesses in the rock until part of the rock detaches.

• Associated landform: (not specified)

  

  Biological Weathering Process

  
  

Chemical Weathering

Occurs in warm, moist climates.

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

2. Rotting vegetation releases organic acids that chemically react with rocks.

Limestone solution (aka carbonation)

• Carbon dioxide in the air reacts with rainwater, forming carbonic acid/acid rain.

• Reacts with calcium carbonate (limestone or chalk).

• Water seeps through cracks.

• Forms calcium bicarbonate (calcium hydrogen carbonate), which is soluble in water.

• Widens and deepens cracks.

• Associated landform: dry valleys and limestone pavement

  
  

Oxidation

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

2. One of the most common minerals to oxidise is iron, forming iron(III) oxide (Fe₂O₃) or rust. This gives the rock a reddish-brown colour.

3. The rock is weakened and eventually crumbles away.