Physical and Chemical Weathering

This document covers physical and chemical weathering topics for CSEC Geography.

1. Oral Johnson
External Forces- Weathering
4
th
Form CSEC Geography
Everywhere in the world rocks are worn away by many different processes. Depending on the
exact location the processes that operate vary because they are affected by climate, rock type
relief and slope angle. For example, rock types are different in hardness (resistance); the more
resistant ones wear away more slowly than softer ones. The hotter and wetter the climate the
faster weathering and erosion happen
Denudation
Denudation simply means the wearing away of the land by weathering and erosion. It includes
all natural agencies, for example sun, rain, frost, wind, rivers, sea, ice, temperature change and
even the actions of plants and animals. This set of major processes is responsible for the creation
of the Earth’s varied landscapes.
Weathering
Weathering is the wearing away (disintegration and decomposition) of rocks by the effects of the
weather and the atmosphere. No movement is involved in this, so the breakdown of the rock is
said to be ‘in situ’- in other words, ‘in that place’. Sometimes, after the break-up of the rock,
fragments are moved but only by gravity, for instance slipping down a slope.
Erosion
Water, ice and wind also wear away the earth’s surface. Water can mean either rivers or the sea.
Ice is in the form of glaciers. Wind erodes especially when it is carrying something to help it
wear away rock, usually sand. (Imagine how a blast of wind carrying sand would feel against
your skin!).
The difference between erosion
The key difference here is movement. In the case of weathering, no movement is involved
(remember the term in situ). The agent that weathers the rock does not move the debris. Any
movement of loose fragments happens due to gravity. However, in erosion, the agent breaking

2. up the rock also removes the debris. For example, the sea attacks cliffs and moves the fragments
out to sea or along the beach. A glacier erodes the rock it moves over and carries fragments
within it, depositing them many kilometers away when it melts.
There are three different types of weathering:
 Physical ( or mechanical) weathering
 Chemical weathering
 Biotic ( or biological) weathering

Physical weathering
Physical weathering – This involve rocks breaking apart but without any chemical change taking
place.
Frost shattering/Freeze-Thaw
Globally, frost shattering, or the freeze-thaw process, is the most important and widespread type
of weathering, although it is less common in the Caribbean due to this region’s tropical latitude.
Only the highest areas could be affected. For this type of weathering to occur the temperature
must fluctuate (change or vary) either side of 00C. There must also be bare rock exposed at the
surface, with little vegetation cover to protect the rock from the weather conditions.
If it rains during the day, or there is moisture from dew or melting ice or frost, water can trickle
into cracks, crevices or pores in the rock. Daytime temperatures are more than 00C. If at night
the temperature drops below zero, the water in the cracks or pores freezes. Ice take up 9% more
space than equivalent amount of water and so exerts pressure on the rock. Repeated freeze-thaw
conditions continue throughout the winter in the cool temperate zones of the world, slowly
widening the cracks until pieces of rock break-off. Frost shattering is most likely to happen on
steeper bare rock slopes, so the broken pieces will slip downhill easily under the force of gravity.
They collect at the bottom of the slope in a fan-shaped pile. This is called scree or talus. Where
the land is more level then boulders ad smaller stones litter the surface. These too have been
broken off by frost shattering. They are known as blockfields or felsenmeer.

3. In colder regions frost shattering is less common, as temperatures are too low; the fluctuation
around 00C does not happen very often. It rarely becomes warm enough, even during the day,
for melting to occur. In Polar Regions it is very unusual for temperatures even to rise above zero.


Temperature changes

4. Rocks expand when heated and contract when cooled. Regular heating and cooling occurs in
some climates. In hot deserts the diurnal range of temperature (that is temperature differences
within a 24-hour period) can be as much as 50
0
C
At midday over 400C is not uncommon, but at night temperatures may drop below freezing
(O
0
C). The outside layer of rock therefore expands and contracts regularly. This weakens it until
eventually it peels off. This process is known as exfoliation or onion peeling. After one layer has
broken off the next one is attacked by temperature change.
Rocks are made up of different minerals such as granite. These minerals can expand and contract
at different rates which causes the rock to disintegrate- a process known as granular
disintegration.

5. Pressure release
Pressure release also results in exfoliation, but here the cause is different. Rocks deep
underground, such as igneous intrusions for e.g. granite, have the weight of other rocks above
pressing down on them. The granite of Dartmoor in south-west England was formed 6km deep
below the surface, so had a huge mass of rock pressing down on it. Such pressure sets up stresses
within the lower rock. Over time, the upper rocks are weathered and eroded away, so the
pressure is reduced and the stresses are less. The rock expands as joints and other cracks open
up, almost as if they were breathing a sigh of relief! This widening of cracks is called dilation.
The cracks formed are roughly parallel to the surface, so the layers of rock peel away one at a
time, like the skin of an onion. This is called exfoliation. Pressure release is responsible for
causing large-scale rounded landforms, such as Half Dome in Yosemite National Park, USA. In
the Caribbean, pressure release has been partly responsible for some of the granite landforms on
Tobago.

Chemical Weathering

6. Chemical weathering is where a chemical change causes rocks to dissolve or decay.
Carbonation
Carbonation is the most important form of chemical weathering in limestone areas, whether they
are in tropical or temperate zones of the world. Rainwater containing carbon dioxide absorbed
from the atmosphere is able to dissolve calcareous (calcium–based) rocks. Carboniferous
limestone and chalk are the types of limestone affected most because they are made of almost
pure calcium carbonate.
Here are the stages of the process:
1) Rainwater dissolves carbon dioxide from the air as it passes through it
H2O + CO2 = H2 CO3
Rain water + Carbon dioxide = Mild carbonic acid
2) The mildly acidic rainwater reacts chemically with the calcium carbonate in the rock

H2 CO3 + CaCO3 = Ca (HCO3)2
Mildly acidic rainwater + calcium carbonate = Calcium bicarbonate
3) Calcium bicarbonate is soluble in water so it is easily washed away

The higher temperatures in tropical areas increase the speed of carbonation. Chemical reactions
are faster as temperature increases. Higher rainfall also plays a part because the rain is the
weathering agent.
Few minerals are soluble in pure water. It is important to understand than it is the weak acid that
allows this natural process of carbonation to take place. However, the carbon dioxide in
rainwater is not the only source of acid that can attack limestone. Any acid will attack calcium
carbonate, including those coming from industrial pollution. Nitrates and sulphates make nitric
and sulphuric acids and both of these can dissolve in rainwater too. The carboniferous limestone

7. of the Pennine Hills in Northern England may have been dissolved faster due to the pollution
from the nearby industrial areas in that part of the country.
The Cockpit country in Jamaica is a world-renowned Limestone area. Features such as cockpits,
cones, stalactites and stalagmites were formed due to carbonation. The high annual rainfall and
high temperatures combine to cause extremely rapid limestone weathering.
Solution
Solution simply means the dissolving of minerals in water. Rocks other than limestone can be
affected by solution weathering. Many minerals are soluble in water, especially if the water is
slightly acidic as explained in carbonation. The greater the acidity (the lower the pH) the more
effective the rainwater will be in dissolving minerals in rocks. Rock salt (halite) is one mineral
that is particularly vulnerable to this process and can be dissolved easily ‘in situ’.
Rocks made up of a mixture of different minerals can be weathered by one or more of them
being dissolved, leaving the rock structure weaker. Other agents of weathering and erosion can
then attack more easily.
Oxidation
Oxygen occurs when rocks are exposed to oxygen in air or water. It is chemical addition of
oxygen to compounds in the rock and it can weaken the structure of the rock. The process of
oxidation has a similar effect on iron minerals within rocks. The chemical changes turn the solid
iron minerals into powdery red or brown clay. This then causes the rock to crumble and break
apart. This weathering process is also known as rusting, and the rock discolours to the reddish-
brown colour seen on rusty metal.
Iron can combine with oxygen to make two different compounds, ferrous iron oxide (FeO) and
ferric iron oxide (Fe2O3). Ferric oxide clearly contains more oxygen per atom of iron than
ferrous oxide.
4FeO + O2 = 2Fe2O3
Ferrous iron oxide + oxygen = Ferric iron oxide

8. Evidence of oxidation can be seen in many regions of the world, particularly in hot deserts and in
parts of the tropics where iron-rich rocks are readily weathered in the warm and wet conditions.
Hydrolysis
Hydrolysis is the chemical breakdown of a substance when combined with water. The prefix
‘hydro’ means ‘water’ and the suffix ‘ lysis’ means to ‘break down’. With chemical weathering
of rock, we see a chemical reaction happening between the minerals found in the rock and rain
water.
The most common example of hydrolysis is feldspar, which can be found in granite changing to
kaolinite clay. When it rains, water seeps down into the grounds and comes in contact with
granite rocks. The feldspar crystals within the granite react with the water and are chemically
altered to form clay minerals, which weaken the rock.
Biotic weathering
Biotic weathering refers to the role plants and animals play in breaking down rocks. Plants and
animals can promote both the mechanical and chemical breakdown of rocks.
Biotic (physical)
By plants
Plants can grow anywhere as long as there is water. Plant roots can enter joints or cracks in order
to find moisture and nutrients. As the tree grow the roots become larger. Overtime when the root
size increase it exerts pressure or force on the crack. Overtime the small joint/crack will become
wider and deeper. This will eventually cause the rock to break apart.
You may have observed before tree roots can split into sections or driveways sufficiently to raise
and crack the concrete.
By animals
a) Burrowing animals like rabbits, moles, earthworms and even ants can contribute to biological
weathering. These animals can move rock fragments to the surface. As a result, these fragments
become more exposed to other environmental factors that can further enhance their weathering.

9. b) These burrowing animals also create holes which makes an easy passage of water and other
weathering agents deeper into the soil into the ground where it can cause further disintegration of
rocks.

Biotic (chemical)
Animals living on the surface of the rocks on the coast secrete chemicals that dissolve rocks.
Some micro-organisms get nutrients by taking minerals from rocks. By removing these minerals
the rock becomes weaker. An animal called the Piddock shell drill into rocks in order for it to
protect itself. The Piddock shell secrete an acid that helps to dissolve the rock. Overtime the rock
gets weaker due to this process.
Another example is Lichen. Lichen is fungi and algae living together in a symbiotic relationship.
The fungi release chemicals that break down the minerals in the rock. The minerals released
from the rock are consumed by the algae. Over time the rock becomes weaker due to this
process.