Weathering is the breakdown of rocks through chemical and mechanical processes. There are two main types of weathering: chemical weathering involves changes to a rock's mineral composition, while mechanical weathering breaks rocks into smaller pieces without changing their composition. The rate and type of weathering depends on factors like climate, rock type, and rock structure. Limestone is particularly susceptible to weathering due to its permeability and solubility, which can form distinctive karst landscapes.

1. ASGEOGRAPHYREVISION
ROCKSANDWEATHERING
3.2 WEATHERINGANDROCKS

2. KEYTERMSANDDEFINITIONS
Biological weathering is the form of weathering caused by the
activities of living organisms. There are two types: chemical and
mechanical (physical).
Chemical weathering occurs with the release of organic acids as
vegetation decays.
Mechanical weathering includes the breakdown of rocks by the growth
of roots in joints and cracks.
Block disintegration is the breakdown of rock due to mechanical
processes along well-defined lines, often joints, to produce an end
product of large–medium angular pieces / blocks. It occurs in rocks like
granite.

4. KEYTERMSANDDEFINITIONS
Carboniferous limestone (karst) is formed of the remains of organic
matter, mostly plants and shells. Limestone scenery is unique because
it:
• is very resistant but also permeable, and
• dissolves very slowly in acid water after the process of carbonation because
the process is two stages: carbonation followed by solution.
Chemical weathering is the form of weathering brought about by
chemical attack of rocks, usually in the presence of water.
Chemical weathering involves the ‘rotting’ or breakdown of the
original minerals within a rock to produce new minerals. Some
chemicals are dissolved and carried away from the weathering source.

6. KEYTERMSANDDEFINITIONS
Denudation is the wearing away of the Earth’s surface. It includes
erosion, weathering and mass movements.
Freeze–thaw is the form of physical weathering, common in mountains
and glacial environments, caused by the expansion of water as it
freezes. Water in a crack freezes and expands in volume by 9–10 per
cent as it turns to ice. This expansion exerts great pressure on the rock,
causing the crack to enlarge. After many cycles of freeze–thaw, rock
fragments may break off to form scree slopes.
Granite is coarse-grained, intrusive, igneous rock, typically consisting of
the minerals quartz, feldspar and mica.

8. KEYTERMSANDDEFINITIONS
Granular disintegration is the result of mechanical weathering
processes which produce an end product of small particles because the
weathering affects the individual minerals in the rock.
Joint is a vertical crack in a rock, often formed by compression; it is
usually several metres in length. The weathering of joints in rocks such
as limestone and granite is responsible for the formation of features
such as swallow holes and tors.

9. KEYTERMSANDDEFINITIONS
Karst is the characteristic scenery of a carboniferous limestone region
with underground streams, caverns and potholes forming a drainage
system.
Mechanical (physical) weathering includes such processes as freeze–
thaw and exfoliation or onion-skin weathering. Mechanical weathering
leads to the breakdown of rocks but does not change the composition
of the rocks.
Permeability is a measure of the ability of a rock (or soil) to allow
liquids to pass through it. This can be through interconnected pores or
through joints, bedding planes and fissures within a rock.

11. Porosity is the percentage of pore space, or that volume within a rock
that can contain fluids.
Scree is angular sediment that collects at the foot of a mountain range
or cliff. The rock fragments that form scree are usually broken off by the
action of freeze–thaw weathering.
Solution occurs on rocks such as chalk and limestone that contain
calcium carbonate. When dissolved in rain, carbon dioxide forms weak
carbonic acid. This acid changes the limestone to calcium bicarbonate,
which is soluble, and can be carried away by flowing water.
KEYTERMSANDDEFINITIONS

12. Spheroidal weathering is a form of chemical weathering in which
concentric or spherical shells of decayed rock are successively separated
from a block of rock, which commonly results in the formation of a
rounded boulder of decomposition.
Tor is an isolated mass of rock, often granite, left upstanding on a hilltop
after the surrounding rock has been broken down and removed.
Weathering takes place along the joints in the rock, reducing the outcrop
into a mass of unweathered stones surrounded by weathered material.
Weathering is the process by which exposed rocks are broken down on the
spot by the action of rain, frost, wind and other elements of the weather. It
differs from erosion in that no movement or transportation of the broken-
down material takes place. Two types of weathering are recognised:
physical (or mechanical) and chemical. They usually occur together.
KEYTERMSANDDEFINITIONS

14. TOPICSUMMARY
Weathering is the decomposition and disintegration of rocks in situ.
Decomposition refers to chemical weathering and creates altered rock
substances.
Disintegration or mechanical weathering produces smaller, angular
fragments of the same rock, such as scree.
There are four main types of mechanical weathering: freeze–thaw (ice
crystal growth), salt crystal growth, disintegration and pressure release.
There are four main types of chemical weathering: carbonation-
solution, hydrolysis, hydration and oxidation.
The type and rate of weathering varies with climate.

15. TOPICSUMMARY
Rock type and rock structure influence the rate and type of weathering
in many ways due to chemical composition, the nature of cements in
sedimentary rock and joints and bedding planes.
Limestone scenery is unique on account of its permeability and
solubility in rain and groundwater.
The main processes to affect limestone are carbonation solution,
freeze–thaw, and river erosion underground.
Limestone features include clints and grikes, limestone pavements and
caves, stalagmites and stalactites.
Granite is an igneous, crystalline rock. It has great physical strength and
is very resistant to erosion.
Tors, a characteristic feature of granite, may be the result of
equifinality.

16. ADDITIONALWORK
1. Outline the importance of weathering.
2. ‘Climate is the most important factor affecting weathering.’ Discuss.
3. Examine the influence of rock type on weathering.
4. To what extent do geomorphic processes vary with climatic regions?

17. SUGGESTEDWEBSITES
Fundamentals of Physical Geography:
www.physicalgeography.net/fundamentals/10r.html
How you can use gravestones to find out about weathering:
www.es.ucl.ac.uk/schools/Pancras/intro_weathering.htm
Visit National Geographic for some stunning images of erosion and
weathering:
http://science.nationalgeographic.com/science/photos/weathering-
erosion-gallery/#/nevada-snakerange_309_600x450.jpg
A cartoon of weathering and erosion:
www.youtube.com/watch?v=lyysL02ZvQ8