Geography Reference
In-Depth Information
off the surface rock. Crystallisation occurs when temperature
increases lead to the growth of salt crystals. Moisture inputs then
cause the salt volume to increase and thermal expansion also occurs
to the salts upon warming. This type of weathering is more
common where coastal fogs can bring sea salts into desert areas
such as Namib desert in Namibia.
Thermal weathering may also be important in deserts where
daily temperature ranges may be high. This leads to the regular
expansion and contraction of rock. Different minerals expand by
different amounts when warmed. This creates internal stresses that
weaken rocks and loosen particles. Rocks can often crack if you
light a fire around them due to these stresses upon expansion.
Chemical weathering
Water acts as a solvent to dissolve rock. Rocks are made up of
bases
(calcium, magnesium, sodium and potassium), silica and
sesquiox-
ides
(mainly with aluminium). Silica, is at least ten times less soluble
than bases, but is at least ten times more soluble than the sesquiox-
ides. Therefore, chemical weathering reduces the proportion of bases
the most, followed by the proportion of silica. As material is exposed
close to the surface, the atmosphere is able to assist weathering of
minerals. The gases of the atmosphere such as oxygen, water vapour
and carbon dioxide aid weathering (e.g. iron and oxygen can
produce iron oxide (rust)). Small amounts of carbon dioxide from
the atmosphere dissolved in rainwater make a weak carbonic acid
which acts to weather rock. More intense rainfall combined with
warmer temperatures and higher carbon dioxide concentrations may
increase chemical weathering rates under climate change. However,
when the carbonic acid reacts with the rock in the weathering
process it produces other dissolved chemicals which then get trans-
ported away in solution. This can result in loss of the carbon dioxide
from the atmosphere into oceans via river channels and could poten-
tially act as a negative feedback on climate change.
Roots from vegetation put organic acids into the soil which
help them to extract the nutrients needed by the plant and this also
weathers soil. Many of the small animals in the soil (e.g. earth-
worms) pass soil material through their bodies, altering it both bio-
chemically and mechanically as they extract nutrients from it.
