Presentation on theme: "Weathering Breaks Down Rocks. WEATHERING Weathering is a set of physical, chemical and biological processes that alter the physical and chemical state."— Presentation transcript:
Weathering Breaks Down Rocks
WEATHERING Weathering is a set of physical, chemical and biological processes that alter the physical and chemical state of rocks and soil at or near the earth's surface in situ. Rock and soil is altered physically by disintegrating and chemically by decomposing. Nearly all weathering involves water, mostly directly in processes such as frost shattering, wetting and drying, and salt weathering, In all chemical weathering, water is in solution. Because weather and climate occur at the earth's surface, the intensity of weathering decreases with depth and most of it occur within less than a metre of the surface, creating a layer of weathered material called regolith.
You need to know the chemical, physical and biological weathering processes including: Oxidation Oxidation Carbonation Carbonation Solution Solution Freeze-thaw Freeze-thaw Pressure release Pressure release Thermal expansion Thermal expansion The actions of tree roots and organic acids. The actions of tree roots and organic acids.
Physical Weathering Physical weathering is the disintegration of rock by mechanical forces concentrated along rock fractures
Freeze-Thaw or Frost Shattering Freeze-Thaw or Frost Shatteringrost Shatteringrost Shattering The force of water in rock, fractures it as it freezes and expands, or is forced into the rock by the pressure of freezing water. This is the most common physical weathering process, given the widespread distribution of frost (even in the tropic at high elevations). It is most effective in coastal Arctic and Alpine environments where there are hundreds of frost cycles per year. The specific volume (vol./unit mass) of water increases by 9% upon freezing producing stress that is greater than the strength of all common rocks.
The process of frost shattering and rock displacement on steep slopes
Ice crystals forming in a rock
Pressure (Stress) Release Exfoliation of a rock mass occurs as it expands in response to the removal of adjacent rock. The most common mechanism of stress release is the erosion of overlying rock by erosion. The rock disintegrates along dilation (expansion) fractures that conform to the surface topography and increase in spacing with depth. The separation of rock into concentric layers is called exfoliation in rock masses and spheriodal or onion skin weathering in boulders.
Evidence of exfoliation as a result of pressure release
Exfoliation in the Rockies
Salt Weathering The growth of salt crystals in rock fractures with the evaporation of saline groundwater causes granular disintegration. The salt is in solution and precipitates in the form of crystals in the matrix of the rock as the groundwater evaporates. It is most effective in dry landscapes where water tables are near the surface (e.g., saline seeps). The salt may be derived from a number of sources including sea water, the chemical weathering of marine or evaporite sediments, snow and rain.
Hydration The wetting, swelling and disintegration of soil aggregates, minerals such as biotite mica, and fine grained rocks such as shale or clay. It is caused by the expansion and contraction with wetting and drying, also the pressure of air drawn into pores under dry conditions and then trapped as water advances into soil and rock. Minerals such as biotite may expand 40% by volume contributing to the weathering of granite.
Insolation (Thermal) Weathering The expansion and contraction with heating and cooling The surface temperature of dark coloured rock can vary from 0-50 o C between day and night, since rock (especially jointed rock) has low thermal conductivity. The differential stresses of expansion and contraction of the outer 1-5 cm of rock causes separation of concentric shallow layers called spheroidal weathering in boulders
Chemical weathering The decomposition of rock by chemical reactions occurs in water, (especially if the soil water and groundwater are rich in dissolved carbon dioxide produced during the decomposition of plants).
Hydrolysis Mineral cations (e.g., Ca+, Fe+, Na+, K+, Al+) are replaced by hydrogen ions (H+) from acidic water. This is the most common weathering process. Pure water is a poor H+ donor, however biogenic CO 2 dissolves in water to produce carbonic acid: the weathering products are in solution or a residue such as clay. This is, the first stage of soil development: the soil water solution becomes more basic as H+ is consumed.
Early stages in the chemical weathering of a granite surface by hydrolysis.
Carbonation The dissolving of calcium carbonate (limestone) in acidic groundwater The process is similar to hydrolysis but the all the products are ionic, there is no residue. Bicarbonate (HCO 3 - ) is a product of carbonation and a major part of the dissolved load of most rivers. The carbonation of limestone results in karst topography: caves, sinkholes etc. (see later notes).
Oxidation The loss of an electron to dissolved oxygen. Iron is the most commonly oxidized mineral element Fe +2 (ferrous iron) ——> Fe +3 (ferric iron) or 2FeO + O 2 ——> Fe 2 O 3 Other readily oxidized mineral elements include magnesium, sulphur, aluminium and chromium.
Biological Weathering Biological weathering would include the effect of animals and plants on the landscape. This is more than roots digging in and wedging rocks. Biological weathering may involve the molecular breakdown of minerals. Plants such a lichens (combinations of fungi and algae), which live on rocks, slowly eat away at the surface of rocks. The amount of biological activity which breaks down minerals depends on how much life is in that area. You might find more lichen activity near oceans where the air is more humid and clean.
Mechanical biological weathering as a tree root lifts a slab of rock
Plants that colonise a rock surface can mechanically break the rock apart.
Chelation The bonding of mineral cations and organic molecules produced by plants. These chelates are stable at a pH at which the cation would normally precipitate and thus they are leached in seeping soil water. H + released during chelation from organic molecules is available for hydrolysis thus plants, such as the lichens on bare rocks, contribute to the decomposition of soil and rock.
Evidence of weathering on grave stones
You need to know that physical factors, including climate, geology, vegetation and relief affect the rate of weathering. Human factors also influence weathering rates through acid rain, removal of vegetation and the protection of buildings.
Peltier’s diagram to explain the global distribution of weathering types. Variation in weathering types with rainfall and temperature in o Celsius Temperature
Weathering is an important process in providing the mineral content of soils. This includes stones, sand, silt and clay particles that create the soil’s structure and soluble minerals in the soils water.
Consider how human activity can either decrease or increase rates of weathering. Consider: Acid rain. Removal or reduction of surface vegetation. Planting coniferous forests that produce acidic leaf litter. Protecting buildings with weather resistant materials. Global warming.
Physical Weathering in Granite Granite is a course grained igneous rock which consists of the minerals - quartz (grey/white), feldspar (cream), and mica (black). It may be pink or grey, depending on the composition of the feldspar. Granites are mainly used for building materials. Granite areas have characteristic moorland scenery. In exposed areas the bedrock may be weathered along joints and cracks to produce a tor, consisting of rounded blocks that appear to have been stacked upon one another. A Case Study of Weathering in Granite
Granite Tors on Dartmoor
Theory of Chemical Weathering in the Tertiary It has been suggested the process most likely to produce the tors is the sub-surface intensive chemical weathering of the rock by acidic groundwater percolating along joints and from them into the body of the rock, later erosion leaving a structure surrounded by the fine-grained products of rock decay known locally on Dartmoor as growan (Linton). These ideas are supported from studies of different climatic environments, but in its application to the British Isles more generally it has met with several problems. Foremost amongst these was its inability to rest easily alongside existing theories which took account of the destructive nature during later glacial periods. Various explanations, including snow cover and river rejuvenation have been suggested to explain the shapes of tors, but crucially, there is no evidence of deep chemical weathering within the immediate vicinity of tors, neither around the intensively studied granite tors of Dartmoor nor the gritstone tors of the English Pennines. First Theory of Granite Tor Formation
The development of a model for granite tors was developed on Dartmoor based on a periglacial cycle, i.e. a period of freeze-thaw activity followed by solifluction in a periglacial climate. The researchers envisaged an initial stage involving the loss by solifluction of soil from the tops of hillsides and rounded hilltops. The action of frost on the granite is the next stage and would be accentuated on the joints and natural partings. The break-up of such a bedrock by frost produces blockfields. Large scale freeze-thaw would be possible on hilltops where permafrost is absent, while permafrost at the foot of the hill limits the effects of frost shattering, in a shallow active layer. The downhill movement of blocks freed by frost action would be facilitated on steeper slopes, but tors are best developed on intermediate slopes. On gentle slopes movement is restricted and there is little scope for the differential weathering necessary for tor isolation. On steep slopes, movement is easy. This is the currently accepted explanation of the formation of tors. Second Theory of Granite Tor Formation
Granite has many uses - most commonly it is used for building owing to its hardwearing nature. Kaolin, the product of the chemically weathered feldspar in granite, has many uses form pottery to the whitening agent in toothpaste. The peaty soils which cover large areas of granite are acidic in nature and are often saturated, forming large blanket bogs. Granite areas are consequently of limited value for farming, although they provide excellent terrain for grouse and for military training. The impermeable nature of granite results in large amounts of surface water, granite areas therefore provide suitable areas for reservoirs. Granite Country like Dartmoor has a considerable economic value, for example tors can become tourist attractions.
The formation of China Clay (Kaolinisation). Acidic soilwater helps to break down the feldspars within the granite to form the alumino-silicate, kaolinite which is a pale white soft clay mineral. The process is essentially hydrolysis, along with the removal of alkalis and silica. K-Feldspar Kaolinite 3KAlSi 3 O 8 + 6H+ Al 2 Si 2 O 5 (OH) 4 + 3K+ + 6Si ions K-Feldspar The abundance of feldspar rich granite in Cornwall has provided the basis for the large scale deposits of china clay that have been commercially exploited
Limestone is a broad term that refers to many different types and purity of rock in which calcium carbonate is the major constituent. In general, limestones in Britain are about million years old and were formed from compressed layers of calcareous sea deposits. Many qualifying terms are used with limestone such as dolomitic, high calcium and fossiliferous, all referring to purity or origin. Most limestones consist mainly of calcite crystals, but there are also some limestone deposits containing aragonite. These occur mainly in deposits of recent origin where the stone formed from warm ocean waters or from seabeds covered with seashells made mostly of aragonite. Since aragonite is not as stable as calcite, it changes into the more stable calcite over geological time or upon heating in the Earth's crust. A Case Study of Weathering in Limestone
Carbonation is the main process operating on the Limestone pavement in the Yorkshire Dales. The well jointed rock weathers along joints and bedding planes to form clints (blocks) and grykes (wide scars) forming limestone pavement. On steep slopes, mechanical weathering such as freeze-thaw is also important leading to extensive scree formation. Grykes can become enlarged so that drainage may disappear underground in swallow holes or sink holes. Abandoned sink holes are called potholes. Their continued weathering can lead to large underground caverns. The diversion of drainage underground can leave dry valleys or gorges. Some of the calcium may be re-deposited as drip-tone forming stalactites, stalagmites and columns or pillars. Caverns may collapse creating additional narrow gorges or shakeholes. Carbon-dioxide in the soils can increase the strength of acid rain to accelerate weathering rates. Limestone Landforms in the Malham Area of Yorkshire
Limestone pavement at Malham Tarn
Clints and grykes
Ivy Growing in the grykes where is sheltered from the weather and from sheep.
A limestone cliff with evidence of scree resulting from freeze-thaw.
Malham Cove: a former waterfall, now abandoned as drainage has moved underground.
View from the top of Malham Cove
Gaping Gill – A Swallow Hole on Ingleborough
Underground Caverns below Gaping Gill
The limestone towers of Guilin, in northern Guangxi, are perhaps the most famous of all Chinese landscapes. The pictures in this set show the scenery first from the Gui River, which ultimately joins the Xi River and flows into the South China Sea near Macao.