Module 2832 (B) the Rock Cycle Processes and Products Revision Notes s1

MODULE 2833/1 (C) ECONOMIC AND ENVIRONMENTAL GEOLOGY REVISION NOTES

3.1 Water Supply

Candidates should be able to:

(a) Define and explain the following terms: porosity, permeability, hydrostatic pressure, hydraulic gradient, aquifers and the water table.

Porosity – the amount of pore space, usually expressed as a % of total rock volume. The porosity is only effective if the pore spaces are interconnecting. E.g. Clay has a porosity of 50% but the effective porosity is virtually zero; Sandstone may typically have a porosity of 15% but most of the pores interconnect. Percentage porosity = total volume of pore space x 100 total volume of rock/sediment Permeability – the ability of the rock to transmit fluids, expressed as a rate of flow. The permeability may be the result of fracture/joint voids rather than interconnecting pores. Permeability = distance water has travelled time taken Surface water – Water on the surface in lakes, rivers and streams. Groundwater – Water held in pore space of rocks, below the water table. The Earth’s largest accessible store of fresh water – a very important water resource especially in dry, arid areas. Water table – Surface separating saturated rock below from unsaturated rock above; above the water table pores hold air and water, below water table water only. It generally follows surface topography, but with less relief, intersecting the ground surface at lakes and most rivers. Aquifer – Body of porous and permeable rock capable of storing and yielding significant quantities of water, which makes it suitable for groundwater abstraction, e.g. aeolian sandstone, limestone, chalk. Types of Aquifer:  Unconfined – Open to the atmosphere, not covered by impermeable strata, under atmospheric pressure.  Confined – Sandwiched between impermeable strata above and below, under hydrostatic pressure.  Perched – Above the regional water table. The aquifer may also be:  Live – Currently being recharged by rain water from the surface = renewable water resource.  Fossil – Not being recharged – a relic of a wetter climate = non-renewable water resource. Aquiclude – Impermeable rock that will not transmit water, e.g. clay, shale. Hydrostatic pressure – Water pressure at any one point in a body of water due to the weight of overlying column of water. Hydrostatic head – Height of overlying column of water. Water will always flow down the hydraulic/pressure gradient from high pressure to low pressure – important for predicting the movement of water in aquifers. Hydraulic gradient – The difference in hydrostatic pressure or hydrostatic head between two points. This pressure difference causes water to flow through an aquifer. Hydraulic gradient = difference between hydrostatic pressure or hydrostatic head (quoted as a ratio) the distance between two points Potentiometric surface – Height to which water under hydrostatic pressure will rise.

(b) Describe and explain the geological conditions leading to the formation of springs as a result of lithology, faults and unconformities.

Springs of ground water occur where the water table intersects the surface – usually where permeable and impermeable rocks are in contact. Types of Springs:  Lithological (rock type) springs occur where a porous and permeable rock overlies an impermeable rock, e.g. chalk overlying clay in south-eastern England.

1  Faults may produce springs if they have brought porous/permeable rocks into juxtaposition with impermeable rocks. Springs associated with joints may occur in limestones – the water flows along the joints and the spring’s location is influenced by the outcrop of joints.  Unconformities and igneous intrusions may also influence the location of springs if they juxtapose porous/permeable rock and impermeable rock.

(c) Explain the geological conditions necessary for artesian basins and water supply from wells.

 Artesian basin – Large down-folded (syncline/basin), confined aquifer under hydrostatic pressure. Has a recharge area at the edges of the syncline/basin. A borehole/well sunk in the basin will be artesian. E.g. London Basin – synclinal structure, chalk sandwiched between clay above and below.  Artesian well – Normally water has to be pumped out of a well. If a well is sunk in an artesian basin (confined aquifer, under hydrostatic pressure), the water will rise under its own pressure to the potentiometric/peizometric surface. The pressure will be highest in centre of the artesian basin – saves on pumping costs.  Water supply – Water is abstracted from the ground by sinking wells/boreholes and pumping it to the surface. If water extraction is well managed the supply can be renewable and sustainable.  Cone of depression – Formed as the water is pumped out because the water table falls in the vicinity of the well.  Draw down – Difference between water level in the well and the water table.  Ground water mining/overdraft conditions – If extraction is greater than recharge.  Problems of groundwater abstraction – If cones of depression overlap, shallow wells become dry as the water table is lowered; Subsidence on the surface; Salt water encroachment in coastal areas – seawater is drawn into the aquifer, water becomes saline (brackish).

(d) Describe water supply in relation to river, reservoir and underground sources. Understand the advantages and disadvantages of surface and underground supply. Understand that water resources are both renewable and sustainable if carefully developed. Advantages & Disadvantages of Groundwater Advantages Disadvantages Rock acts as natural filter purifying water. Requires sedimentary rocks and presence of No loss of water through evaporation. aquifers. No requirement for expensive/unsightly Problem of subsidence at surface. dams. Problem of salt water encroachment at coast. If artesian – cheap pumping costs. Pollutants have a long residence time. Sustainable providing recharge = extraction. Pumping costs. Groundwater is not always suitable for drinking. Unsustainable if groundwater mining.

Advantages & Disadvantages of Surface Water Advantages Disadvantages Readily available – no pumping costs. Water will need treatment. May be used for H.E.P. generation. Requires construction of expensive dam. Reservoir may be used for recreation and Requires flooding of area for reservoir. other purposes. Seasonal/water loss through evaporation. Reservoir will silt up. May trigger earthquakes. Requires sufficient rainfall and large catchment – no backup in drought conditions.

 British water supply – A useful diagonal dividing line can be drawn from the mouth of the River Tees (Middlesborough) to the mouth of the River Exe (Exeter). To the north-west, the area is underlain by older, impermeable mainly igneous and metamorphic strata, 2 whilst to the south-east, the area is largely composed of younger porous and permeable sedimentary rocks separated by impermeable clays.  Rainfall is much greater in the north-west compared to the south-east. The result is that in the north-west (SW England, Wales, Isle of Man, Northern England, Ireland and Scotland) water supply is dominated by abstraction of water from rivers and storage of water in surface reservoirs. In the south-east of England the water supply pattern is dominated by the abstraction of water from underground sources, either springs or wells.

3.2 Energy Resources

(a) Describe and explain the origin of oil and natural gas and migration from source-rock to reservoir-rock under a cap rock. Define and recognise the trap structures: anticline, fault, salt dome, unconformity and lithological.

Requirements for the formation and accumulation of petroleum:  Source rock – An organic-rich mudstone/shale formed in shallow marine conditions, with abundant zooplankton whose remains accumulated in low energy, anoxic conditions on the seafloor.  Maturation – The source rock is subjected to burial, compaction and temperatures of between 50 and 200ºC (the oil window) to generate fluid hydrocarbons. Below these temperatures biogenic gas forms but is usually lost. Above these temperatures the oil denatures and is destroyed.  Migration – The oil and gas migrate down the temperature and pressure gradients from the source rock to a reservoir rock. This requires permeable rocks. The oil and gas will migrate upwards until they encounter an impermeable layer or they reach the surface and are lost.  Reservoir rock – A porous and permeable rock, with good interconnected pore space, so petroleum can accumulate and be extracted easily, e.g. desert sandstone, limestone. A cap rock overlies the reservoir rock.  Cap rock – An impermeable rock – either crystalline sedimentary (e.g. evaporites) or very fine-grained (e.g. clay, shale) – which prevents the petroleum rising further and escaping.  Trap – A structure that concentrates the petroleum in one place. This may be structural (anticline, fault, salt dome) or stratigraphic (unconformity, lithological, e.g. reef, wedge- edge). All require the juxtaposition of a porous/permeable reservoir rock under an impermeable cap rock. You must be able to recognise and draw fully annotated diagrams of each of the trap types. Oil may be lost from a trap by erosion of the overlying cap rocks or it may escape upwards through a fault. Oil may be destroyed by the heat from igneous intrusions/volcanic activity or during metamorphism.

(b) Describe the methods of primary and secondary recovery of oil and natural gas from suitable reservoirs and the environmental and technological problems of oil and natural gas extraction.

The trap is drilled and an oil well established. Extraction may be by primary or secondary recovery.  Primary recovery consists of drilling production wells to exploit the reservoir. In most cases the production wells are in lower parts of the reservoir so that gas pressure forces the oil to the surface – a gusher. But 75% of the oil is left in the reservoir.  Secondary recovery involves the injection of water below or steam/carbon dioxide above the oil to encourage more to rise towards the surface by increasing the pressure and breaking down surface tension, which holds the oil onto grains in the reservoir rock. Oil may need to be pumped to the surface if there is insufficient pressure within the reservoir. 20% of the oil is still unrecoverable.  Problems – blow outs, explosions, fires, pollution.

3 (c) Describe the occurrence of oil and natural gas in and around the British Isles and its social and environmental implications. Understand that oil and natural gas are examples of non-renewable energy resources.

Most oil and gas deposits are offshore, particularly in the North Sea, which is a shallow continental shelf sea. Each geological combination of reservoir, cap and trap is called a hydrocarbon play.  Northern North Sea has a variety of oil and gas fields – Source rock = Kimmeridge Clay in almost all cases; Reservoir rock = sandstones (e.g. Brent) or fractured chalk (e.g. Ekofisk).  Southern North Sea has extensive natural gas deposits – Source rock = Carboniferous Coal Measures; Reservoir rock = Permian desert sandstone; Cap rock = Permian evaporites. The traps are mainly anticlines – associated with salt doming.  Other offshore gas fields occur in the Irish Sea and there may be extensive deposits to the west of Scotland, but current technologies do not allow them to be fully exploited.  Onshore there a number of small oilfields – mostly in Mesozoic rocks – e.g. Wytches Farm, Dorset, and a number in Lincolnshire, e.g. Scampton and Gainsborough.

 The social implications of oil/gas exploitation are mainly that we are very dependent on fossil fuels for energy, plastics, etc., and the industry provides many jobs.  The environmental consequences of oil/gas exploitation are mainly the risks of pollution and fires from oil spills, blow outs, etc.  Oil and natural gas are examples of non-renewable energy resources because they are fossil fuels that formed millions of years ago and once they are burnt they are lost as gases such as carbon dioxide and cannot be recycled in human timescales.

(d) Describe and explain the origin of coal and coal seams as part of a cyclothem. Describe the development of rank and the physical properties of lignite, bituminous coal and anthracite.

Coal forms from the partial decay and preservation of plant material in anoxic conditions, associated with swamps and deltas in a hot, tropical climate where there is luxuriant vegetation. In deltaic environments (cyclothems) the rapid sedimentation and subsidence ensure the peat is preserved. The peat deposits are gradually changed to coal seams by heat and pressure - coalification. As the peat undergoes compaction, the volatile constituents are driven off and the thickness of the deposit is reduced.  Rank of a coal = The percentage of carbon in the coal – this increases as the degree of burial/compaction increases.  Coal series = The sequence of increasing rank: peat  lignite  bituminous coal  anthracite brown coal, black coal, black, hard, iridescent, dull appearance, dull & shiny layers shiny coal plant matter visible formed by different types of plant matter Increasing temperature and pressure ------> Increasing carbon content, calorific value and density ------> Decreasing water, volatiles (H, O, N) and ash ------>

(e) Describe the methods of extracting economic deposits of coal by opencast and underground mining. Describe the geological problems that can make coal mining uneconomic.

Open cast mining is used in exposed coalfields to depths of 200m. Dragline excavators are used and a stripping ratio of overburden:coal of up to 20:1 is profitable. Advantages – cheaper and safer. Underground mining is used in concealed coalfields. Longwall retreat mining is the underground coal mining method used in Britain – it is highly mechanised. Two tunnels/roadways are driven out to the furthest point from the shaft. A coal face is established and the coal is mined as the face is moved back towards the shaft. This allows the geological conditions to be determined in advance of mining. The coal face is up to 100m in length and is only economic if the seam is thick (about 2m). The coal is cut by a rotating shearer and then falls onto a conveyor belt. The roof is held up by hydraulic chocks 4 and as the coal is mined the area behind is allowed to collapse (causes subsidence on the surface). Geological problems in underground coal mining include:  Faults – offset the seam; juxtaposes different rock types, including hard sandstone; zone of permeability – may cause flooding.  Folds – machinery cannot cope with steep dips.  Methane gas may be present – highly explosive.  Porous and permeable strata may be present in deltaic sequences – problems of flooding.  Washouts – result of channel switching on the delta top – a distributary channel changes course and erodes away the peat, depositing river channel gravels and sands in its place.  Seam splits – where one thick seam splits into several thinner unworkable seams – the result of differential subsidence of the delta.

(f) Describe the broad structure and distribution of coalfields in the British Isles. Describe the effects of mining and understand the environmental consequences of mining operations. Understand that coal is an example of a non-renewable energy resource.

Coal in the British Isles is almost all of Carboniferous age – Britain was at the Equator and had a hot tropical climate. Most coal deposits are located onshore in deltaic sedimentary sequences (cyclothems) that formed at this time. There are two types of coalfield:  Exposed coalfield – where the coal-bearing strata outcrops at the surface.  Concealed coalfield – where the coal-bearing strata is below younger sedimentary “cover” rocks. The current location of British coalfields is the result of folding during the Variscan/ Hercynian Orogeny (mountain building) at the end of the Carboniferous Period.  South Wales Coalfield – Synclinal structure, closest to the mountain building event to the south, suffered deformation and deeper burial → anthracite. Folding and faulting has made this coalfield difficult to mine.  Yorks, Notts, and Derbys – East Pennine Coalfield – Formed in the Pennine sedimentary basin. The Variscan Orogeny resulted in the formation of the Pennine anticline and erosion of the coal seams in this area of the original basin, leaving the Lancashire and East Pennine coalfields on either side.  Coal is an example of a non-renewable energy resource because it is a fossil fuel that formed millions of years ago and once it is burnt it is lost as gases such as carbon dioxide and cannot be recycled in human timescales.

(g) Describe geothermal energy extraction from volcanic sources around the world and potential 'hot' rock sources in the British Isles. Understand that geothermal energy is an example of a renewable energy resource.

Three main types:  Volcanic sources – High enthalpy (T and P). Mainly located at plate margins, e.g. Iceland (constructive), New Zealand (destructive), not applicable to the British Isles. Groundwater is converted to steam and this can be used to drive turbines to generate electricity.  Geothermal aquifers in sedimentary basins – Low enthalpy. Similar requirements to oil – needs a reservoir (aquifer) covered with an impermeable cap rock. Insufficient heat for driving turbines, but can be used for ‘space’ heating, e.g. homes, greenhouses, etc.  Hot dry rock sources – Granites – The heat source is the decay of radioactive elements. The rock will need to be artificially fractured by explosives and cold water could be pumped in and hot water extracted. However, the possibility of obtaining energy from ‘hot dry rock sources’ in the granites of SW England has now been abandoned as uneconomic.  Advantages – renewable; cheap if in the right location; doesn't produce carbon dioxide emissions.  Disadvantages – wells only have a lifespan of ~30 years; water may contain toxic/corrosive elements/salts; geographically restricted to volcanically active areas (mainly plate margins).

5  Geothermal energy is an example of a renewable energy resource because there is a constant supply of heat/magma from the Earth’s interior produced by the decay of radioactive heat-producing elements.

3.3 Metal Deposits

(a) Show an understanding of concentration factors to produce economic deposits from low crustal abundances of metallic minerals; calculate concentration factors.

 Resource – A naturally occurring material or substance in the Earth’s crust that is useful and/or valuable to man. Can be divided into energy and mineral resources.  Reserves – Amount of the resource that can be economically exploited using existing technology. These may change due to: exploration; technological advances; World demand and prices; politics, war and stock-piling.  Ore – Rock which contains metal(s) of interest that can be mined at a profit. A mixture of ore minerals and gangue minerals.  Ore mineral – Mineral containing the metal of interest, usually oxides or sulphides. E.g. Haematite = iron oxide Galena = lead sulphide Cassiterite = tin oxide Magnetite = iron oxide Sphalerite = zinc sulphide Siderite = iron carbonate Bauxite = hydrated Chalcopyrite = copper-iron Ilmenite = iron-titanium oxide aluminium oxides sulphide  Gangue mineral – Worthless mineral mixed in with the ore mineral(s). E.g. Quartz, Calcite, Pyrite (iron sulphide) - “fool’s gold”.  Grade – Percentage/amount of metal in the ore deposit.  Tonnage – Weight of ore (volume x density), i.e. the size of the deposit. E.g. 100,000 tonnes of copper ore, grading 3.2% copper.  Average crustal abundance – Amount of the metal in average continental crust.  Concentration/enrichment factor – Amount by which the metal in an ore deposit is concentrated above its average crustal abundance.  Cut-off grade – Minimum grade required for an economic ore deposit.

Concentration factor = Concentration of metal in ore (or cut-off grade) Average crustal abundance E.g. Metal Average Crustal Abundance Cut off grade Concentration Factor Iron 5 25 5 Tin 0.0002 0.5 2500 Lead 0.001 4 4000 Gold 0.0000004 0.0001 250

(b) Describe the concentration of magnetite by gravity-settling in igneous intrusions.  Magmatic segregation is the result of gravity settling of magnetite and other dense ore minerals in fluid basic/ultrabasic igneous intrusions. Magnetite is an early-formed, high temperature, dense mineral – it sinks to the bottom of the magma chamber to form a cumulate layer, thus concentrating the ore. Example: Bushveld Igneous Complex, South Africa.

(c) Describe the secondary enrichment of chalcopyrite copper deposits.  The main ore mineral of copper is chalcopyrite. Many primary copper deposits are high tonnage, but low grade.  Secondary enrichment is a process by which low grade copper deposits, which may not be economic, can concentrated by chemical weathering. Groundwater percolating downwards from the surface leaches soluble copper minerals from the pre-existing veins in the zone above the water table (oxidising). The copper is carried downwards in solution and copper minerals such as chalcopyrite and bornite are re-precipitated just below the water table, where the chemical conditions change (reducing). The ore is concentrated

6 into a smaller volume and the enriched deposit may now be economic. Example: Secondary enrichment of porphyry copper deposits such as Bingham Canyon, Utah.

(d) Describe the hydrothermal processes associated with igneous intrusions forming veins of galena, sphalerite and cassiterite.  Hydrothermal processes require a source of heat, aqueous fluid and metal ions.  Hydrothermal fluid – A hot, aqueous fluid capable of transporting significant quantities of metals in solution.  Hydrothermal ore deposits may be associated with: 1. Igneous intrusions, e.g. tin, lead and zinc deposits in Cornwall. The source of heat, water and metal is the igneous intrusion. Associated with acid magmas – rich in water and volatiles. At the end of crystallisation the water and “incompatible” metal elements are concentrated at the top of the intrusion. The hydrothermal fluid then moves out from the intrusion.  Veins/disseminations of cassiterite, galena and sphalerite are formed. The veins are formed by ore minerals precipitating in fractures in the host/country rock, as the fluids cool or as a result of chemical changes. Requires porous and permeable (+/- chemically reactive) country rock, e.g. limestone, sandstone. Bedding and joint planes often control the distribution of the mineral veins in the country rocks.  The mineral deposits often show a zonation with higher temperature minerals (e.g. cassiterite > 400ºC) close to the heat source and lower temperature minerals (e.g. galena and sphalerite < 300ºC) further away. They are precipitated in order of solubility/temperature – least soluble first at highest temperatures. 2. Sedimentary basins, e.g. lead and zinc deposits in the Peak District. The source of heat is the geothermal gradient, the source of water is the pore fluid in the sedimentary basin, the source of metal is from the pre-existing sedimentary rocks. 3. Mid ocean ridges – black smokers, e.g. copper and nickel deposits in the Troodos Mountains ophiolite complex, Cyprus. The source of heat is the igneous activity, the source of water is seawater, the source of metal is from the pre-existing pillow lavas.

(e) Describe how residual deposits of bauxite are formed as the insoluble product of chemical weathering.  Residual deposits of bauxite form when rocks containing aluminium are subjected to extreme chemical weathering in hot, tropical/equatorial climates. The source rock is often limestone or granite. The aluminium minerals are broken down to insoluble aluminium hydroxides. These can be smelted to obtain the aluminium – this is not possible with the original minerals. The aluminium ore represents the insoluble residue from the chemical weathering process. Example: Jamaica.

(f) Describe the formation of placer deposits of cassiterite and gold in rivers and beaches and the characteristics of these minerals which make them suitable.  Placer deposits of cassiterite, gold and other minerals form when grains of the ore minerals are separated by weathering of pre-existing veins, transported by water or wind and are then preferentially deposited when the current velocity drops. This may occur on the inside of meander bends, downstream of confluences, in plunge pools and potholes, on beaches, etc. The ore minerals must be dense, chemically resistant and usually resist abrasion (N.B. gold is the exception in this instance). Ancient placers are rare – most are recent deposits. Usually small operations. Examples: Tin deposits in the Kinta Valley, Malaysia, Gold deposits in the Witswatersrand palaeo-conglomerate, South Africa.

(g) Describe the effects of mining and understand the environmental consequences of mining operations. Understand that metal mining is an example of unsustainable resource exploitation.  Environmental consequences of mining operations include pollution (including contaminated land), noise, dust, creation of spoil heaps, acid mine water drainage, subsidence.  Economic consequences of mining operations include employment opportunities, mining operations will depend on the current price of the metal, mining operations may be short term, and significant capital investment is required before mining operations can begin.

7 Many developing countries are heavily dependent on mining operations for generating foreign income. 2.4 Applied Geology

(a) Describe the geological factors affecting major construction or engineering projects: (i) road cuttings and embankments: (ii) tunnels through both hard rock and unconsolidated material; (iii) waste disposal in quarries; (iv) dams and reservoirs. Describe methods which can be used to stabilise rocks for these projects: grouting, gabions, rock drains, rock bolts, retaining wall.

This is a very important aspect of Geology as it impacts on many Human activities including building and civil/structural engineering, motorway construction, slope stability, tunnelling, quarrying, mining, land restoration, waste disposal, and dam construction. Please refer to your handout on Applied/Engineering Geology for more specific detail on each application.

Main Geological Considerations in any construction or engineering project:  General considerations - Best if aseismic – no likelihood of earthquakes to cause collapse.  Rock type - Mechanical strength is very important (high load-bearing strength) – competent rocks are best. Texture – grain/crystal size, homogeneity, etc. – fine-grained best. Mineralogy – hardness; chemical stability; calcite and swelling clay minerals are problematic. Porosity and permeability – problems of leakage (dams); problems of flooding/lubrication (tunnels, slope stability); pollution of groundwater (waste disposal) – impermeable best.  Condition – Weathering (weakens rock). Degree of consolidation/lithification, compaction/cementation of sedimentary rocks – if variable this is problematic. Attitude – flat-lying or dipping? Flat-lying best. Dipping – may be unstable, water/fluids can migrate down dip.  Structure – Jointing – increases permeability, lines of weakness. Faulting – zones of weakness, increases permeability, may juxtapose contrasting rock types. Folding – changes in dip, slippage along bedding planes, etc. Presence of structures are generally problematic!  Groundwater - Position of water table – may vary if aquifer. High water table – risk of flooding in tunnels; easier for pollution (leachate) to reach groundwater in waste disposal, etc. Water adds weight and lubrication to rocks/slopes – can lead to failure. Limestone – caves (karst) are problematic, may cause collapse.  Availability of construction materials – Bulk commodities, expensive to transport, best if locally available.  Ground stabilisation/improvement techniques can be used if necessary.

Main Techniques for Ground Stabilisation/Improvement:  Profile/slope modification – Regrading/recontouring slopes to lower angle – increases stability (e.g. strong, competent rocks – vertical cuttings are possible; incompetent rocks such as clay – cutting angle must be less than 10°!). Berm ledges – level steps in slopes ~5m wide, 10m high – increases stability. Lateral toe support (at bottom). Cut and fill – to reduce slope angle.  Drainage – Pore fluid pressure is critical in providing weight and lubrication, promoting slips, etc. Drains, relief wells, pipes and tunnels, etc., can be constructed.  Support – Concrete retaining walls. Gabion walls and boxes (wire mesh boxes filled with rocks). Sprayed liquid concrete (shotcrete) – increases strength, reduces permeability, protects against weathering. Wire meshing – prevents rock falls.  Rock bolts – Pin loose rock to sound rock behind. Only useful for competent rocks.  Ground improvement – Vegetation – fixes soil, prevents erosion and slippage. Geotextiles – artificial materials used to increase stability. Weathering protection – shotcrete.  Grouting – Holes are drilled into the rock and liquid cement is pumped in to reduce permeability and increase strength.  Lining – Clay, or geomembrane (plastic) if hazardous waste, used to reduce permeability.  Cut off curtain – Specifically used for dams – an impermeable barrier, usually made of concrete, is constructed below the dam to prevent leakage. 8 9 Geological factors affecting the construction and siting of dams and reservoirs:  General conditions Requires a lack of seismic activity and mass movement – may have landslides into reservoir.  Catchment considerations Requires a large catchment with sufficient rainfall; with a lack of sediments in feeder streams – possibility of silting up of the reservoir; and impermeable rocks to promote surface runoff. No mineral veins containing toxic elements, e.g. lead, zinc.  Rock type for foundations Dams require sound/strong foundations to withstand the stresses from the mass of water dammed behind them. Dams made of concrete usually require strong/impermeable rock foundations, which may have to be improved by grouting. Earth fill/embankment dams are more ‘flexible’ and spread their load over a much greater area. There may need to be a ‘cut- off curtain’ to prevent leakage from the dam – this may be clay or concrete, depending on the type of dam. Rock should be homogeneous – if sited on two rock types differential subsidence of the dam may occur. Depth of weathered material is important – weakens rock, increases permeability. The construction of the dam may cause problems with the river channel downstream, which now has less water in it. The construction of the reservoir may cause micro-earthquakes to occur as the crust adjusts to the increased load.  Attitude/Structures Land slipping may occur if strata/beds in valley dip towards reservoir. Horizontal strata is stable, strata dipping upstream is stable, strata dipping downstream is unstable – potential for slippage and collapse of dam. Faults are zones of permeability and weakness and old faults may be reactivated by added pressure/lubrication by water. Joints can cause leaks. Synclines may permit leakage as the water bypasses dam through permeable strata. Anticlines are unstable as slippage may occur on the limbs and tension joints may be present on the crest.  Ground improvement strategies can be employed – Grouting; Clay lining; Cut off curtain – all designed to reduce permeability, prevent leakage of water. There have been some notable dam/reservoir ‘failures’ resulting form poor siting of the reservoir or from poor design of the dam – ‘Classics’ include the Vaiont dam/reservoir in Italy; Dol-y-Gaer dam in Wales – built on Carboniferous Limestone – it leaked!; The Teton Dam in Idaho failed because the clay core of the embankment dam was eroded away by seepage.

(b) Describe the geological factors that cause landslips and slumping hazards.  Geomorphological hazards are usually related to mass movement of material under the force of gravity. They are seldom attributable to one cause – a series of events lead to failure.  The main geological factors involved in slope processes and failure are: rock type (strength, porosity and permeability); presence of structures; slope angle; and the presence of water (rainfall, climate). Addition of water is often a key component in slope failure – adds weight and lubricates the rocks – failures often happen after heavy rain or snow melt.  Human contributions include: adding weight (building at top of slope); poor drainage – leaking water mains, sewage disposal and cess pits; removal of material at base of slope, e.g. cuttings for roads; removal of natural vegetation that intercepts water and binds soil together; and the creation of impermeable surfaces that promote surface runoff.

(c) Describe how geophysical exploration techniques are used for finding hydrocarbons, coal and metals: seismic reflection and refraction, gravity surveys, magnetic survey using proton magnetometer, electrical resistivity and down hole logging surveys.  Seismic survey – Both seismic reflection and seismic refraction techniques are used. Seismic reflection is the most widely used. An artificial source (explosion/compressed air gun/vibrations) generates seismic waves that are transmitted through the ground and the reflected/refracted waves are detected by geophones/hydrophones. The technique is very effective for showing subsurface layering and finding traps. Used for oil exploration.  Gravity survey – Gravimetric surveys are useful for detecting areas of unusually high (e.g. metallic minerals) or low density (e.g. salt domes) rocks. A gravimeter is used and the data is plotted on gravity anomaly maps. Used for both oil and metal exploration.

10  Magnetic survey – Useful for detecting metal deposits which are rich in magnetic minerals. A proton magnetometer is used to detect variations/anomalies in the Earth’s magnetic field. Data can be plotted on magnetic anomaly maps  Resistivity – Used to measure the electrical resistance of the rocks. Two electrodes are placed in the ground and an electric current passed between them. Can show the presence of conductors (metals, water). Used for metal exploration, but also in down-hole logging.  Gamma Radiation Spectrometry – Used in metal, particularly uranium, exploration.  Wire-line/down-hole logging techniques – Instruments are mounted on a sonde which is passed down the drill hole – measures resistivity, magnetic susceptibility, gamma ray count, pH, etc. – used to characterise and identify rock types in oil exploration.

(d) Describe how the geochemical exploration methods: soil sampling and stream sampling are used for metal exploration. Geochemical exploration methods include stream sampling and soil sampling. Samples are analysed to detect anomalous concentrations of metal ions. Used exclusively for metallic mineral exploration.  Geochemical anomaly – A concentration of a metal above its normal background value.  Stream sediment sampling – Used for regional surveys. Sediments from rivers are collected and analysed to allow anomalies/targets to be identified and located. Stream sediments downstream of the target will have anomalous concentrations. Stream sediments upstream will have background levels. The size of the anomaly will decrease downstream of the target. Where tributaries meet catastrophic dilution will occur. The best sampling strategy is to sample each tributary immediately upstream of each confluence.  Soil sampling – Used for local surveys, once a target area has been identified. Soil samples down slope of the target will have anomalous concentrations. Soil samples up slope will have background levels. © D. Armstrong, 2004.