WEATHERING & EROSION
PHYSICAL WEATHERING
Physical weathering is more effective in areas which have:
~ a little vegetation
~ a large diurnal (day to night) range of temperature or
~ temperatures fluctuating around 0 degrees Celsius
Contact Forces
It happens when a force is applied to a rock and it breaks into smaller pieces. A larger rock falling onto another
rock below or wind whipping sand against other rocks thus breaking tiny pieces off (sand blasting) are examples of
contact forces.
Pressure Release
It takes place when the overlying rocks are removed by erosion. With the release of pressure, the rock expands.
causing stress within the rock. Cracks are formed parallel to the rock surface. Over a period of time, the outer layers
of the rock break away in sheets.
Exfoliation
It occurs in places like hot deserts where the day temperature often soars to 40 degrees Celsius and night
temperature falls to below 10 degrees Celsius. The repeated heating by day and cooling by night cause the outer
layers of the rock to expand and contract alternately. Stress is created in the rock, producing cracks and gradually,
the outer layers of rock will peel off.
Frost Shattering (Ice)
It occurs in cold temperate regions and mountain tops where temperatures fluctuate above and below freezing
point. Water fills the cracks or joints in the rock and freezes at night or during winter. Repeated freeze- thaw will
eventually shatter the rock along the cracks or joints into angular pieces. The broken materials collect at the foot of
the slope to form a scree.
Crystallization
or weathering by growth of salt crystals takes place in deserts where evaporation draws ground water containing
dissolved salts upwards into the pores of the rock. When the water evaporates , the salts are left behind as crystals.
The build-up of the salt crystals creates stress in the rock, weakens it and breaks it down into grains.
Biological Weathering
roots growing into the cracks and joints in rocks
earthworms and termites making tunnels in the ground
man building roads and cultivating the land , exposing the rocks to other agents of weathering.
CHEMICAL WEATHERING
Chemical weathering refers to the breakdown of an object into particles with a different mineral
composition than the original object. Water is perhaps the most powerful agent of chemical weathering: Over
time, it can dissolve many kinds of rocks into a solution that has a different chemical makeup than the original
substance. Other types of chemical weathering involve more complicated chemical reactions with oxygen,
carbon dioxide, water or other compounds.
The primary agents in chemical weathering are water, oxygen, and acids.
Oxidation takes place when oxygen reacts with earth materials. Oxygen dissolved in water combines with atoms of
metallic elements abundant in silicate minerals. Attacking metals in the soil, oxidation causes them to rust leaving the
soil a brownish red to red color. When oxygen combines with iron, the reddish iron oxide hematite (Fe2O3) is formed:
4Fe+3 + 3O2 -> 2Fe2O3
Hydrolysis is an exchange reaction involving minerals and water. Free hydrogen (H +) and hydroxide (OH)- ions in
water are able to replace mineral ions and drive them into solution. As a result, the mineral's atomic structure is changed
into a new form. It is a process whereby silicate minerals like potassium feldspar are weathered and a clay mineral is
formed.
2KAlSi3O8 + 2H+ + 9 H2O -> Al2Si2O5(OH)4 + 4H4SiO4 + 2 K2+
Hydration involves the absorption of water like which occurs during the conversion of hematite to limonite:
2Fe2O3 + 3H20 -> 2Fe2O3 . 3H20
Some geoscientists question whether hydration is a true chemical weathering process because the process is readily
reversible and the new product is not chemically different from its precursor. Some would rather call hydration a physical
weathering process.
Carbonic acid action involves combination of carbon dioxide and water. Though present in pure water, carbon
dioxide dissolved in water provides ions that produces free hydrogen. Carbon dioxide in the atmosphere combines with
rain water to form carbonic acid (H2CO3):
H2O + CO2 -> H2CO3
Though weak, when carbonic acid is combined with a mineral like calcite (CaCO 3) common to limestone, calcium and
bicarbonate ions are released and carried off by groundwater.
CaCO3 + H2CO3 -> Ca+2 + 2 HCO-3
Biological Agents secreting enzymes (such as bacteria, fungus, lichens, etc.) can release sufficient
amounts of enzymes that over time, they break down the rock.