The weathering subsystem

Introduction Weathering is the breakdown of rock which is in contact

with the atmosphere

It includes physical weathering and chemical weathering

Physical disintegration of rock masses into smaller particles

Chemical weathering alter the particular materials into dissolved ions or clay material

These sand and rock fragment form regolith

The depth of the regolith will depend on the nature of the rock,the intensity of atmospheric process and the rates of removal

Weathering is the process by which rocks adjusted to their environment

It prepare rocks for the process of slope movement

Causes of weathering Erosion uncovers underlying rocks Faulting and folding push rocks that were deep

within the crust to the surface Volcanic eruptions may cause magmas to be the

subject to weathering A fall in sea level may expose marine sediments Changes in the climate Man changes the weathering environment by

putting pollutants into the atmosphere

Factors affecting weathering 1) The nature and structure of rock materialMechanical structure: Many sedimentary rock posses bedding planes ( 層面 ).They are

used by the weathering process Joints are line of weakness. They are formed under cooling and

contraction,compression,tension and dilation( 膨脹 ) Cleavage ( 裂痕 )is the small parallel cracks formed in platy min

erals such as mica Some rocks such as sandstone or conglomerate ( 礫岩 )may con

tain pores and allow water to enter

Chemical composition

o As rock is a collection of various materials,each of which has a different chemical changes through weathering than others

o For example : limestone composed of CaCO3( 炭酸鈣 ) is slowly dissolved by rainwater

o Quartz is very resistant

Climatic condition The importance of water - Changes in the volume caused by the addition or rem

oval of water produce mechanical stresses in a rock and may cause it to split apart or crumble( 碎裂 )

- Most chemical changes requires the presence of water: solution , carbonation, hydration and hydrolysis, oxidation ( 溶解、炭化、水化、水解、氧化作用 )

Temperature

- a wide temperature range causes the surface layers of exposed rock to expand and contract.

- The rate of chemical reaction tend to rise if the temperature increase

- This is also same to biological activities

The vegetation cover,the topography and the length of time may all affect the weathering process

Mechanical weathering Process Effect

Sheeting, unloading, spalling 片狀剝落 Release of pressure by removal of overlying rocks

Crystal growth ( 結晶作用 ) The expansion of crystal exerts pressure ,as the volume also change too

Insolation Temperature changes at the surface causing cracking

Fire Forests fires lead to the cracking of rocks

Moisture swelling ( 濕氣膨脹 ) Changes in volume

Wetting and drying Cause changes in volume and the rock peel off

Freeze and thaw action ( 凍融作用 ) Freezing cause expansion then thawing cause contraction,disintegrating rocks

Solution Some materials will dissolve in water,and then carried away by rivers

Oxidation Adding or removing of oxygen: oxidation occurs in the aerated zones above the water table or form bacterial action

Carbonation The reaction of carbonate or bicarbonate ions with minerals ,causing the breakdown of feldspars

Hydration Adding water,often with a large volume change,preparing surfaces for carbonation or oxidation,important with clays

Hydrolysis Reaction between a mineral and water ,increasing the concentration of hydrogen ion,making silica dissolve

Chemical Weathering

Biotic weathering

Process Effect

Particles broken Animal burrowing and growing root exert pressure

Transfer,mixing Animal movements moves materials

Chemical effects Solution enhanced by carbon dioxide from respiration

Weathering Environments

Humid tropical Chemical activity is dominant.Thick regolith and rapidly decaying vegetation release acids.Silica may be dissolved,leaving bauxite / laterite. Solution is common

Seasonal tropic (with a dry season)

The wet season leaches salts, but they are bought near the surface in the dry season by high evaporation rates

Arid Capillary action ( 毛細管作用 ) leads to the surface concentration of salts forming desert varnish ( 荒漠漆皮 ) or calcrete. Water causes few chemical processes,and produces little clay or solutes

Humid temperate Water is available because of lower temperature.Low evaporation rates produce little capillary action

Periglacial areas Freezing through the long winter,and freeze thaw in the spring and autumn. Water is available for chemical activity in the summer

Arctic Snow-cover and river frozen.Carbonation is common beneath ice when it melts

Weathering and Man

Weathering produced by weathering have economic value

Bauxite ( 鋁土礦 ) provides aluminium, kaolinite( 高嶺石 ) is used in paper and ceramics industries

Weathering is a fundamental process in soil formation,thus influencing man’s use of land

Weathering alters the stability of materials,affecting construc

tion work and road building

Man is reinforcing the weathering process.For example,salt is used to keep roads free of ice

Weathering in the tropical rain forest Chemical weathering - The high rainfall and temperature favours chemical weathering - There is break down of silicates into regolith, consist of sand and

clay

- The rock which is highly permeable allow chemical weathering of fresh rock

- Hydrolysis and oxidation results in igneous rocks being disintegrated to form laterite soil ( 磚紅壤 ) and bauxite,while sedimentary rocks are converted to soft clay

Biological Weathering in tropical rain

forest

Plants root wedging cause the joint block separate Fauna and burrowing animal help to disintegrate t

he rockTransport - The rocks are decomposed to great depths over 30

meter - That means the TRF has a thick regolith- The river also carry a lot of solute load too

Weathering in tropical desert Mechanical weathering - It is important because of the diurnal range of temper

ature- Minerals expand when heated and contract when cool

ed- The continue expansion and contraction can break up

the rock - This can be seen in granular disintegration( 粒狀崩解 )

Exfoliation

- it occurs when there is release of the pressure on granite rocks when the overlying material is removed. The rock expands,producing sheeting layers

- More resistant rock ,like gneiss, stand up as inselbergs( 島山 ). This is produced by the process of parallel retreat.

Chemical weathering - no frost weathering - There is little chemical weathering as the supply of wa

ter is limited - The upward movement of water by capillary action m

ay occur after rain - Honeycomb( 蜂窩狀 ),deep tafoni( 風化窗 ),niches( 雪

凹 ),shallow caves ,rock arches ( 拱頂 )and pit ( 坑 )are produced by chemical weathering

- There is also hydrolysis of exposed granite ,creating egg-shaped boulders and pinnacles ( 尖石 )

- The end product is sand and the regolith is thin. Little clay and solute will result

- - Alluvial fan ( 冲積扇 )will be form by this deposit

Weathering in tundra Frost action is the most important

As water freeze, its volume will be 10% larger

The repeated growth and melting of ice crystal produce frost-shattering ( 凍裂作用 )

Talus slope ( 岩屑堆 )may accumulate along the cliff

Heaving ( 隆起 )

- The cold in winter will cause the permanently frozen subsoil to heave upwards

- There is the growth of ice lenses/needles( 冰晶狀體 ), water beneath the stones freeze faster than the surrounding silt

- Water from the silt migrates to the crystals and begins to form an ice lens or perpendicular ( 垂直的 ) ice needles

- Such growth can push the stone upwards,sorting soil material

Freeze and thaw action assists the flow of debris by solifuction ( 融凍泥流 )

- the meltwater cannot penetrate the permafrost,it mixes the semi-frozen mud and flow down the slope

- The thawing of frozen ground can produce karst scenery( 岩溶景色 )

- The intense contraction of the frozen ground produce a serious of cracks.These cracks may fill with water which freezes at depth to form an ice-wedge. Continue contraction and expansion may produce hexagonal pattern – stone polygon ( 石多邊形 )