GY1004 Principles of Physical Geography B

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• GY1004 Principles of Physical Geography B
• Lecture 4
• Weathering products and process controls

DEPARTMENT OF GEOGRAPHY
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Weathering profiles
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The products of weathering
Physical weathering - rock fragments
Chemical weathering - material that is lost in
solution - the production of new compounds -
the residue of stable compounds
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The products of chemical weathering
1. Material that is lost in solution Na, K, Mg,
Ca ions taken up by plants flushed into
rivers, lakes and seas form precipitates/cements
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The products of chemical weathering

• New compounds
• (secondary minerals)
• Clay minerals

Compounds of iron and aluminium
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What are clays?
Complex hydrated alumino-silicates Formed through
the recombination of silica, aluminium and metal
cations during chemical weathering
processes Different clays result from the
composition of the circulating pore water the
mineralogy of the bedrock the intensity of
leaching Eh-pH conditions
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Clays and leaching
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The products of chemical weathering
3. The residue of stable compounds mainly
silica forms a large proportion of soils,
sandstones and other sedimentary rocks.
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Weathering profiles
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Weathering and Landforms
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Weathering and Landforms
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Karst

• Karst bare stony ground
• - limestone terrain characterised by a lack of
  surface drainage, a discontinuous or thin soil
  cover, abundant enclosed depressions and a well
  developed system of underground caves.

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Karst

• Develops best where
• limestone is pure
• limestone is thick, strong, massive
• climate is humid
• relief is high
• Classic Area - former Yugoslavia.

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Karren
1 m
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Dolines
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Tower Karst Guilin, China
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Factors influencing weathering
1. Climate 2. Parent material 3. Topography 4. Veg
etation 5. Time
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1. Climate
Intensity of chemical weathering in relation to
rainfall and temperature Peltier (1950)
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Variation in the depth of the weathered mantle in
relation to climate (Strakov, 1967)
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Intensity of physical weathering in relation to
rainfall and temperature (Peltier, 1950)
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Intensity of physical and chemical weathering in
relation to rainfall and temperature Peltier
(1950)
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2. Parent material
Mineralogical composition Physical
characteristics
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Proportion of area covered by major rock
types Shale 52 Sandstone 15 Granites 15 Li
mestone 7 Others 11
Mineralogical composition affects both the rate
and products of chemical weathering
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Physical characteristics (particle size,
porosity, permeability etc.) mainly affects the
rate rather than the products of weathering.
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3. Vegetation

• Release of organic acids
• Supply of CO2

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4. Topography

• Influences movement of water through the regolith

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5. Time
Duricrusts - hard layers formed in the weathering
zone at or near the land-surface as a consequence
of mineral accumulations.
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Summary

• The products of physical weathering (fragments
  of chemically unaltered rock) and chemical
  weathering (secondary minerals and the residue of
  resistant minerals) accumulate in the regolith.
• The accumulation of organic matter in the
  regolith leads to the development of soil.

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Summary

• In general, landforms related directly to
  weathering are minor and the relationship between
  lithology and landscape is weak.
• Certain lithologies, however, can influence
  landscape development to such an extent that
  distinctive landform assemblages can be
  associated with specific types of rock (e.g.
  landscapes of karst and granitic terrains).

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Summary

• Weathering rates and processes are controlled by
  five factors climate, parent material,
  vegetation/organic activity, topography and time.
• The factors influence the spatial distribution
  of soils, landforms and landscapes at a range of
  different scales, from the local to the global.