Weathering
Weathering is a general term describing all of the changes that result from the
exposure of rock materials to the atmosphere. It is also the breakdown and
alteration of rocks at Earth’s surface through physical and chemical reactions with
the atmosphere and the hydrosphere.
Types of weathering processes
Physical weathering
Physical weathering is the mechanical fragmentation of rocks from stressacting on
them. Ice wedging may be the most important type. Physical weathering is caused
by physical changes in the environment.
Insolation Weathering/ Thermal expansion (exfoliation)
The Process: A rock expands when heated and contracts when cool. Outer layers of
the rock heat up and cool down more rapidly than the inner ones, resulting in
stresses set up in the layers. Repeated differential heating and cooling sets up more
stress within the boulder, causing the outer layers to peel off like the layers of an
onion(Exfoliation: Sheeting).
Conditions: It usually happens in deserts, for the day is intensely hot and the nights
very cold. There is also minimal cloud and vegetation cover there, exposing the
rocks to the elements. The diurnal range of temperatures can exceed 50 degrees
Celsius. It also occurs where rocks splits along joints/cracks/lines of weaknesses due
to/resulting in block disintegration, where rock composition/minerals are of diff
colours, rocks will disintegrate by granular disintegration. It is found most commonly
in well-jointed rocks such as basalt and granite. Outer sheets are also easy to erode
which helps perpetuate the process.
This exfoliation process creates arch-shaped and dome-shaped features on the
exposed landscape
Example: Rock bursts in mines is evidence of rapid expansion of rock by pressure
release
Wind, rain and waves
The wind can blow tiny grains of sand against a rock. These wear the rock
away and weather it. Rain and waves are pretty self explanatory.
Freeze-thaw/Ice wedging
The Process: As the day becomes warmer, water enters the joints of rocks,
which then freezes at night. Water expands by 9% when it freezes into ice. This is why
water pipes sometimes burst in the winter. If water from rain or snow gets into a
cracks/bedding planes/other openings in a rock and then freezes, the expanding
wedge forces the crack further apart exerting great pressure on the rock walls,
similar to the pressure produced by driving a wedge into a crack. This occurs quite
often in areas where temperatures fluctuate around 0 degrees Celsius. Alternating
freeze thaw or frost shattering slowly widens the joints and in time, cause piece of
rock to shatter away from the main body.
This block disintegration occurs on steep slopes. The large angular rock
fragments gather, forming talus cones and screes at the foot of the slope. The
process usually occurs in crevices and joints of rocks where there is limited
vegetation cover and is the most widespread form of mechanical weathering. It is
common in upland regions of temperate regions like Britain where temperatures
fluctuate around the freezing point for several months in winter. However, this does
not occur in Polar Regions as temperatures rarely rise above 0 degrees Celsius.
Example: The effects of ice wedging in the Teton Range in Wyoming are seen
in both the rugged surface of the mountain peaks and the accumulation of
fragmented debris at the base of the cliff. The rock that forms the mountain range is
massive granite cut by numerous fractures. Ice wedging, controlled in part by the
fractures, produces the sharp, angular texture of the mountain peaks. The debris
derived from ice wedging has accumulated in conical slopes near the base of the
cliff.
Conditions that allows ice wedging:
1. Adequate supply of moisture
2. Pre-existing fractures, cracks or voids
3. Located where temperatures fluctuates beyond and before the freezing
point
a. Impt as stress applied with each freeze.
Ice wedging is most effective where water is permanently frozen. It occurs
more frequently above the timerline and is especially active on the steep slopes
above valley glaciers where melt water produced during the warm summer days
seeps into cracks and joints before freezing in the night.
Salt Crystallisation  Tafoni
The Process: In arid regions, dry weather draws moisture to the surface of the
rocks, encouraging the growth of salt crystals in pores and cracks can also pry apart
rock. When water that enters the rock begins to evaporate, dissolved minerals in the
water grow crystals. Can occur next to salt lakes or just the big ole sea itself. As the
crystals grow larger, they exert a force great enough to disintegrate the rock. Rocks
like sandstone are broken down by granular disintegration. This process is vividly
expressed in the shattering of fence posts near the shore of the Great Salt Lake.
Tafoni: Tafonis are honeycomb weathering pits that are commonly
associated with salt weathering. Basically its holey.
Common locations:
1. Deserts
2. Semiarid regions
3. Near coasts where sals can precipitate easily
Pressure Release/Unloading
The Process: Intrusive igneous rock formed when magma cools and solidifies
deep within the ground. (The slow cooling of the magma produces coarse-grained
crystalline granitic rocks.) They are under great confining pressure from the weight of
thousands of meters of overlying rocks. As the landscape is subjected to uplift, the
regolith overburden is weathered, eroded and transported away, eventually
exposing the granite. As the tremendous weight of overburden is removed from the
granite, the pressure of deep burial is relieved. This release in pressure weakens the
rock, allowing other agents to enter it and other processes to develop. Where cracks
develop parallel to the surface, layer after layer of rock peels off in curved slabs
(sheeting ). This exfoliation process creates arch-shaped/dome shaped features on
the exposed surface exfoliation domes.
The same process occasionally causes rock bursts in mines and tunnels, when
the confining pressure is released during the tunnelling operation. It can also be seen
in many valley walls and in excavations for roads, where rock slumping, due to
sheeting, can cause serious highway problems.
Funfact! Talus
The products of physical weathering are best seen in high mountain country, where ice
wedging dominates and produces a large volume of angular rock fragments. This material
commonly accumulates in a pile at the base of the cliffs from which it was derived. Because
most cliffs are notched by steep valleys and narrow ravines, the fragments dislodged from the
high valley walls are funnelled through the ravines to the base of the cliff, where they
accumulate in cone-shaped deposits.
Talus cones are built up by isolated blocks loosened by physical weathering. The blocks
commonly fall separately, but large masses of the material on steep slopes may be moved by
an avalanche.
Earthquakes may also suddenly activate large numbers of blocks loosened by many
seasons of ice wedging.
Hydration
The process: A process involving water, but little chemical change. IN this process,
water becomes part of the chemical composition of the mineral such as gypsum,
which is hydrous calcium sulfate. When some minerals absorb water, they expand,
creating a strong mechanical effect that stresses the rock, forcing grains apart.
They also work together in Carbonation and Oxidation to convert feldspar, a
common mineral in rocks, to clayminerals and silica.
Biological weathering  Physical and Chemical Weathering
Burrowing animals mix up the soil and loose rock particles which promotes
further break down by chemical means. Lichens grow on the surface of bare rocks
and extract nutrients from its minerals by ion exchange, causing physical and
chemical alterations to the rock.
Humic acid from the decomposition of vegetation is released by a process
called chelation
The action of bacteria and respiration of plant roots increases the CO2 levels
which accelerates the solution process, esp carbonation
Note: presence of vegetation cover significantly reduces the extent of
mechanical weathering
Human economic activities release more carbon dioxide, sulphur dioxide and
nitrogen oxides into the atmosphere. These gases then form acids in solution in
rainwater. Acid rain readily attacks limestone and to a lesser extent sandstones,
contributing to chemical weathering.
Humans are one of the most important geomorphic agents and causes of
mass removal of materials. While critters like termites or prairie dogs excavate below
the ground, human’s pollution, waste and mining/quarrying digs deep into the
ground and helps speed up weathering. (eg. Bingham Mine)
The waste of humans and animals also adds to the weathering of the area, as
most of them are slightly acidic and can undergo solution or carbonation.
Tree and plant roots often considered as wedges. But, roots follow paths of
least resistance, so probably not that important. However, it can burrow through
decaying rocks to speed up the weathering process  Roots growing through
pavements and uplifting them. Trees swaying in the wind might have some prying
capability as well.
Chemical weathering
Chemical weathering is the breakdown of minerals by chemical reactions
with the atmosphere or hydrosphere. Rocks are decomposed and the internal
structure of the minerals destroyed for the coming of new materials. This results in a
significant change in the chemical composition and physical appearance of the
rock. They attack minerals selectively, and usually occur in places of alternate
wetting and drying.
The major types of chemical weathering are dissolution, acid hydrolysis, and
oxidation.
Dissolution
Dissolution is a process whereby a mineral passes completely into a solution,
like salt dissolving in water. Some minerals dissolve directly in water and the ions
leached/flushed away.
Halite (salt) is best known example.
Gypsum is not as soluble, but it easily dissolves by surface water.
Most of large outcrops of minerals occur in non-humid regions as water is the
most effective and universal solvents known. The structure of the water molecule
requires the 2 hydrogen atoms to be positioned on the same side of the larger
oxygen atom, giving it a concentration of positive charges on one side and a
negative change on the opposite. Hence, the molecule is a polar and behaves like
a tiny magnet, loosening the bonds of ions at the surface of the minerals it comes in
contact with. Due to the polarity of the water, pretty much all minerals are soluble to
a certain extent in water, though ionic bonds dissolve more easily.
Carbonation and Solution
Carbonation and solution occur when a mineral dissolves into solution, with
water being the universal solvent.
Carbonation = Minerals dissolved by carbonic acid. Water readily dissolves
carbon dioxide, thereby yielding precipitation containing carbonic acid (H2CO3).
Carbonic acid preferentially dissolves certain rocks and minerals like limestone,
marble.
All rain is mildly acidic (average pH ~5.6) but the pH decreases significantly
with the addition of pollutants generated from the burning of fossil fuels. This more
acidic solution is termed acid rain and typically occurs downwind from large
industrial cities or from coal-burning power plants. (Sulfur monoxide and Nitrogen
dioxide mixing into the water vapour in the rain that condenses into acid rain)
Solution= minerals dissolving into the water
A prominent product of carbonation and solution would be the Karst
Landscape.
Karst Topography
Carboniferous limestone is well-jointed and bedded, which results in the
development of Karst topography. It is a landscape that is characterized by
numerous caves, sinkholes, fissures, and underground streams. Usually, it forms in
regions of plentiful rainfall where bedrock consists of carbonate-rich rock, such as
limestone, gypsum, or dolomite, that is easily dissolved. Calcium carbonates
dissolves and is removed during solution by running water. Thus, surface streams are
usually absent from karst landscapes.
Features of Karst Landscapes
Limestone Pavements: Clints and Grikes (Temperate)
Clints = slabs.
Grikes = lines of weakness  vertical depressions
Clints and grikes form under relatively deep cover of soil where water, carrying
carbonic acid (from carbon dioxide dissolved In water and organic acids from
decaying vegetation) pick at the joints. Carbonic acid reacts with limestone
pavement, causing a chemical change. Calcium bicarbonate is removed by
solution along the joints. As the process repeats itself overtime, the joints will deepen
and widen. It can be as wide as 0.5m and as deep as 2m.
Overtime, the soil on the top of the limesone platform will disappear down the
newly eroded grikes and taken away from the tops of the crints. Some of the
materials will be washed deep into drainage systems of the pavements through
connecting fissures, leaving open grikes of a metre or more in depth.
Process was increased when forest clearance and grazing was introduced.
Limestone Peak Forests and Clusters (Tropical)
Limestone peaks are the giant teeth like protrusions in karst landscapes. When
there is a piece of hard compact carbonate rock that experiences strong uplift
during the monsoon climate of high moisture, these are formed. The area must not
be plagued in glaciers for this to happen.
Peak forest = isolated towers
Peak cluster = linked-base towers
Examples: China, Guilin
Stalagmites and Stalactites
Stalagmites = stuff that grows from the ground up
Stalactites = stuff that grows from the ceiling down
Columns = when it extends from the ground to the ceiling
These are just calcium deposits. As the water carrying the calcium carbonate
evaporates on the ceiling (stalactites), or drips to the ground before
evaporation(stalagmites), the calcium deposits in it is left behind. Over time, the
accumulation of calcium forms these structures.
Sinkholes and fissures
Sinkholes are collapsed chambers. There was once a top soil. However, as the
limestone below it experiences weathering, after some time it could no longer
support the soil on top, causing it to collapse in on itself. Boomz
Advantages and Disadvantages
Advantages: Construction and Tourism
1. Limestone can be used as building material cement/mine safety
dust/glass/animal feed filler/limestone tile
2. Paint pigment
3. Bring in tourists/ Recreational purposes
a. Umpherston Sinkhole: The Sunken Garden
4. Water source wells and springs aquifers ssupply water
5. Unique ecosystem
6. Limestone agriculture (mostly livestock)
Disadvantages
1. Possibility of popping into a sinkhole
2. Unstable land and irregular space stuff moving around constantly
3. Drainage issues
a. Pipes and underground hazards
4. Hummocky terrain makes it hard to build things on it
5. As water travels through the limestone, it makes it very alkaline 7-14 pH
tastes gross apparently
a. Infertile farmland (hence mostly livestock)
Limestone landforms
Tropical
Closed
Cockpit karst, tower karst
depression
Temperate
Dolines, uvalas
Karren
features
Pinnacles (spitzkarren)
Caves
Less well-developed
Limestone pavements
(need glacial / lateral erosion)
More well-developed
(higher rainfall, but not much (drizzle)
seeps in to form caves / enter
Stalagtite (more spread out)
ground. Most flow off to rivers)
Stalagmite (more concentrated)
Drainage
Shallow holes (resurgence stream) + dry valleys + blind valleys
*Cross question between geomo and hydro: drainage density falls
because stream length falls but area of drainage still the same
Acid Hydrolysis
Naturally (slightly) acidic water erodes rocks.
Carbonic acid is common in natural environments. It can be created when:
1. Water combine with carbon dioxides in the atmosphere and in the root zones
of plants where carbon dioxide is released into the soil
2. Bacteria in the soil combine oxygen with decaying organic materials. Water
seeping through organic remains become more and more acidic, thus
increasing its effectiveness as a weathering agent
3. Human activities
a. Sulfuric and nitric acid in acid rain
b. Sulfuric acid from mining coal or sulphide materials
Effects of these acids are seen in the corrosion of buildings and acidification
of lakes and rivers and occasionally in the destruction of their biota.
Hydrolysis = chemical reaction where water and another substance
decompose into ions in water. It can occur in pure water but in nature, it usually
includes reactions with acids. The reaction between a mineral and an acid is called
acid hydrolysis.
A good example of the production of secondary minerals is the chemical
weathering of feldspar. Feldspar is an abundant mineral in a great many igneous,
metamorphic, and sedimentary rocks. It is therefore important to understand how
feldspars weather and decompose to make clay minerals. In turn, these clay
minerals are transported and deposited to form the most abundant sedimentary
rock, shale (or, strictly speaking, mudrocks).
Oxidation
Oxidation is the chemical combination of oxygen, in the atmosphere or
dissolved in water, with certain metallic elements to form oxides Oxygen reacts with
iron in minerals to form iron oxide minerals, e.g., hematite (rust), that give rocks a red
or yellow coloration. Oxidation causes rocks to crumble more easily
Of the elements that have variable charges, iron is the most important in
weathering reactions on Earth. In most silicates, iron is present as Fe2+, but in the
presence of Earth’s modern oxygen-rich atmosphere,Fe3+ is the favored oxidation
state. Therefore, oxidation is especially important in the weathering of minerals that
have high iron content, such as Basalt.
Most alkali (e.g., Na and K) and alkaline earth (e.g., Ca and Mg) elements
are removed into solution by weathering reactions (Table 10.1) and eventually
become enriched in seawater. On the other hand, the solid mineral residue
becomes enriched in Al, Si—incorporated in clays—and Fe— incorporated in oxides.
These minerals are stable in the surface environment
Comparison: Limestone vs Granite
Class:
formation
Limestone
Granite
Sedimentary
Igneous (intrusive) (pressure
release – exfoliation)
Non-clastic
- Crystallisation of magma
- Calcium from decomposition of
marine organisms
- Granitic (viscous) magma
- Gaps filled in with mud and
clay
- Lithification: cementation and
compaction and drying
Chemical
>50% generally calcite / calcium
composition carbonate (carbonation and
solution): homogenous (little
regolith left behind) but pure
limestone should be >90%
50-90%: impure limestone
Calcite: natural cementing
agent so not all rocks with
calcite are limestones
Rock
structure
*Only for carboniferous
limestone
High secondary permeability
- Bedding planes / joints / faults
(selective weathering – block
disintegration)
- Rock texture – coarse-grained
(phaneritic) because more time
for crystals to grow before
solidifies into rock
Feldspar (hydrolysis – clay),
quartz (sand – granular
disintegration) (clay, sand and
corestones due to spherodial
weathering = residual debris):
heterogenous (insolation
weathering – granular
disintegration) (many types of
elements)
Mica, biotite
High secondary permeability
- Joints / faults (selective
weathering – block
disintegration): sheet joints,
shrinkage joints: during formation
stage of rock cooling process
during formation of granite
- Shrinkage joints: drying –
tensional (folding) / shear joints
- Older: probability of
experiencing tectonic forces
Low primary permeability
- Small pore spaces: water
Low primary permeability
- Crystals close together and
interlock during crystallization
cannot penetrate through easily
- Older: more compaction and
cementation – less pore spaces
Physical
hardness
Hard (not prone to physical
weathering) – compaction
Hard (not prone to physical
weathering) – feldspar and
quartz – hard minerals and
interlock tightly
Spheroidal Weathering
Spheroidal weathering is a form of exfoliation.
There is a universal tendency for rounded surfaces to form in decaying rock as
sharp edges and corners of blocks tend to weather more quickly than flat surfaces. It
could be because rounded shapes have the least amount of surface area per unit
volume. This is especially noticeable when the parent rock fractures into a blocky
framework. Unweathered cores are called corestones.
The main chemical processes involved are hydrolysis and oxidation.
Weathering attacks all the edges and corners of the block of rock into a sphere or
ellipsoid, causing it to reduce in size. Due to its nature, Spheroidal weathering can
occur both above and below the ground level.
An excellent example will be that of Granite.
Granite Landforms: Tors, Inselburgs and More
Tors: large free-standing residual mass (rock outcrop) that arises abruptly from
the ground in a smooth gentle sloping surroundings of a rounded hill summit/ridge
crest.
Tors vs Inselburgs  Tors = temperate climate  inselburgs = arid/semi-arid climate
Formation:
1. Decay begins underground. Granite is made of feldspar, mica and quartz,
which forms clay (hydrolysis), clay (oxidation) and more quartz (sand)
respectively when weathered. Deep weathering occurs when the climate is
wetter and hotter. Decaying vegetation can also contribute through organic
acid weathering.
a. Ground surface lack vegetation cover to stop the rain from entering
the ground
2. Allows rainwater to percolate through the unexposed granite as though
granite is non-porous, it is well jointed, and therefore permeable
a. Water can go through well-jointed granite
b. Joints formed during cooling and contracting
3. Chemical weathering (hydrolysis and oxidation) takes place.
a. Widens joints form rectangular blocks and corestones
4. Surrounding rock (clay/regolith) removed by mass movementwashed away
by rain etc
a. Exposes more resistant outcrops of granite tors
b. Granite is formed with high temperature and pressure at great depth.
Release of burden decreases pressure, allowing for expansion of the
joints.
5. Spheroidal weathering^. Granite rounded and form corestones.
a. Granite still connected together despite being broken down.
Why would some parts experience Spheroidal weathering faster than others?
1. Experiences more water running down it  more percolation  increased
erosion and weathering
a. Other rocks are parallel to rainfall and hence, harder to weather by
the falling rain
2. Rocks at the area is more jointed/angular
a. More surface area increases the rate of weathering
Funfact!
Deep weathering profiles of decaying can’t usually be seen. Sometimes, the cross
section is exposed because the construction of roads made the people do road
cutting, which chops up the road and reveals the profile.
Singapore’s top soil is only 5cm thick! Everything below is the regolith.
Decaying Rock brown reddish brownpinkish/orange-ishwhite
Factors affecting weathering
Factors:




Rock composition
o Permeability/ porosity of rocks
o Stability of rocks
o Soft/hardness of rock
o Susceptibility of rock
o Type of grains
o No. of joints/fractures
Presence of vegetation
Climate of area
o Temperature
 10°C increase in temperature doubles reaction rates
 Rate of evaporation
o Precipitation
o Seasonal changes
Depth of rock in crust
Joints and fractures facilitate weathering because they permit water and gases in
the atmosphere to attack a rock body at considerable depth. They also greatly
increase the surface area on which chemical reactions can occur.
Features or products of weathering process
The major products of weathering are spheroidal rock forms, a blanket of
regolith, and dissolved ions. Soil is the upper part of the regolith—a mixture of clay
minerals, weathered rock particles, and organic matter.
1. Block disintegration
a. Basalt cooling to form hexagonal basalt columns. Chunks of rock fall
out together when weathered
2. Bedding Plane Seperation
a. Breaking down rocks into slabs
3. Jointing
a. Breaking down pieces of rocks into smaller pieces
b. Edges rounded by Spheroidal weathering
4. Granular disintegration
a. Common in granite
b. Produces crumbly Spheroidal boulders
c. Material consists of clay (weathered from feldspar) and quartz grains
d. Dissolution of calcite cement in sandstone has similar effects
5. Exfoliation
a. Layers of rocks parallel to the rock’s core falls out due to expansion
and contraction creating these fractures and fissures in it
6. Shattering angular rock fragments
a. Rocks subjected to severe stress  ruptures rock into sharp, irregular
and angular blocks
b. Ice wedging, insolation, blasting bedrock with explosives (hur hur
humans)
Impacts??????????????????????