External Forces and Limestone
PROCESSES OF WEATHERING
Denudation is the long-term sum of processes that cause the
wearing away of the earth’s surface leading to a reduction in
elevation and relief of landforms and landscapes.
Weathering is the breaking down of rocks, soils and minerals as
well as artificial materials through contact with the Earth’s
atmospheric , biota and waters. Weathering occurs in situ, or "with
Erosion refers to the processes which break up, transport and
deposit weathered materials and rocks. Its agents includes :
water, ice, snow, wind, waves and gravity.
Mass Wasting is the movement of weathered materials (with or
without water or ice) downslope by the force of gravity
The processes of weathering are facilitated by
two important elements of climate:
temperature and rainfall
The processes of weathering may be placed
into three categories:
1. Physical / Mechanical Weathering
2. Chemical Weathering
3. Biotic / Biological Weathering
The form of weathering results in the break-up of rock into
small pieces with no change in the chemical structure and
physical appearance of the rocks. This type of weathering is
most common in cold and dry climates. Physical weathering can
occur due to temperature, pressure, frost etc.
Certain frost-susceptible soils expand upon freezing as
a result of water traveling via capillary action to grow
ice lenses near the freezing front. This same
phenomenon occurs within pore spaces of rocks. The
ice accumulations grow larger as they attract liquid
water from the surrounding pores. The ice crystal
growth weakens the rocks which, in time, break up. It is
caused by the approximately 10% (9.87) expansion of
ice when water freezes, which can place considerable
stress on anything containing the water as it freezes.
A rock in Abisko, Sweden fractured along existing joints possibly
by frost weathering or thermal stress
Pressure release could have caused the exfoliated granite sheets
shown in the picture.
When rocks form under great pressure (volcanic rocks for example)
are exposed on the surface, the release of the pressure of overlying
layers causes them to expand, crack and breakup. Pressure release
forms areas of large broken blocks of rock.
Intrusive igneous rocks (e.g. granite) are formed deep beneath the
Earth's surface. They are under tremendous pressure because of
the overlying rock material. When erosion removes the overlying
rock material, these intrusive rocks are exposed and the pressure
on them is released. The outer parts of the rocks then tend to
expand. The expansion sets up stresses which cause fractures
parallel to the rock surface to form. Over time, sheets of rock break
away from the exposed rocks along the fractures, a process known
as exfoliation. Exfoliation due to pressure release is also known as
Chemical weathering changes the chemical structure and
physical appearance of the rock. The rock decomposes. It
occurs rapidly in hot , wet climates such as the Caribbean.
Limestone which is made of calcium carbonate is especially
vulnerable to the chemical weathering process known as
Rainwater absorbs carbon dioxide in the atmosphere to produce a
weak carbonic acid. This acid acts on the calcium carbonate in
limestone to form calcium bicarbonate. Calcium bicarbonate is
dissolved by water. This results in pits and holes common in
limestone areas and known as karst landforms.
The reactions as follows:
This is the process of oxygen being added to a
compound. Rocks rich in iron react with oxygen in water
to form iron oxide as in the rusting of iron. The effect is
a thin soft crust on the outer part of the rock that is
easily broken down.
This occurs in rocks containing salts such as
sodium and potassium that are soluble in water.
Rainwater falling on these rocks will eventually
Biotic/ Biological Weathering
Biotic weathering is the result of the physical or
chemical effects of plants and animals on a rock; they
may cause disintegration or decomposition of rock.
Plant roots may get into cracks and spaces of a rock and break it
apart as they grow. This is a physical process, leading to
disintegration of rocks.
These may release organic acids which chemically alter rocks
Biological weathering of lava by lichen, La Palma.
Activity 1. Answer the following questions
1. Explain how rocks are affected by
(i) ONE physical weathering process
(ii) ONE biotic/ biological weathering process
2.Give two characteristics of Limestone
3. Differentiate between chemical weathering and physical
4. Explain two ways in which weathering is affected by climate
5. State clearly the meaning of the terms- ‘weathering’ and