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Drainage System
Weathering Subsystem
Denudation
The physical landscape is the combined
result of plate movements, the composition
of the rocks, and the processes of
denudation.
Rocks in or on the crust is attacked by the
following processes:
- weathering
- mass wasting / movement
- erosion
- transportation
The process of Weathering
Weathering is the disintegration and
decomposition of rocks at or near the
Earth’s surface caused by physical and
chemical changes.
It results in the formation of pieces of rock
called regolith (風化層).
Weathering processes do not involve any
movement. So the regolith is produced insitu (原位).
The process of Denudation
The agents of erosion use regolith to break
up surface rocks still further to form a layer
of rock particles called rock debris (岩屑).
The process of Denudation
Mass wasting / movement is the movement
of regolith and rock debris down the slopes
under the pull of gravity.
Erosion refers to the wearing away of rocks
by agents of erosion, e.g. rivers, waves, ice
and wind.
Except for processes of hydraulic action and
solution, all erosion results from agents of
erosion dragging or blowing regolith and
rock debris over the Earth’s surface.
The process of Denudation
Transportation refers to the removal of
regolith and rock debris from where it is
produced, by weathering and erosion, to
the place of deposition.
Weathering
It is the first process to operate in shaping
the landscape.
Rocks on the Earth’s surface are exposed to
temperatures that range from –30oC to 35oC,
and in some regions to rain and frost as well.
Weathering
Water and temperature changes cause
rocks to break up by processes called
physical disintegration (物理崩解) and
chemical decomposition (化學分解).
The process of Weathering
Weathering is confined to a relatively
narrow zone of the Earth’s surface.
It extends from the soil surface to the max.
depth of penetration of mobile water, gases
and energy derived from the atmosphere.
These processes turn exposed rock surfaces
into a layer of rock fragments (regolith) that
extends from the surface to the bedrock
(基岩).
The reasons for Weathering
It is a response to a disequilibrium.
What are the possible reasons?
Reasons for disequilibrium
1. Exposure by uplifting e.g. tectonic
processes like folding and faulting
2. Exposure by vulcanicity / volcanic
activities
3. Erosion
4. A fall in sea level
5. Fluctuation in climate
Reasons for disequilibrium
6. Changes in vegetation
7. Impacts by human activities
e.g. pollution
The process of Weathering
The base of the active zone is generally
marked by the water table, beneath which
ground remains permanently saturated.
Beneath that level, weathering may operate,
but a a much reduced rate.
How the landscape is
shaped by external
processes?
What are external processes?
Denudation:
the external
processes
which
down
External processes
include
denudation
andwear
deposition.
rocks on the earth’s surface.
Rocks are broken into fragments by moisture and
temperature changes
Fragments move down the slope under gravity
Debris is removed by moving agents,
e.g. river
Load is carried by moving
agents
Weathering
Mass wasting
Erosion
Transportation
Sea or lake
What are external processes?
When river and wave lose their energy, deposition
occurs.
Load is settled when
moving agents lose
their energy
Deposition
Sea or lake
How do rocks become weathered?
Weathering: rocks disintegrate or decompose in situ
due to changing temperature and pressure, moisture
and action of living organisms.
Weathering
Mechanical
Chemical
These two types of weathering usually work together,
not alone, on rocks.
How do rocks become weathered?
1
Mechanical weathering
It is the breaking of rocks into pieces without changing
their chemical composition.
Mechanical weathering
Pressure
release of
rock
Thermal
expansion and
contraction
Biological
activity
1
Mechanical weathering
A
Pressure release of rocks
Pressure from
overlying rocks
Deep plutonic rock
Rocks deep in
the crust
(plutonic
rocks) are
under great
pressure
exerted from
the overlying
rocks.
1
Mechanical weathering
A
Pressure release of rocks
Removal of overlying materials
reduces the pressure on the
rock below
Crack
Pressure release
causes expansion
and sheeting
The expansion of the
plutonic rocks causes
the outer layers of
rocks to break away,
and peel off like
onions (sheeting).
The exposed
rock is further
subject to
weathering.
1
Mechanical weathering
A
Pressure release of rocks
Unloading is a form of physical weathering
that occurs when the removal of overlying
layers causes the rock to expand, cracking in
layers parallel to the surface that break away
from the rock in sheets.
B
Thermal expansion and contraction
1
Block disintegration (on rocks with cracks)
RockRepeated
releases heat
and and
expansion
contracts
contraction causes rock to
break into blocks
Cracks
enlarged
Night-time
Daytime
Rock is
heated and
expands
Features in Hong Kong
A
From weathering
Describe the appearance
of the rock and identify the
associated weathering
process.
The rock breaks
like slices of bread.
Block disintegration
(mechanical
weathering)
A rock structure at Tai Lang Shui, Tuen Mun
(Credit: Association for Geoconservation, Hong Kong)
B
Thermal expansion and contraction
2
Granular disintegration (on rocks with
different minerals)
Mineral
AA
expands
Mineral
contracts faster
faster
adjacent
Repeated
expansion
thanthan
adjacent
onesand
ones
(Mineral
B) and
contraction
all mineral
(Mineral
B)causes
and
becomes
pushes
apart
grains
tothem
loosen
and detach
detached
Rocks
break off
into grains
Night-time
Daytime
B
Thermal expansion and contraction
3
Exfoliation (on a uniform mass of rock)
expansion
and
TheRepeated
outer layer
absorbs more
The outer layer
loses heat
causes
outer
heatcontraction
and expands
morethe
than
faster
and
contracts
more
layer to
peel
off. A new
surface is
the inner
layer
than
the inner
layer and cooling
exposed
to heating
Concentric
cracks
develop
Radial
cracks
develop
Rock
breaks
into slabs
Night-time
Daytime
1
Mechanical weathering
C
Biological activity
As tree roots grow, they can
force open the joints in rocks.
The large physical pressure
helps break the rock along the
joints.
Plant roots enlarge the joints in rocks
(Credit: Tom Bean/DRK Photo)
How do rocks become weathered?
2
Chemical weathering
It is caused by the reaction between the rocks and
the gases in the air, water or other solutions (for
example, acids) with which they come into contact.
Chemical reaction causes certain minerals to
change into more vulnerable ones.
Four processes of chemical weathering may work
together, and result in two special forms.
2
Chemical weathering
Four processes and two forms of chemical
weathering
Chemical weathering
Oxidation
Hydration
Honeycomb
weathering
Hydrolysis
Solution and
carbonation
Spheroidal
weathering
A
Four processes of chemical weathering
1
Oxidation
(Credit: Richard Cummins/Getty Images)
The process by which
minerals react and
combine with oxygen. It
is also called metal
rusting.
Example
Iron oxidizing into
reddish-brown iron oxide
Oxidation occurs on the rocks
used in this construction
Features in Hong Kong
A
From weathering
Describe the
appearance of the
rock and identify
the associated
weathering
process.
Cracks occur on
the rock. Reddish
brown colour is
found at the two
sides of the cracks.
Mudstone on Tai Tsan Chau, eastern New Territories
(Acknowledgement: The Head of the Geotechnical Engineering Office and
the Director of Civil Engineering and Development, the Hong Kong SAR
Government, for the permission to publish the photograph)
Oxidation
(chemical
weathering)
A
Four processes of chemical weathering
2
Hydration
The absorption of water into the mineral structure,
turning it into new but more vulnerable minerals.
Example
Calcium
sulphate
+
Water
becomes
Gypsum
A
Four processes of chemical weathering
3
Hydrolysis
The process by which minerals react with hydrogen in
water and break down the original structure.
Example
Silicates in
feldspar
+
Hydrogen
turns to
Clay
A
Four processes of chemical weathering
4
Solution and carbonation
The dissolving of soluble
minerals in pure water
or weak acid.
Example: carbonic acid
(rainwater + carbon
dioxide) and acid rain
(rainwater + sulphur
dioxide)
Calcium carbonate in
limestone can be
dissolved and removed
easily from the rocks in
solution.
Acid rain has caused chemical
weathering on stone statues
(Credit: Ray Roberts/Topfoto)
A
Four processes of chemical weathering
4
Solution and carbonation
A
Four processes of chemical weathering
4
Solution and carbonation
Growing of plant
roots
Decomposition of
plant
releases
releases
Organic acid
Humic acid
Both acids can attack the underlying rocks and
weaken their structure.
B
Two forms of chemical weathering
1
Honeycomb weathering
The removal of minerals in solution produces a
honeycombed effect on the rock face.
Honeycomb
weathering on
the rock face in
Ping Chau
Features in Hong Kong
A
From weathering
Describe the appearance
of the rock and identify the
associated weathering
process.
Deep holes like
honeycombs are found
on the rock surface.
Honeycomb weathering
(chemical weathering)
A rock at Cheung Chau
B
Two forms of chemical weathering
2
Spheroidal weathering
As water seeps through joints, weathering works from
the joints towards the interior of the rock.
Original joint planes
Water seeps through joints
More water favours
faster weathering at
the intersection of
joint planes
Unweathered
corestones
Unweathered bedrock
Weathered material
which will be
removed eventually
2
Deep weathering profiles
Granite is vulnerable to chemical weathering. It will
develop into a deep weathering profile beneath the
earth’s surface.
Soil surface
Thick profile
(may reach
up to 60
metres)
Bedrock
The cross-section of the weathering profile of granite
Percolation
of water
along
previous
joints
2
Deep weathering profiles
In a mature profile, spheroidal weathering turns all the
rock materials into soil at the top layer.
Soil surface
Fine regolith
Thick profile
(may reach
up to 60
metres)
Bedrock
The cross-section of the weathering profile of granite
Percolation
of water
along
previous
joints
2
Deep weathering profiles
__________
Spheroidal weathering forms rounded corestones.
More and bigger corestones are found nearer the
bedrock.
Soil surface
Fine regolith
Thick profile
(may reach
up to 60
metres)
Bedrock
The cross-section of the weathering profile of granite
Percolation
of water
along
previous
joints
Angularity and
size of
corestones
increase with
soil depth
B
Two forms of chemical weathering
2
Spheroidal weathering
The corestones get smaller and rounded as time
goes by.
(Credit: Chlaus Lotscher/Peter Arnold Inc.)
Unweathered
corestones
3
Tor formation
When the overlying materials
above the granite corestones
are removed, the corestones
are left as tors.
Amah Rock is an example of
tors
(Credit: Association for
Geoconservation, Hong Kong)
Features in Hong Kong
A
From weathering
Describe the appearance
of the rock and identify the
associated weathering
process.
Outer layers of
rock have peeled
off like onions.
Reddish brown
patches are found
on the rock.
A rock at Ninepin
Group
Exfoliation
(mechanical
weathering)
Oxidation
(chemical
weathering)
Comparison – weathered vs unweathered
Chemical weathering
Limestone
Which of them is under
chemical weathering?
Which one is fresh?
Other forms of physical weathering
A
Salt-crystal weathering
It operates extensively in dry climates and is the
result of the growth of salt crystals in rock pores.
Groundwater moves to the surface through
capillary action and evaporates, leaving the salts
behind producing grain by grain breakup of
sandstone.
Salt weathering of building stone on the
island of Gozo, Malta
Other forms of physical weathering
A
Salt-crystal weathering
Crystallized salt on the rock surface and in rock fissures in granite
Other forms of physical weathering
B
Frost action
It is one of the most important physical
weathering processes in cold climates. It occurs
when water freezes in joints in the rock, and the
expansion of the water during repeated freezing
and thawing to cause the joints to be enlarged.
Other forms of physical weathering
B
Frost action
How does rock materials help
weathering processes?
1. Structure
- the pathways of percolating water
2. Texture / particle size
- the structure of rock
- the size and surface area
- e.g. fine grains are more easily weathered
due to the presence of more porous
space (i.e. porous rock / high porosity)
How does rock materials help
weathering processes?
3. Rock composition
- chemistry of the rock
- e.g. quartz vs feldspar
How about other factors?
1. Vegetation cover
- both physically and chemically
2. Topography / gradient
- drainage & moisture content
- transportation & mass wasting
3. Human activities
- both positively and negatively
4. Time
- for which the rock has been exposed to
new conditions
What are the factors affecting the
rate of chemical weathering?
1. Chemical composition of the minerals
- e.g. quartz: the least reactive
ferromagnesian materials & calcium
carbonate: most active
2. Structure of the rock
- e.g. rocks with highly fractured or porous
are the most prone to decay
What are the factors affecting the
rate of chemical weathering?
3. Climate
- many chemical reactions are faster in
warm than in cool climates
- the rate of weathering is further
accelerated by continual percolation of
rainwater that is still chemically active
Weathering in different
landscapes
Practical
Conclusion
Useful Websites on Weathering
地理入門—風化作用
http://ihouse.hkedcity.net/~hm1203/lithosph
ere/decay-factor.htm
Weathering
http://www2.hkedcity.net/sch_files/a/rhs/rhs
-so/public_html/AL-GeogNote/Weathering.html