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SLCSS/GEOGRAPHY/CE/LANDFORM/81 Denudation From the last two chapters, we learn that compression and folding of sediments along destructive plate margin will give rise to the formation of fold mountains. Also, the intrusion of lava will supply volcanic rocks on the mountains. We know that the Earth movement is continuous, so if there are continuous supply of sediments and lava, these mountains should keep on rising and never ends. However, the fact tells us that the heights of mountains are quite constant over time. Why? Because of denudation. In fact, no landforms on the Earth’s surface are permanent. All are being changed very slowly and continuously by various processes as shown below: Weathering Mass movement Denudation Erosion by Processes changing the shape of earth surface Transportation by Deposition River Marine (wave) Wind Glacier Physical Chemical Biological Creep Flow Fall Slide River Marine (wave) Wind Glacier River Marine (wave) Wind Glacier Definitions: Denudation The wearing away of land surface by natural agents, including the process of _____________________, _________________________________, _______________________ and _______________________________. Weathering The actual loosening, ________________ and _________________ of rocks largely by various climatic agents, such as temperature change, rainwater, snow and ice. The weathered rock materials can become the inputs for the processes of mass wasting, erosion, transportation and deposition. SLCSS/GEOGRAPHY/CE/LANDFORM/82 Mass wasting The ________________ movement of weathered materials in mass mainly due to the force of ____________________. The breaking down of rocks and the removal of the weathered materials by Erosion various agents such as running ______________, moving __________, _____________, ________________ and currents. Transportation The process which _____________ the loose materials from one place to another place where it is laid down as new deposit. The setting down of transported materials. This takes place when the Deposition transportation agent does not have enough _________________ or ________________ to carry the load. A. Weathering 1. Physical (mechanical) weathering - _____________________________ of rocks into smaller and angular fragments without change in their chemical composition - mainly caused by __________________________ changes which cause repeated expansion and contraction of rock minerals I. Types of physical weathering - Four types of physical weathering are studied below, their resultant landscapes are more commonly found in Hong Kong. a. Block disintegration Favourable conditions: – barren areas with large ________________________________ range, e.g. desserts – in __________________________ rocks Process and resultant landscape : – in daytime, intense solar heating causes rocks to _________________ – at night, the temperature falls so rocks cool and ___________________ – repeated expansion and contraction produces stress along _______________ joints are then widened and deepened and finally break down the rocks block by block SLCSS/GEOGRAPHY/CE/LANDFORM/83 Differential rates of expansion and contraction of different mineral grains causing them to break away from the parent rock. Accumulation of sand grains derived from the rock block b. Granular disintegration Favourable conditions: – barren areas with large daily temperature range, e.g. ________________ – in ____________________________ rocks with different structure, e.g. granite and sandstone Process and resultant landscape : – individual minerals __________________ and ________________ at different rates due to their different structure – repeated expansion and contraction causes different minerals to loosen grain by grain sand grains are accumulated at the base of rocks c. Exfoliation (Onion weathering) Favourable conditions: – barren areas with large daily temperature range, e.g. desserts – in fine-grained rocks with ______________________________, e.g. rhyolite Process and resultant landscape : – in daytime, intense solar heating causes the ___________________________ of rocks to expand more than the interior part – at night, temperature falls and the outer layers cool and _________________ – repeated expansion and contraction creates stress in rocks and produces radial and concentric cracks in the outer layers in time, the ______________________________ peel off SLCSS/GEOGRAPHY/CE/LANDFORM/84 d. Frost shattering Favourable conditions: – areas with temperature fluctuating __________ and ____________ freezing point, e.g. on mountain areas, or temperate latitudes in winter Process and resultant landscape : – water in the cracks or joints of rocks may _______________ at night and expand, this will ___________________ the cracks – in daytime, ice ______________, water fills the cracks more deeply and more water can be held – repeated freeze and thaw action causes the cracks to widen and deepen, and finally rock break down into fragments rock fragments accumulate at the foothills to form screes II. Other processes responsible for physical weathering a. Wetting and drying – commonly found in coarse-grained granite and other permeable rocks – certain minerals in rocks absorb rain water and expand – these minerals contract when they dry up – alternate expansion and contraction of minerals causes rocks to break down b. Growth of salt crystals – commonly found in desserts – dissolved minerals (e.g. salt solution) in rocks re-crystallize when water evaporates – growth of crystals produces stress in rocks – cracks and joints in rocks are widened, then rocks break down 2. Chemical weathering - ________________________ of rocks through various chemical processes it changes the original minerals of rocks into new minerals which could be easily removed, so the rocks become less resistant against weathering and erosion active in hot and wet regions, e.g. equatorial rain-forest I. Processes of chemical weathering a. Solution / Carbonation - dissolved carbon dioxide in rain water acts as a mild carbonic acid to dissolve minerals in rocks - e.g. carbonation often occurs on limestone region, by which can dissolve and remove limestone minerals along joints by rain water combined with carbon dioxide, joints are then deepened to form karst landscape. CaCO3+H2O+CO2 Ca(Hco3)2 SLCSS/GEOGRAPHY/CE/LANDFORM/85 b. Hydration - minerals in rocks absorb water and then expand to become hydrated minerals, which are soluble in water and are more easily affected by other processes - the expansion of minerals will weaken the rock structure c. Hydrolysis - combination of water with minerals brings chemical changes which weaken rock structure d. Oxidation - some rocks contain iron compounds which can be oxidized to form iron oxide which may be soluble II. Types of chemical weathering - Chemical weathering is most common in Hong Kong, comparing with physical and biological weathering. - It is due to its heavy ____________________, high __________________ and the presence of granite, which occupies one-third of the total land area in Hong Kong. - All these factors facilitate chemical weathering. (will be explained later) - There are two common types of chemical weathering in Hong Kong: a. Honeycomb weathering Favourable conditions: – in __________________ region where sea water sprays onto rock surface – in rocks with different structure which contain _________________ minerals Process and resultant landscape : – ___________ rain water or __________________________ falling on rocks will dissolve some minerals by solution – dissolved minerals are then removed, leaving __________________on rocks acidic water and chemical weathering concentrate at the hollows and make them larger, rocks with honeycombed surface are formed SLCSS/GEOGRAPHY/CE/LANDFORM/86 b. Spheroidal weathering Favourable conditions: – on land with _____________________ and exposed rocks, e.g. granite land in Hong Kong – in area with __________ temperature and ______________ rainfall Process and resultant landscape : – acidic ______________________ goes into _______________ of granite, ______________________________starts – rocks decay along the joints while the inner parts of blocks remain ____________________ – the inner parts become rounded ____________________, which are surrounded by _____________________ materials when weathered materials are ______________________ by agents of mass wasting or erosion, e.g. rainfall or wind, _______________are exposed on the ground called ________________ SLCSS/GEOGRAPHY/CE/LANDFORM/87 3. Biological (Organic) weathering - weathering by _____________, _________________, man and decomposers (e.g. bacteria) by mechanical and chemical means I. Mechanical - plant ___________________ grow deeply into rocks, then widen their cracks and joints, and rocks break down - burrowing animals like _________________ will loosen the soil, allowing acidic rain water enter the rocks and speed up chemical weathering II. Chemical - decay of organic matter by ______________________ produces organic acid which weakens rock surface 4. Factors affecting weathering Factors Humidity Physical weathering Chemical weathering Alternate wetting and drying Various chemical processes cause stress on rocks requires the presence of water Temperature Large temperature range and High temperature speeds up freeze-thaw changes cause chemical reaction alternate expansion and contraction of rocks and finally split rocks along widened joints Rock Well-jointed or coarse-grained rocks are easily weathered structure Rock Limestone contains soluble composition minerals while granite contains felspar which is easily decomposed Vegetation Sparse vegetation cover causes land surface expose to various agents of weathering Relief Biological weathering High temperature speeds up biological activity which produces organic acid to decay rocks Plant roots grow into joints and break up rocks mechanically Plants give out organic acid which help to decay rocks Lowland helps accumulation of weathered material, which protect the underlying rock from further weathering SLCSS/GEOGRAPHY/CE/LANDFORM/88 5. Weathering in different climatic regions Regions Types of weathering Equatorial regions, e.g. TRF ________________ and Explanations ___________ temperature and _________________ _____________________ rainfall weathering is active Dense vegetation provide large amount of organic acid _____________ weathering is _____________ temperature range due less significant to continuous hot seasons and cloud cover Tropical regions, ______________ weathering ________ and _________ summer e.g. HK is active in summer Arid regions, e.g. ______________ weathering _______________ daily temperature dessert is much significant, however, range _____________ weathering is active when water is available Temperate and polar regions ______________ weathering Freeze-thaw actions is important is much significant Mass movement (Mass wasting) 6. Factors affecting mass movement I. Slope steepness - slow movement is commonly found on gentle slopes while rapid movement is found - on slopes with _______________________ hilly relief or slope undergone slope cutting usually facilitate mass movement SLCSS/GEOGRAPHY/CE/LANDFORM/89 vegetation rain weathered material gravity relief/slope gradient II. Degree of weathering - the higher the degree of weathering, the more the ____________________________ provide for mass movement III. Amount of rainfall - rain water acts as a _______________________, which reduces the cohesion and friction between soil particles and speeds up mass movement - rain water add ______________________ to weathered materials and increase the force of gravity IV. Presence of vegetation cover - plants can absorb rainwater and decrease the pressure of downslope movement - _____________________ can bind loose weathered materials together V. Human impact - digging at the base of slopes like mining and tunneling results in unstable slope, road cutting will cause _______________________ slope - _____________________ cover may be cleared for construction VI. Climate - climate is an important factor, it affects some of the factors shown above: - e.g. hot and humid climate facilitates ____________________ weathering, large - daily temperature range favours __________________ weathering, all these provide - large amount of ________________________materials for mass movement - e.g. regions with prolonged ___________________________supply large amount of rain water 7. Types and processes of mass movement I. Slow movement a. Soil creep Process: – alternate _____________________ and __________________ of soil particles in response to changing ___________________ and ____________________ contents – these lead to the loosening of soil particles and they move dowslope slowly due to the force of ____________________ SLCSS/GEOGRAPHY/CE/LANDFORM/90 Evidence of soil creep: – ____________ trees and fences, _____________ walls with accumulation of soil, terracettes Solifluction (Solifluxion) Process: – occurs in arctic regions with great depth of frozen ground – in warmer summer, only the top soil thaws while the subsoil remains frozen and known as permafrost – impermeable permafrost hinders meltwater from infiltration – the top soil becomes saturated with water and increases in weight, then moves downslope slowly under the force of gravity Evidence of Solifluction: – Solifluction lobes and terraces SLCSS/GEOGRAPHY/CE/LANDFORM/91 II. Rapid movement a. Rock fall - rocks suddenly fall vertically from a cliff b. Mudflow and earthflow - rapid and sudden downslope movement of waterlogged soil particles in the form of a stream after sporadic intense downpours, e.g. desert c. Landslide - landslide is common in Hong Kong Types of landslide: – includes _______________ and __________________ – slide is the movement of rock materials downslope along a slide plane, while slump occurs along a curved slide plane Causes of landslide in Hong Kong: – Presence of deep weathered profile for mass movement because: __________ and _____________ climate favours chemical weathering _______________ is common in Hong Kong, it is easily weathered by physical and chemical processes – High downslope force for weathered materials because: ____________ relief with many steep ______________ SLCSS/GEOGRAPHY/CE/LANDFORM/92 abundant __________________ in summer reduces cohesion and friction between soil(_________________ effect), and add ___________ to soil thus increase the force of gravity ____________________ and _____________________ cause bare slope and remove the support from plant roots __________________ or unstable slopes are caused by road cutting and constructions. Effects of landslide: – – – – – roads blocked by rock materials / affects traffic loss of life and property destroy of natural landscape spoon-shaped scar on hill slope is left top soil is removed, so regrowth of vegetation is difficult Measures to prevent mass movement, especially landslide: – reduces water in soil by: ____________________ (planting of trees and grass) spreading ______________ or ______________ on steep slopes construction of _________________ channels and _____________ – reduce ________________ of steep slopes – _________________ unstable rock materials – construct ______________________ to hold back possible landslides – install instruments on slopes to provide early ____________________ – carry out regular _____________________ on danger slopes – restrict ____________________ and _________________ on steep slopes SLCSS/GEOGRAPHY/CE/LANDFORM/93 Landscape formed by weathering, erosion and mass movement – badland on granite slope in Hong Kong Formations: 1. Removal of __________________ cover by human or hill fires cause the topsoil expose to various agents of weathering and erosion. 2. _________________ weathering takes place under sunlight and enlarges ______________ of granite. 3. __________________weathering takes place after rainfall when _____________ rain water concentrate at joints, joints are then enlarged into ___________________. SLCSS/GEOGRAPHY/CE/LANDFORM/94 4. As rills are widened by _________________ erosion and __________________________ of weathered materials, __________________ are formed. 5. When an area is full of gullies and become bare surface without vegetation cover, ________________ is formed.
