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2.2.1 Weathering Processes in the Tropics (Chemical Weathering…
2.2.1 Weathering Processes in the Tropics
Weathering
The chemical alteration and mechanical breakdown of rocks and sediments at or below the earth's surface when they are exposed to temperature fluctuations, air, moisture and organic matter
Weathered rocks break up to form
regolith
(A layer of loose broken, rocky material that covers the surface of unweathered bedrock)
The boundary between weathered and unweathered rock is
basal surface of weathering
Weathered materials remain
in situ
(at the original site) and can only be transported away by agents of erosion;
wind and water
or by
mass movement
Strength
of weathering varies in different climatic regions
Physical Weathering
Salt Crystal Growth
(Salt Weathering)
Most effective in arid coastal locations
Occurs in
hot deserts and salt-rich
environments, located at cliff foots where
groundwater seepage is concentrated
or in
coastal locations
where pores and joints in the exposed cliffs can trap sea spray and NaCl crystals can grow
High temps lead to high evaporation, leaving salts accumulated on the surface
Salt solutions are constantly drawn through permeable rocks by
capillary action
to the surface
These solutions precipitate in the rock pores during slow evaporation and expansion of salt crystals through hydration
Due to repeated and regular volume changes from repeated precipitation and crystallisation, stress applied to joint boundaries causes granular disintegration resulting in
honeycombed surfaces
Requires porous rocks that can expand or else process cannot take place (Eg: Sandstone / Limestone)
Pressure Release/ Dilatation
Occurs in areas where surface layers of rock are eroded substantially over time, causing subsurface rocks to be exposed
Large-scaled process and does not result in direct weathering, but
aids other weathering processes
When surface rocks are weathered and removed by erosion, exposed underlying rocks will experience a
fall in pressure
and expand, as compared to its previous state of stability confined under pressure by its overlying rocks
Sheet joints would appear near and parallel t the surface of the exposed rocks which can aid other processes to peel off the rocks in layers (insol weathering)
Insolation Weathering
(Thermal Weathering)
Occurs in
hot desert regions with extreme temperature fluctuations
(large diurnal range)
Rocks are poor conductors of heat so at high temps, the outer layer of rock heats up faster and expands more rapidly than its inner layers
(vice versa for inner layers where outer cools and contracts more rapidly)
Eg: A ↑ in temp of 83°C in a sheet of granite 30m in diameter would cause it to expand 25mm. A fall in temp of the same amount leads to a corresponding contraction
A
sharp thermal gradient
forms and stress builds up within the rock so joints
(both parallel; sheet joints, and perpedicular)
are formed
These joints facilitate the removal of the outer surface layers of the rock, causing the rock to peel off layer by layer over time as
concentric shells
, leaving behind successively
smaller spheroidal bodies
[Process is called exfoliation]
Rock composition and exposure of rocks (little vegetation) can cause this to occur
Can also occur when rock is composed of
different coloured mineral grains
, eg: when granite is exposed to insolation weathering, the dark mica crystals heat up more than light coloured feldspar or quartz
Leads to
differential rates of expansion and contraction
thus leading to
granular disintegeration
:red_cross: However, rocks are bad conductors of heat so the effects of differential heating can only penetrate a few mms into the rock so exfoliation only occurs on a
small and shallow scale
. Griggs also subjected rock specimens to temp changes of
110°C
equivalent to
244 years
of diurnal heating and cooling, where no detectable disintegration was observed. When cooled by water, surface cracking was observed after
2.5 years
of weathering
Biophysical Weathering
Faunal
Animal burrowing
provides an easy path for water to reach the rocks underneath
In humid regions, burrowing is done by macro soil organisms
earthworms
Micro organisms like lichens excrete humic and organic acids
that promote
hydrolysis
and
chelation
but also retard
carbonation
due to the oxalic acids they produce which can react with limestone to produce an insoluble coating that protects the limestone from dissolution
Floral
Tree Root Wedging/ Root Penetration
occurs when tree roots break up rocks by wedging into rock cracks and enlarging them as the roots grow longer and thicker
Occurs in places with
vegetation
, especially where its dense, producing blocks or smaller fragments of rocks
Freeze-Thaw Action
(Frost Wedging)
Occurs in places where
temp fluctuates frequently above and below 0°C like high altitude mountains
In the warm season, rain falling on glaciated mountains which contains dissolved Co2 provides weak acids which can attack rock minerals on the mountain
Water derived from thawing of snow contains lots of dissolved Co2 as solubility of Co2 increases with lower temperatures, thus ↑ the effectiveness of reactions involving
carbonation
At low temperatures, water freezes in rock cracks, forming ice wedges and volume ↑
[Temp must be at least
-5°C
and freezing must be
rapid
and
frequent
]
These ice expands, widens and deepens the cracks, weakening it
This maximum pressure generated (2115 kg/cm2) is easily able to exceed the tensile strength of rocks
The rocks break down as blocks or as coarse angular fragments which accumulate as a
talus cone
at the foot of slopes
Chemical Weathering
Oxidation
Reaction of a mineral with atmospheric or soil oxygen dissolved in water to form oxides or hydroxides
Minerals are commonly Fe, Manganese and Sulphur
Weathered materials are loosened from main rock and weakens the structure of the rock, producing
crumbly iron oxide grains
Often accompanies other processes of chemical weathering as it operates most effectively with water
Reduction
Reverse of oxidation , removes oxygen and is important in
changing soil colour
to grey, blue or green as ferric iron is converted to ferrous iron compounds
Occurs when there is excess water or water logged conditions
Hydrolysis
Affects minerals in igneous and metamorphic rocks (Eg:
Granite
)
Hydrolysis of feldspar in granite produces KOH (carbonated and removed in solution) and alumina-silicic acid (broken down into clay minerals)
End product: residual clay
Dissolution
Occurs when plain water comes into contact with dissolvable rock minerals creating a solution
Solution leaves the rock bringing along with it the dissolved rock minerals, weakening rock structure and enlarging rock joints
pH of groundwater
determines solubility of minerals (highly alkaline - ↑ silica solubility , highly acidic - ↑ alumina solubility)
Hydration
Occurs
when rock minerals absorb water
and chemical changes occur
Rock ↑ in volume and causes stress within the rock, resulting in the rock breaking down over time
Results in no more than a change in colour in some rocks
Carbonation
The mildly acidic solutions formed when Co2 dissolves in rainwater reacts with Calcium Carbonate in limestone to form soluble bicarbonates
The limestone is dissolved and carried away as solution, into underground caves where with a change in temp , its minerals can be precipitated out of its solution to form limestone features
Acts only on limestone rocks
Biochemical Weathering
Chelation
A biochemical process which ↑ the solubility of iron compounds before allowing them to be leached from the regolith or absorbed by plants
Occurs when metallic cations in rocks and minerals are removed by organic acids
When plants decompose, organic acids like humic acids are formed which facilitates the breakdown of rocks
Respiration
By plant roots or living organisms underground can produce Co2. This can form carbonic acid which will react with rock minerals and break them down
The physical breakdown of rocks into smaller pieces
Associated with differential expansion due to temperature fluctuations
Only affects exposed rock surfaces
Regolith resembles original rock in colour and texture and are normally blocky, concentric shells, angular fragments or granular sediments
In Tropics , mostly high altitudes where temp fluctuates and in tropical deserts (Bwh) with large diurnal temp ranges
Descriptions of a Weathered Product
Colour
Texture: coarse, fine-grained, honey-combed
Lustre: shiny, glassy
Shape: poorly/well rounded, angular
Size: large, small
The breakdown of rocks by the chemical alteration of original rocks/ minerals by agents like water, oxygen and mild acids
Affects both surface and subsurface rocks
Can occur anywhere as long as there is
water
and
temp >0°C
Rate of CW ↑ with ↑ temps or rainfall (Speed of a chemical reaction ↑ 2.5x with each ↑ in 10° of temp)
May alter the
composition
of rock materials,
volume
of rock and strength/coherence of rock
Regolith differs from original rock mass in colour and texture
Weathered materials generally softer, composed of
clays and sand
, oxides, quartz. Some in solution form like calcium salts
Unweathered rocks remain as corestones
(large unweathered resistant boulders)
Attacks some minerals
but not in crystalline rocks
, it is either concentrated at the rock surface or along joints and bedding planes where agents such as water or air can easily penetrate
Physical and chemical
complements each other
and often operate together
Physical ↓ size of particles, thus ↑ SA for cw to take place
Chemical creates stress and ↓ strength of rock making them more susceptible to pw
Requires rock examples
, which process requires which kind of rock
Sedimentary gets weathered faster than igneous and metamorphic as it has more joints
In Arid, weathering is affected by
Lack of Vegetation (exposed land)
Rock composition
Climate: Temperature Fluctuations (Insol)
Surface Characteristics (Thin soil layer)
(PW) Common Processess:
Insolation Weathering
Salt Crystal Growth
Biophysical Processes
In Humid, weathering is affected by
Seasonality of rainfall (monsoons)
Frequency of rainfall
Surface Characteristics (Thick Regolith and Soil layer)
Dense vegetation (helps water infiltrate to cause subsurface CW)
Rock Characteristics
Common Processes: (CW , some PW)
Freeze-Thaw Action (High altitudes & water)
Salt Crystal Growth (Coastal)
CW like Hydrolysis