drainage system weathering subsystem. denudation the physical landscape is the combined result of...

Drainage System

Weathering Subsystem

DenudationThe 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 in-situ (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 ).


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.


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 ).


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 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?

Sea or lake

External processes include denudation and deposition.

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 movingagents

Weathering

Erosion

Transportation

Mass wasting

Denudation: the external processes which wear down rocks on the earth’s surface.

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

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.

Thermal expansion and

contraction

Mechanical weathering

Pressure release of

rock

Biological activity

1 Mechanical weatheringA Pressure release of rocks

Deep plutonic rock

Rocks deep in the crust (plutonic rocks) are under great pressure exerted from the overlying rocks.

Pressure from overlying 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.


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 contraction1 Block disintegration (on rocks with cracks)

Cracks enlarged

Rock is heated and expands

DaytimeNight-time

Rock releases heat and contracts

Repeated expansion and contraction causes rock to break into blocks

Features in Hong KongA 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)

(Credit: Association for Geoconservation, Hong Kong)

A rock structure at Tai Lang Shui, Tuen Mun

B Thermal expansion and contraction2 Granular disintegration (on rocks with different minerals)

Mineral A expands faster than adjacent ones (Mineral B) and pushes them apart

DaytimeNight-time

Mineral A contracts faster than adjacent ones (Mineral B) and becomes detached

Repeated expansion and contraction causes all mineral grains to loosen and detach

Rocks break off into grains

The outer layer absorbs more heat and expands more than the inner layer

Concentric cracks develop

Daytime

The outer layer loses heat faster and contracts more than the inner layer

Radial cracks develop

Night-time

Repeated expansion and contraction causes the outer layer to peel off. A new surface is exposed to heating and cooling

Rock breaks into slabs

B Thermal expansion and contraction3 Exfoliation (on a uniform mass of rock)

1 Mechanical weatheringC 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 weatheringFour processes and two forms of chemical weathering

Chemical weathering

Oxidation Hydration Hydrolysis Solution and carbonation

Honeycomb weathering

Spheroidal weathering

A Four processes of chemical weathering1 Oxidation

The process by which minerals react and combine with oxygen. It is also called metal rusting.

ExampleExample

Iron oxidizing into reddish-brown iron oxide

Oxidation occurs on the rocks used in this construction

(Credit: Richard Cummins/Getty Images)

Features in Hong Kong

A From weatheringDescribe the

appearance of the rock and identify the associated

weathering process.

(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)

Cracks occur on the rock. Reddish brown colour is found at the two sides of the cracks.

Oxidation (chemical

weathering)Mudstone on Tai Tsan Chau, eastern New Territories

A Four processes of chemical weathering2 Hydration

The absorption of water into the mineral structure, turning it into new but more vulnerable minerals.

ExampleExample

Calcium sulphate

Water Gypsum+ becomes

A Four processes of chemical weathering3 Hydrolysis

The process by which minerals react with hydrogen in water and break down the original structure.

ExampleExample

Silicates in feldspar

Hydrogen Clay + turns to

A Four processes of chemical weathering4 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)


Growing of plant roots

Organic acid Humic acid

Decomposition of plant

releasesreleases releasesreleases

Both acids can attack the underlying rocks and weaken their structure.

B Two forms of chemical weathering1 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 KongA 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 weathering2 Spheroidal weathering

As water seeps through joints, weathering works from the joints towards the interior of the rock.

Water seeps through joints

Unweathered bedrock

Original joint planes

More water favours faster weathering at the intersection of joint planes

Unweathered corestones

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.

Percolation of water along previous joints

The cross-section of the weathering profile of granite

Thick profile (may reach

up to 60 metres)

Bedrock

Soil surface

2 Deep weathering profiles

In a mature profile, spheroidal weathering turns all the rock materials into soil at the top layer.




up to 60 metres)

Bedrock

Soil surface

Fine regolithFine regolith

2 Deep weathering profiles__________ weathering forms rounded corestones.




up to 60 metres)

Bedrock

Soil surface

Fine regolithFine regolith

More and bigger corestones are found nearer the bedrock.

Spheroidal

Angularity and size of corestones increase with soil depth

B Two forms of chemical weathering2 Spheroidal weathering

The corestones get smaller and rounded as time goes by.

Unweathered corestones

(Credit: Chlaus Lotscher/Peter Arnold Inc.)

3 Tor formation

Amah Rock is an example of tors

(Credit: Association for Geoconservation, Hong Kong)

When the overlying materials above the granite corestones are removed, the corestones are left as tors.

Features in Hong KongA 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.

Exfoliation (mechanical weathering)

Oxidation (chemical

weathering)A rock at Ninepin Group

Comparison – weathered vs unweathered Chemical weathering

Which of them is under chemical weathering? Which one is fresh?

Limestone

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 water2. 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 chemically2. Topography / gradient- drainage & moisture content

- transportation & mass wasting3. Human activities- both positively and negatively4. 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/lithosphere/decay-factor.htm

Weatheringhttp://www2.hkedcity.net/sch_files/a/rhs/rhs-so/public_html/AL-Geog-Note/Weathering.html

drainage system weathering subsystem. denudation the physical landscape is the combined result of...

Documents

external processes

surface rocks

weathering processes

processes of denudation

following processes

tectonic processes

earths surface

rock debris