Dear Geology 11 Class, These are the notes for the second exam on September 4, 2008. Topics covered: sedimentary rocks, sedimentary environments, groundwater, mass wasting, metamorphic rocks Exam type: multiple choice (op chors), true or false, matching type Mag-aral ng mabuti, ok? After the exam fieldtrip sa Taal, yehey! :) Good luck! Rakenrol, Ma'am Jill >SEDIMENTARY ROCKS FORMATION OF SEDIMENTARY ROCKS 1. Weathering 2. Erosion 3. Transport 4. Deposition 5. Diagenesis External processes shaping the Earth: 1. Weathering – the physical breakdown (disintegration) (disintegr ation) and chemical alteration (decomposition) of rocks at or near the Earth’s surface - Occurs when a rock is mechanically fragmented (disintegrated) and/or chemically altered (decomposed) - Two types of weathering: Mechanical and Chemical 2. Erosion – the removal of material by mobile agents such as water, wind, ice or man Mechanical Weathering - The physical breaking up of rock into smaller pieces leads to an increase in surface area - requires the application of some physical force or stress to be applied to the rock; no accompanying changes to the composition of rocks - Prevails in: cold climates, high altitudes, dry regions →
1. Frost wedging – repeated cycles of freezing and thawing; the expansion force of water as it freezes is sufficient to split any mineral or rock. 2. Heating and cooling - Differences in temperature in a rock give rise to differential expansion (heating) and contraction (cooling). 3. Wetting and drying - The disruption of soil results in the swelling and contracting of soil particles. 4. Organisms- Action of organisms, including animals and plants, reduces the size of rocks and minerals. 5. Unloading - the removal of thick layers of sediments overlying deeply buried rocks by erosion or uplift. Chemical Weathering - breakdown of minerals by chemical reactions with water, with chemicals dissolved in water or with gases in the air progression from less stable minerals to more stable minerals 1) Dissolution Dissoluti on - the dissolving of a solid in a liquid 2) Hydrolysis Hydrolysi s - process of minerals reacting with water to form hydroxides, which usually are more soluble than the original mineral. example - pyroxene to Fe oxide 4FeSiO3 + H2O + O2 4FeO(OH) + 4SiO2 →
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3) Acidification Acidificati on - Weathering is accelerated by the presence of hydrogen ion in water, such as that provided by carbonic and organic acids. 4) Hydration - combination of a solid mineral or element with water. 5) Oxidation and Reduction - used in mineral weathering, is both the chemical combination of oxygen with a compound and the change in oxidation number of some chemical element (Reduction is the chemical process in which electrons are gained.) 6) Ion-exchange - involves the transfer of charged atoms (ions) of calcium, magnesium, magnesium, sodium, and potassium between waters rich in one of the ions and a mineral rich in another (Most effective in clays.) Susceptibility of minerals to chemical weathering - Goldich Stability Series - describes the order in which silicate minerals weather. Minerals which form at high temperatures and pressures are least stable, and weather most quickly because they are farther from their "zone of stability“ or the conditions under which they formed. - The order of mineral stability in the weathering environment is the same order as Bowen's Reaction Series. The main products of weathering 1. Soluble Ions - Na, Ca, K, Mg 2. Clay minerals – kaolinite, montmorillonite 3. resistant minerals – quartz, hematite, magnetite, garnet, gold, diamond Alterations due to Chemical Weathering: **1. Decomposition of unstable minerals. **2. Generation or retention of those minerals which are in equilibri equilibrium um with the Earth’s surface. Factors Controlling Weathering 1. Source Composition Specifically involves the mineralogy, texture and rock structure Source rock lithology Fine-grained rocks decompose chemically more readily than coarse-grained rocks 2. Climate Temperature fluctuations determine importance of ice-wedging and insolation Precipitation governs the extent of hydrolysis, hydration and solution 3. Topographic Relief Influence the amount of rock exposed to the forces of weathering Slope steepness controls the rate at which weathering products are eroded to be transported elsewhere Product of weathering – SOIL Soil – combination of rock, air, water and organic material Soil type – varies in color, texture, mineral content Types (orders) of soil: Alfisols, Aridisols, Entisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, Gelisols, Andisols, and Vertisols Soil Profile O Horizon - organic layer of soil, made up mostly of leaf litter and humus (decomposed organic matter) nd
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A Horizon - made up of humus (decomposed organic matter) mixed with mineral particles E Horizon - leaching layer; light in color; made up mostly of sand and silt, having lost most of its minerals and clay as water drips through the soil. B Horizon – contains clay and mineral deposits (like iron, aluminum oxides, and calcium carbonate) that it receives from layers above it when water drips from the soil above. C Horizon –it consists of slightly broken-up bedrock. Plant roots do not penetrate into this layer because of very little organic material.. R Horizon - unweathered rock layer
Erosion - comes from old word meaning “eat away” involves movement of rock or soil Agents of Erosion: 1. gravity 2. ice 3. organism 4. water 5. wind Transportation Agents of sediment transport: 1. ice 2. water 3. wind Distance of sediment transport affects clast: Roundness (measures how rounded corners are) and Sphericity (measures sphere-like shape of clasts) Sorting - measure of variation of grain sizes Ways of physically transporting particles: nd
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Suspension Saltation Traction Deposition Transporting sediment requires energy Grain size has relationship with energy Smaller grains take less energy Bigger grains take more If river slows down, sediment will drop out If river speeds up, water can pick up sediment Larger sediments are deposited in higher energy environments Examples: Gravel - needs fast moving water or rock slides Sand - wind and wave action (beaches) Silt and clay - lakes, swamps and deep oceans Diagenesis - physical, chemical, and biological processes which collectively result in transformation of sediments into sedimentary rock modification of the texture and mineralogy of the rock Stages of diagenesis: Early diagenesis - takes place from sedimentation until shallow burial Late diagenesis - from deep burial to subsequent uplift The different types of diagenetic processes are: Compaction Recrystallization Cementation Replacement Bioturbation The simple ideal model for the evolution of sedimentary rocks says there are three basic end products, that all sedimentary processes are working to reach - quartz sandstone, shale, and limestone. The three end products in the simple ideal model are not isolated, each one stands for a class of weathering products. Quartz sandstone = all visible grains , including such ones as incompletely weathered feldspar from the granodiorite in the simple ideal model. grains (clay is a generic name; there are many kinds of clay minerals as Shale = all clay sized grains (clay well as other minerals that are clay sized) Limestone = all dissolved minerals , including not only calcite CaCO3, but also halite (table salt; NaCl), and gypsum (CaSO4 . H2O) among others. CLASSIFICATION OF SEDIMENTARY ROCKS siliciclastic (or (or simply, clastic) clastic) rocks - Clastic rocks (sandstones, (sandstones, shales, shales, etc.) are classified classified on: texture (grain size), and composition - Clastic particles are divided into size categories based on the WENTWORTH SCALE. The Wentworth scale is straight forward, and with a ruler for scale it is relatively easy to classify the rock. - They have a clastic (broken or fragmental) texture consisting of: nd
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Clast (larger pieces, such as sand or gravel) Matrix (mud or fine-grained sediment surrounding the clasts) Cement (the glue that holds it all together, such as calcite, iron oxide, or silica) Breccia - Grain size = 2 cm or greater - Clasts are angular Conglomerate - Grain size = 2 cm or greater - Clasts are rounded Sandstone - Grain size = 1/16 mm to 2 mm - Arkose Sandstone - 75% greater feldspar - Lithic Sandstone - mostly rock fragments - Quartz Sandstone - 95% quartz content chemical rocks Carbonates composed of the mineral calcite (CaCO3 - calcium carbonate) they form by both chemical and biochemical processes tend to be mixed together in various combinations in the rocks They are extremely abundant and important. Chert a siliceous rock (composed of SiO2) forms from the recrystallized skeletons of "animals " (single-celled radiolarians and glass sponges) or single-celled " plants " (diatoms, silicoflagellates) Rock salt (halite; NaCl) and gypsum (CaSO4 . H2O) originally are dissolved in the sea water, thus making the sea salty sea water evaporates in a closed area, such as a lagoon, the salt concentration becomes very high, supersaturated, and precipitates out biochemical rocks Carbonates - made up of carbonaceous remains of organisms Peat and coal come from plant remains are biochemical rocks always form in the presence of clastic rocks - sandstones and shales Common Sedimentary Structures - One of the most obvious features of sedimentary rocks and sediment is the layered structure which they exhibit. The layers are evident because of differences in mineralogy, clast size, degree of sorting, or color of the different layers. Layer Thickness
Names
> 300 cm
Massive
100-300 cm
Very thickly bedded
30 - 100 cm
Thickly bedded
10 - 30 cm
Medium bedded
3 - 10 cm
Thinly bedded
1 - 3 cm
Very thinly bedded
0.3 - 1 cm
Thickly laminated
<0.3 cm
Thinly laminated nd
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- cross bedding, mud cracks, ripple marks, raindrop imprints, bioturbation Resources from sedimentary rocks Sediments are used as: a. sand and gravel: used as road base, components of concrete, fill, sand for sandboxes; if pure sand, for glass making b. pure clays for ceramics (dinner ware/plate ware) Fossil fuels (e.g. coal, petroleum, natural gas) Food additives (e.g. sodium and potassium salts) Placer deposits (grains of economic minerals) e.g. gold, diamond, garnet Building stone, filtering materials Lecture - Sedimentary Environments Terrestrial Fluvial Environment Lacustrine Environment Eolian Environment Glacial Environment Transitional Environment Delta Beach Marine Environment Reef Shelf Slope Rise Abyssal Plain SEDIMENTARY ENVIRONMENT - A part of the earth’s surface, physically, chemically, and biologically distinct from adjacent terrain. - Defined by fauna and flora, geology, geomorphology, climate, weather, temperature, and if sub-aqueous, the depth, salinity, and current system of the water. SEDIMENTARY FACIES - A mass of sedimentary rock which can be defined and distinguished from others by its geometry, lithology, sedimentary structures, paleocurrent pattern and fossils (Selley 1970). Methods of Environmental Diagnosis 1. Geometry - a function of pre-depositional pre-depositi onal topography, geomorphology and its post depositional history. 2. Lithology - parameters easily observe and has environmental significance - grain size, sorting, shape, and texture often reflect process of the environment 3. Sedimentary Structures - important indicators of sedimentary environment Pre-depositional; Pre-depositi onal; Syn-depositional; Syn-depositional ; Post depositional 4. Fossils - one of the most important methods of identifying the depositional environment of a sediment - to use fossils in identifying the depositional environment of the host sediment two assumption are made: the fossil lived in the place where it was buried; the nd
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habitat of the fossil can be deduced either from its morphology or from studying its living descendants (if there are any) * Trace fossils are useful in environmental analysis due to two reasons they occur in situ (cannot be reworked) some trace fossils characterize particular environments Alluvial Environment lots of coarse particles usually sandstone and conglomerate poor sorting deposited by high energy floods or mudflows cone-shaped typically found in tectonically active regions (rifting continental grabens and foreland basins) What is an alluvium? It is a young sediment (freshly eroded rock particles) that have come off the hillside and been carried by streams. The sediments accumulate at the base of the slope to form an alluvial fan. How is it formed? - When mountains shed sediment off their flanks, streams carry it away as alluvium. - A mountain stream carries lots of alluvial sediment easily when its gradient is steep and energy is abundant. Movement of grains by: suspension, saltation, creep or roll Ability of streams to carry sediments is described by: capacity - maximum load of sediment that a stream can transport competence - measure of the maximum size of particles it is capable of transporting Two types of alluvial system: 1. Meandering System consist of one single channel low gradient and high sinuosity sediments deposited at the inner sides of meander bends deposition of sediments takes place in the channel, on the levees and in the basins gravel and coarse sand are normally found on the channel floor (`lag deposits') finer sand settles along the inner bends of the river, on so-called `point bars' * Evolution of an Oxbow lake (1) On the inside of the loop, the river travels more slowly leading to deposition of silt. (2) Meanwhile water on the outside edges tends to flow faster, which erodes the banks making the meander even wider. (3) Over time the loop of the meander widens until the neck vanishes altogether. (4) Then the meander is removed from the river's current and the horseshoe shaped oxbow lake is formed. Without a current to move the water along, sediment builds up along the banks and fills in the lake. 2. Braided System have one single channel of low sinuosity and high gradient, with multiple `thalwegs' and bars. high sediment load During times of maximum discharge, the channel is completely inundated In times of low discharge, multiple thalwegs and bars reappear within the channel nd
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occur in areas with a highly irregular water regime, and abundant sediment supply Deposits contain alternating areas (lenses) of coarse gravel and sand * Types of drainage patterns (look for figures depicting this) a. Dendritic - Uniform underlying bedrock (Igneous rocks; Flat-lying sedimentary rocks) - Most common drainage pattern on all scales. - Determined by direction of slope of land b. Radial - Develops in isolated volcanic cones and domal uplifts - Often localized. c. Rectangular - Common in faulted or fractured igneous rock - Often control pattern of streams - Guides directions of valleys d. Trellis - Most common in tilted and folded sedimentary or metamorphic rocks - Formed by alternating less resistant and resistant layers. Lacustrine Environment lake - landlocked body of standing, non-marine water Geometry - circular or elongate in plan view; lenticular in cross section Typical sequence - coarsening upward from laminated shales, marls, and limestones to crossbeds of sandstones Eolian Environment wind - a turbulent stream of air like water, it has the ability to erode, transport and deposit two properties: low density - limits competence unrestricted flow - enables spread over wide areas and high atmosphere lack of rain allows more effective wind work deserts (arid environment) eolian (from Aeolus, Greek god of winds) - describes activity of deposits of winds Sand Transport a. Saltation - bouncing and jumping movement of grains. Involves bedload. b. Suspension - occurs when fine dust and dirt are lifted into the wind. Involves suspended load. *Sand particles can be blown away at any height. However, the majority (over 93%) of sand movement takes place at or below one meter. Wind Erosion - needs chemical and mechanical weathering to act effectively two types of wind erosion: Abrasion deflation - erosion of ground when dry, loose particles of dust and salt are lifted and blown away Sandblasting - shaping of solid rock surfaces by constant impact of grains by wind. *Ventifacts - rocks shaped by the wind Deserts concentrated in two regions: subtropics middle-latitudes nd
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areas where rainfall is less than 250 mm (10 in.)/year, or where evaporation exceeds precipitation. Rainfall in deserts may vary from 0.2 cm./yr. to about 40 cm./yr. Rainfalls of 5-20 cm./yr. are common. Temperature extremes can vary from –60 degrees F. in Mongolian deserts to 137 degrees F. in the Sahara Desert. Temperatures in excess of 180 degrees may occur in sands exposed to full solar radiation. Great daily extremes can occur. Due to lack of vegetation, wind velocities are high. Causes of Deserts high mountains cause available moisture to condense and precipitate in their higher parts, reducing moisture available for lowlands in the lee of mountains. Direct blocking of moisture may also occur. Wind Deposits dunes - Sand dunes form when there is (1) a ready supply of sand, (2) a steady wind, and (3) some kind of obstacle such as vegetation, rocks, or fences, to trap some of the sand. Sand dunes form when moving air slows down on the downwind side of an obstacle. Types of dunes (look for figures showing the shape of the dunes) Barchan dunes - crescent-shaped crescent-shap ed dunes. They form in areas where there is a hard ground surface, a moderate supply of sand, and a constant wind direction Transverse dunes- large fields of dunes that resemble sand ripples on a large scale. Consist of ridges of sand with a steep face in the downwind side, form in areas where there is abundant supply of sand and a constant wind direction. Linear dunes - long straight dunes that form in areas with a limited sand supply and converging wind directions. Parabolic dunes - are "U" shaped dunes with an open end facing upwind. Form in areas with abundant vegetation and constant wind. Most common in coastal areas. Star dunes - dunes with variable arms and slip face directions. Form in areas with abundant sand supply and variable wind direction. Glacial Environment glacier - a permanent (on a human time scale) body of ice that shows evidence of downward movement due to gravitational pull. Types of Glaciers Alpine/Mountain Glaciers - Relatively small glaciers at higher elevations in mountainous regions. Ice Sheets: (Continental glaciers): the largest types of glaciers on Earth. cover large areas of the land including mountain areas. Modern ice sheets cover Greenland and Antarctica. Ice Shelves: are sheets of ice floating on water and attached to land. usually occupy coastal embayments. Can also be classified as: Temperate Glaciers - ice near melting point. Polar Glaciers - ice below melting point. nd
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Formation of Glaciers Form at or above snowline Snowline - where ice can be created and remain all year round The snowline, at present, lies at sea level in polar latitudes and rises up to 6000 m in tropical areas. Form by recrystallization of snow due to pressure of overlying compacted snow. Recrystallized snow has decreased air and increased grain size and density forming solid blocks of ice. Glaciation - Changes in landforms due to glacial action, both by glacial deposition and glacial erosion. Landforms produced by mountain glaciers (search for examples of these landforms) Cirques - bowl shaped depressions that occur at the heads of mountain glaciers Glacial Valleys - Valleys that once contained glacial ice become eroded into a "U" shape in cross section Arêtes - If two adjacent valleys are filled with glacial ice, the ridges between the valleys can be carved into a sharp knife-edge ridge, called an arête. Horns - Where three or more cirques are carved out of a mountain, they can produce a sharp peak called a horn. Hanging Valleys - A valley that has greater elevation than the valley to which it is tributary. Since glaciers are solid they can transport all sizes of sediment, from huge house-sized boulders to fine-grained material. All sediment deposited as a result of glacial erosion is called Glacial Drift. Ice Laid Deposits Till - nonsorted glacial drift deposited directly from ice. A till that has undergone diagenesis and has turned into a rock is called a tillite. Erratics - a glacially deposited rock or fragment that now rests on a surface made of different rock Moraines - are deposits of till that have a form different from the underlying bedrock. Depending on where it formed in relation to the glacier moraines can be: TRANSITIONAL ENVIRONMENTS - Transitional environments are those environments at or near the transition between the land and the sea. Delta - prograding depositional bodies that form at the point where a river debauches in a lake or sea. Parts of a Delta: delta plain - composed of meandering flood plains, swamps, and beach complex. delta front - steeper part. prodelta - broadly sloping that grades into the open shelf. Types of Delta: River-dominated large sediment volume lobate shape = moderate sediment supply elongated when sediment supply is large Tide-dominated linear features parallel to tidal flow and perpendicular to shore. nd
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Wave-dominated delta smoothly arcuate; tidal action reworks sediment. much sandier than the other types of delta. MARINE ENVIRONMENT Coral Reefs - composed of carbonate structures formed by carbonate secreting organisms; build up on continental shelves; form limestone deposits Types Fringing Reefs - coral reef that is directly attached or borders the shore of an island or continent. Barrier Reefs - a long narrow coral reef roughly parallel to the shore - separated from it at same distance by a lagoon. Atoll - continuous or broken circle of coral reef and low coral islands surrounding a central lagoon. Question: How do you form barrier reefs and atolls? Beach shore of a body of water formed and washed by waves and tides. usually covered by sandy or pebbly material usually well sorted sand and pebbles, accompanied by mud, cobbles, boulders, smooth rocks and shell fragments. Waves Longshore drift the movement of sediment along a beach by swash and backwash of waves that approach the shore obliquely. Longshore current a current that moves parallel to a shore formed from the momentum of breaking waves that approach shore obliquely. Spit - long ridge of sand deposited by longshore current and drift; attached to land at upstream end (e.g. Cavite) Tombolo - a sand or gravel bar that connects an island with the mainland or another island. Continental Margin Continental Shelf - continuous with the coastal plain sequences of the continents - part of the continental margin that is between the shoreline and the continental slope (~200m). - quartz and clay minerals are dominant; fossils are mostly marine invertebrates Continental Slope - between the continental shelf and continental shelf and continental rise (oceanic trench) Continental Rise - between continental slope and abyssal plain - gentle incline and generally smooth topography - may bear submarine canyons Abyssal Plain - flat region of the ocean floor - covered with pelagic mud with fine sand layers from distal turbidites nd
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THINK HARD 1. Limestones are commonly formed in warm, shallow seas. However, if you go up to Baguio City which is 1400 m. above sea level, you are going to find limestone deposits. How do you explain this? 2. How can a sedimentary deposit contain fragments of basalt and andesite?
3. The beach in boracay is made up of white sand. The beach in Zambales is made up of black magnetite sand. Explain this please. Lecture - GROUNDWATER Waters of the World: Oceans – 97.20% Glaciers and other ice – 2.10% Others – 0.62% (97.40% groundwater, 1.62% surface water, 0.81% soil moisture, 0.16% atmosphere Ground water - Fresh water located beneath the ground; Stored in and transmitted through: a) spaces between grains of Sediments and Clastic rocks; b) cracks or openings in rocks Source of groundwater: precipitation (See water cycle) Water table - upper boundary of groundwater Streams – usually intersect water table - geologic term for channelized flow of water (e.g. rivers, streams, creeks) Vadose Zone/ Zone of Aeration – above water table; air between grains Phreatic Zone/ Zone of Saturation – below water table; water between grains Capillary Fringe – just above water table; water rises due to capillary action Porosity - Measures amount of water that rocks/sediments can hold; Volume of voids / total volume of material - Affected by grain size, sorting and grain packing (Poorly sorted - less porous; cubic packing more porous than rhombohedral) - Well-rounded coarse-grained sediments usually have higher porosity than fine-grained sediments, because the grains do not fit together well. Permeability - Ability to transmit fluids; degree of interconnection of voids in the material
Groundwater transport Aquifer - Stores and transmits sufficient amount of water Confining units Aquitard – stores, but slowly transmits water Aquiclude – stores, but does not transmit water Aquifuge – does not store nor transmit water
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Unconfined aquifer - Bounded at the bottom by a confining unit; Water rises up to the water table Perched aquifer - Unconfined aquifer defined by a discontinuous confining unit; Local water table (usually above the main/regional water table) Confined aquifer - Bounded at top and bottom by confining units; Water rises up to the piezometric water level (also called potentiometric line/surface) Potentiometric surface - level to which water will rise in a well due to natural pressure in the rocks Springs - Form when the water table, confined aquifers or GW-bearing fractures/cavities intersect ground surface Artesian wells - Wells tapping a confined aquifer; Analogy: water supply from elevated water tanks - Water in the well rises above the top of the aquifer under artesian pressure, but does not necessarily reach the land surface (in deep wells tapping confined aquifers, the water does not flow to the surface but the water level is higher than the aquifer level) Flowing artesian well – a well in which the water level is above the land surface; the water will flow out freely Does groundwater run out? Aquifers are recharged by the infiltration of rainwater or snowmelt from the ground surface; When the rate of recharge becomes much slower than the rate at which groundwater is being extracted, groundwater can run out, Geologic Work of groundwater Caverns – created by acidic groundwater dissolving soluble rock (limestone) at or just below the surface in the zone of saturation - features found within caverns from at the zone of aeration (e.g. speleothems – stalactites, stalagmites, column) Karst topography – result of dissolution of limestone by groundwater (e.g. sinkholes) Groundwater-related problems >Pumping from a well in a water table aquifer lowers the water table near the well. This area is known as a cone of depression. Groundwater flows towards the well into the cone of depression. >Pollution/contamination and salt-water intrusion Problems associated with groundwater withdrawal >Ground water depletion >Subsidence (e.g. Mexico: 18 in/yr to supply ~20 million people) >Sinkhole formation >Saltwater encroachment >Groundwater pollution or contamination CAMANAVA enhanced flooding: groundwater overextraction à compaction of aquifers à land subsidence à increased susceptibility to floods Groundwater pollution >Septic systems nd
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>Pesticides, herbicides, fungicides, farm chemicals, fertilizers >Organic waste from farm animals (fecal colliform bacteria in water) >Hazardous and industrial waste >LUST (Leak of Underground Storage Tanks) >Leaking landfills >Storage and disposal of radioactive waste >Acid rain or runoff Groundwater availability >Rocks are classified based on their capacity to store groundwater. This is dependent on the rock type, and its porosity and permeability. Igneous and metamorphic rocks are typically not known to store significant amounts of groundwater. Lecture - Mass Wasting Mass Wasting - downslope movement of rock, regolith (unconsolidated material) and soil under the influence gravity - some mass-wasting processes act very slowly; others occur very suddenly, often with disastrous results Factors Promoting Mass Wasting >Slope angle >Water content >Material type >Triggering mechanisms Slope Angle: Two components of gravity act on a slope: a.) normal force – helps to hold the object in place b.) shear force – pulls the object in the downslope direction On steeper slopes, the shear force is greater than the normal force. Angle of Repose - steepest angle at which a pile of unconsolidated grains remains stable and controlled by frictional contact between grains Amount of Water: Dry unconsolidated grains will form a pile with a slope angle determined by the angle of repose; Slightly wet unconsolidated materials exhibit a very high angle of repose due to surface tension between the water and the solid grains holding the grains in place; But when the material becomes saturated with water, the angle of repose becomes very small material flows like a fluid, because the water gets between the grains and removes grain to grain frictional contact →
Type of Material: > Bedrock is more resistant to slope failure than soil > Presence of fractures increase susceptibility to mass wasting > Orientation of bedding planes – Those that dip downslope (daylight structures) have a higher tendency for slope failure Triggering Mechanism: > Earthquake – seismic and volcanic activity, minor shocks like heavy trucks rambling down the road or man-made explosions > excessive rainfall – saturates material nd
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> undercutting of slopes and infrastructure reduces slope stability Types of Mass Wasting Classified by: Type of Motion – fall, slide, flow Rate of movement – imperceptibly slow to very rapid movement Type of material – rock, debris, earth Fall - Piece of material on a steep slope becomes dislodged and falls down the slope *Accumulation of fallen material at the base of slopes or cliffs is known as talus. Slide - materials slide down pre-existing surface (e.g. bedding plane, foliation surface or joint surface) Two types: >Translational – material moves along a roughly planar surface; Differ from slumps because there is no rotation of the sliding rock mass >Rotational (slump) - downward and outward rotation of material along a concave-upward surface; leaves arcuate scars or depressions on the hill slope Flow - distribution of velocities in the displacing mass resembles that of a liquid a. Mudflow - Highly fluid, high velocity mixture of sediment and water (consistency similar to wet concrete); Move at velocities >1 km/hr *Volcanic mudflows are known as lahars b. Earthflow – involves finer materials, usually slower than mudflows - leaves a tongue-shaped deposit at the base c. Creep - requires years of gradual movement (a few inches to several feet per year) to have a pronounced effect on a slope; due to the expansion and contraction of surface sediment, and the pull of gravity. Mitigating Measures > Engineering intervention – retaining walls with weep holes, wire mesh and bionets, rock bolts, removal of portions susceptible to mass wasting > Revegetation > Hazard preparedness – hazard mapping, information campaign Lecture - Metamorphic Rocks METAMORPHISM - change undergone by an existing rock (e.g. igneous, sedimentary or metamorphic), in the solid state, to another rock Agents of metamorphism – Heat, Pressure, Chemically active fluids Heat Sources > Geothermal gradient - temperature increases with depth (20o – 30oC per km in the crust) > Large bodies of molten rock or intrusive bodies Provides the energy to drive chemical reactions recrystallization recrystallization of minerals →
Pressure nd
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- When subjected to confining pressure, minerals may recrystallize into more compact forms. Two Types: > Confining pressure – equal stress in all directions; from overlying rock > Differential stress – unequal pressure in different directions Chemically Active Fluids Sources? > Water trapped in pore spaces of the original rock. > Water released during dehydration of minerals, such as amphibole or mica. > Water from magmatic bodies (hydrothermal fluids). Types of metamorphism Contact Metamorphism - occurs when magma invades pre-existing rock. A zone of alteration called an aureole (or halo) forms around the emplaced magma - involves high temperature - Takes place at shallow depths (0-6km) and low pressure - Metamorphic rocks produced: non-foliated; fine-grained Regional Metamorphism - Takes place at considerable depths over an extensive area (5-20 km, sometimes more than 30 km) under high pressure and associated with the process of mountain building - When continents collide (A) or oceanic crust subducts (B) Hydrothermal Metamorphism - Chemical alteration at high temperatures and moderate pressures by hot, ion-rich (hydrothermal) fluids that circulate through fissures and cracks. - common in basaltic rocks where hydrothermal metamorphism results in alteration to such MgFe rich hydrous minerals as talc, chlorite, serpentine, actinolite, tremolite, zeolites, and clay minerals. - Rich ore deposits are often formed as a result of hydrothermal metamorphism. Shock (impact) metamorphism - When an extraterrestrial body, such as a meteorite or comet impacts with the Earth or if there is a very large volcanic explosion, ultrahigh pressures can be generated in the impacted rock - These ultrahigh pressures can produce minerals that are only stable at very high pressure, such as the SiO2 polymorphs coesite and stishovite. Classification of metamorphic rocks Based on: 1. Texture – size, shape and relationships of constituent minerals (foliated and non-foliated) 2. Protolith – original rock that was transformed by metamorphism (in low-grade metamorphic rocks, original textures may still be preserved and might allow determination of likely protolith) 3. Mineralogy – mineral assemblage present Textures nd
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> Foliated – exhibits a pervasive planar structure known as foliation which is due to the nearly parallel alignment of minerals and/or compositional and mineralogical layering in the rock > Non-foliated – no preferred mineral orientations; metamorphism leads to transformation of one mineral to another
Resources from metamorphic rocks 1. Marble is used for statues and ornamental building stone. 2. Slate is used for roofing, flooring, billiard/pool tables, and blackboards. nd
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3. Talc is ground into powder. 4. Graphite is used in pencils and lubricants. 5. Garnet and corundum used as gemstones and abrasives. 6. Kyanite, andalusite, sillimanite used as raw materials in the ceramics industry. 7. Sulfide deposits (bornite, chalcopyrite, galena, pyrite, and sphalerite) 8. Iron and tin oxide deposits (hematite, magnetite and cassiterite) 9. Tungsten deposits (wolframite and scheelite) 10. Precious metal deposits (gold)
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Geology 11 2 Gabo ‘O8
exam lecture notes
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