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Earth Sciences
Earth Sciences 1070A/B
Paul Whitehead

Unit Seven Sedimentary Environments (Shorelines & Deserts) 7.0 Waves Wind-generated waves are the dominant modification process of shores Stables waves gradually form an advance if wind exceeds 3km/hour Top of the wave: crest Lowest dip of the wave: trough The vertical distance between the crest and the trough is wave height Distance between two crests: wavelength Wave period: the time taken between the passage of two successive crests at a stationary point Height, wavelength and period are dependent on: Wind velocity Length of time wind has been vbowing The distance the wind has travelled over open water (fetch) When wave height increase to a pint where they begin to topple over, they form breakers known as whitecaps Each water particle moves in a circular motion during the passage of a wave Energy transferred from the wind to the water is also transmitted below the service Wave base: the depth equal to have the wave length where the movement of water particles stops 7.1 Shorelines Water is forced into cracks which causes the air to be highly compressed As the wave retreats, the air expands again Similar to frost wedging, this compression and decompression continues and can cause the dislodging of rock fragments Because of a shoreline’s irregular shape, waves can bend or undergo wave refraction in a surf zone Waves rarely approach a shore at right angles/straight on Most waves approach the shore at angles Because of refraction, the greatest impact of waves is against the sides/ends of a headland The wave meets the headland and is bent around it from all sides 7.2 Tides Waves are pulled by the gravitation attraction between Earth and the Moon The tidal bulge remains consistently in place relative to the Moon as Earth rotates As a tide rises and falls there is a horizontal flow of water called tidal currents When the tide rises, water advances into or toward the coastal zone, this flow is called flood currents Have a profound and important impact at the mouth of a river or estuary that meets the sea 7.3 Shoreline Features Created by erosion or accumulation of sediment Erosional features include wave undercutting coastal land to develop wave-cut cliffs or wave- cut platforms A coast’s profile can be divided into three main components to help separate and define the different sedimentary environments Offshore zone: the base/bottom of the coastal profile, permanently submerged and below the base of fair-weather waves Fair-weather wave base: the depth at which fair-weathered waves (not storm) touch bottom and where breakers start to form (on average) Shoreface zone: lies between the fair-weathered base and low-tide mark and is subject to continuous wave action 7.5 Deserts and Winds Dry environments lacking vegetation, dominated by rock and Sand Receives less than 25cm of rainfall each year 7.5.1 Latitude Air near the equator absorbs moisture and rises up Rising warm are cools as pressure decreases at it moves higher Cooler air can not hold as munch moisture as warm air – water vapour condenses and falls as rain 7.5.2 Mountains: Rain-Shadow Deserts Mountains are obstructions of air flow and shape moisture distribution of air pressure systems As moisture-laden air flows over a mountain range, it begins to rise in order o pass over the mountain As it rise: ability to hold water decreases Moisture condenses, falls on the windward side of the mountain rage Drier, cooler air flows down the leeside of the mountain Rain-shadow desert: the dry zone on the leeside of the mountain range 7.5.3 Polar, Coastal and Interior Deserts In polar regions, cold air is constantly descending and the dry air prevents rain cloud formation 7.6 Processes in an Arid Climate Same sedimentary/weathering processes but operating in different climates Water is sparse, chemical weathering is reduced Limited chemical weathering results in the tinted/stained colours of desert material 7.7 Water in the Desert Periods of time when streams flow after intermittent periods of precipitation These short-lived streams are called ephemeral streams When rain does fall it is usually heavy and occurs over a short period of time Most desert sediment is unconsolidated and not anchored by vegetation so the erosion during a single flash flood is very high No extensive network of tributaries to supply with water – die out and die before they reach the ocean/sea Deserts also contain seasonal lakes fed by ephemeral rivers 7.7.1 Wind Transport and Erosion Moving air can flow turbulently, moving loose debris and transporting it over great distances Majority of bed load carried by wind is sand that moves by saltation (skipping/bouncing) Sand grains do not move very far even in high winds Coarse sand grains that are too large to be driven by saltation are driven forward by the saltation of other grains Dust/small suspended grains can be carried by wind Blowout: an area where a large volume of material has been removed and left a depression behind The removal of lighter material can result in coarse materials being left behind: desert pavement 7.8 Sand Deposits formed by Wind Sand dunes are the most common feature in desert environments Mounds of sand that are deposited by wind in depressions or other areas where wind speeds slow down Commonly grow between 30 – 100 m Most are asymmetrical Several different types of dunes can form dependent on vegetation, velocity, availability of sand Barchan dunes: form in rocky deserts where sand is limited – flat, hard surface and sparse vegetation allow the dune to grow higher in the centre, but the sides migrate more because they are smaller Not connected to each other and migrate separately Transverse Dunes: sand is abundant and evenly disperse, it can accumulate in long ridges, alinged perpendicular to the prevailing wind Vegetation is sparse and dunes are separated by troughs Barchanoid Dune: forms between transverse/barchans dunes, oriented at right angles to the ridge but ridges are scalloped Parabolic Dunes: form when vegetation partially covers the sand/sand is abundant, these dunes form in a similar appearce to Barchan dunes but flipped Longitudinal Dunes: wind is erratic by prevailing wind is from the same general quadrant of the compass – form parallel to prevailing wind Unit Eight Groundwater and Glaciation 8.0 Groundwater Groundwater acts as an erosional agent in the subsurface Can control the variable/seasonal flow of streams, can account for fluctuation of rivers Water table depth fluctuate seasonally Water table is rarely level and undulates, mimicking the profile of the topography Changes in water tale lag after periods of rain/no rain 8.1 The Movement of Groundwater Porosity: the percentage of the total volume of void space in a soil/material Pore space is congrolled by grain size, shape, sorting, how well the grains pack together and amount of cement Permeability: measure of the material that can transmit a fluid If the pores are well interconnected, the medium has a higher permeability Groundwater moves a few cm/day When the water table meets a steam, the groundwater enters the stream from all directions Slope of the water table (hydraulic gradient) is determined by dividing the vertical difference between the recharge – discharge points by the length of flow between these points 8.2 Wells, Springs, Hot Springs and Geysers Spring: is a natural outflow of groundwater where the water table intersects the Earth’s surface Hot spring: mean temperature of 6 – 9 degrees warmer than the mean annual air temperature Most common removal of groundwater occurs through wells When water is pulled from a well, the immediately surrounding water table is lowered by the drawdown effect Unit Eight Groundwater and Glaciation 8.3 Groundwater Formations and Processes Groundwater is an effective medium for dissolving rock – particularly soluble rock like limestone Although groundwater is fresh, it tends to contain carbonic acid from decayed organic material/atmospheric carbon dioxide Carbonic acid is effective at dissolving limestone Caves and caverns are the most common result of groundwater erosion of limestone below the water table Caverns are typically below the water the table When caverns are above the water table, groundwater seeps through the upper limestone to the ceiling of the cave and drips onto the cavern floor Dripping water contains dissolved calcium/carbon dioxide which it leaves behind Produces limestone called travertine Areas that have been shaped by groundwater dissolution are said to exhibit karst topography Typically irregular and contain sinkholes Sinkholes can form in two ways: Gradually when limestone below the solid is dissolved, bedrock is lowered and fractures within the limestone grow Can also form suddenly as the roof of a cavern collapses under its own weight Lack of surface drainages such as streams 8.4. Glaciers and Glaciation Ice sheets are large continental masses of ice/glacier Glaciers are not stationary and flow due to gravity, mass and minor melting 8.4.1 Formation, Movement and Erosion Unit Eight Groundwater and Glaciation Form when snow accumulation compacts to form ice With continued accumulation, air is forced out and the snow recrystallizes in a denser mass called firn Movement of glaciers can be divided into: internal deformation, basal slip and soft bed deformation Internal deformation occurs when the weight of 50 m of ice is exceeded Due to high pressure, ice behaves plastically and begins to flow Basal slip occurs when the base of the glacier slides across the ground beneath it Soft Bed Deformation occurs when the underlying sediment or bedrock deforms under the stress of friction caused by the weight of the glacier Glaciers are capable of significant erosion Glaciers erode by: 1. Plucking: when meltwater penetrates the underlying bedrock fractures and through freeze-thaw action pries loose rock away from the bedrock 2. Abrasion: the rock fragments trapped in the base of the glacier scrape across the underlying bedrock as ice flows overtop 8.4.2 Glacial Erosion and Landforms In mountainous regions, glacial erosion can widen initially v-shaped rivers into u-shaped glacial troughs Glaciers cause a wide range of erosion styles on landforms Erosion can also develop due to the melting of a glacier rather than its movement Glacier can be separated into three zones: 1. Zone of accumulation, where snow fall accumulates 2. Equilibrium line, the elevation of the snowline 3. Zone of ablation or wastage, where all the snow and ice accumulate during the winter melts Unit Nine Metamorphism and Crustal Deformation 9.0 Metamorphism Unit Nine Metamorphism and Crustal Deformation Metamorphism is the change that takes place within a body of rock as a result of it undergoing conditions different than those in which it formed Type and degree of change in the transformation if an existing parent rock depends on type and intensity of the individual/combined metamorphic processes Can be used to describe what happens when rocks are buried under other rocks, subjected to high temperatures and pressures than those at the surface Main factors that influence the mineralogical makeup of metamorphic rock are: a. The bulk composition of the rock b. The pressure and temperature conditions a the time of crystallization c. Composition of the fluid phase in the rock during metamorphism Metamorphic rocks all originate from a parent rock The chemical composition of a metamorphic rock is largely influenced by the parent rock Texture/mineral make up depend on particular processes involved during metamorphic transformation Temperature is one of the most important factors in metamorphism Heat drives chemical reactions for mineral recrystallization and influence the reactivity and mobility of chemically active fluids Pressure is another important factor in metamorphism responsible for changing rock Presence of fluid/water within the rock is critical to the process of metamorphism Much of the water is derived from the minerals themselves but other volatiles are important as well Metamorphic rocks are often described in terms of their metamorphic grade, which refers to the intensity of metamorphism Increase in metamorphic grade is indicated by the appearance of minerals that are stable at progressively higher temperatures 9.1 Types of Metamorphism Burial, region and contact metamorphism Unit Nine Metamorphism and Crustal Deformation Burial metamorphism: is very mild, with slight increases to pressure/temperature Usually related to burial of sediments in a basin where geothermal heat produces very low grade metamorphic reactions Regional metamorphism: affects very large areas More intense than burial metamorphism Metamorphic rocks are products of the high temperatures and pressures resulting from deep burial/tectonic forces Common consequence of the convergence of tectonic plates/mountain building Directed pressure occurs Convergent plate settings: compressive forces, usually resulting the rock being squeezed in one direction and stretched in another Subducting slab is relatively low temperature but high pressure Contact metamorphism: igneous body rising through the crust and baking the adjacent rocks that are in contact with the intruding body 9.2 Textures of Metamorphism Schist: medium – coarse grained rock that is dominated by platy mica minerals Gneiss: medium – coarse grained banded metamorphic rocks in which elongated minerals predominate At low grades of metamorphism, basalts will be metamorphosed to a rock termed greenschist or greenstone 9.3 Crustal Deformation Large scale tectonic forces that deform the crustal rocks of Earth General term that refers to all changes in the orientation and location of rocks is deformation Rocks deform when the are subjected to stress and stain Stress is the amount of force applied to a given area Unit Nine Metamorphism and Crustal Deformation Strain is the visible result of the stress and can be applied in different directions Differential stress that results in a shortened rock body is termed compressional stress Shortens and thickens crust by folding and faulting Tensional stress: pulls rocks apart Occurs at divergent boundaries Shear stress: occurs when two rocks are forced past one another 9.4 How Rocks Deform When rocks are subjected to stresses that exceed their own strength they deform by flowing (folding) or brittle fracturing (faulting) Brittle deformation occurs in near-surface environments of low temperatures/pressures but where rock strength is exceeded, the rocks will break apart into fractures In deep crustal environments with high temperature/pressure cause solid-state flow ductile deformation occurs Permanent changes in the rocks occur Rock type has a great influence on the type/degree of deformation, particularly in near-surface environments Unit Ten Economic Petroleum and Ore Deposits 10.1 Igneous Ore Deposits Magmatic ore deposits that form through crystallization/ gravitational settling are called stratiform or layered mafic intrusions Two types of igneous deposits made of disseminated massive layers of metals in the lower parts 10.1.1 Layered Mafic Intrusions Magma rises to form a pluton like body Magma trapped within the pluton begins to cool and undergoes Bowen’s Reaction series For pluton to be economic, the magma must be enriched or contain abundant amounts of metallic elements 10.1.2 Bushveld Igneous Complex Layered igneous intrusion Vertically it is divided into several different intervals More magnesium-chromium rich rocks near the base Formed when mafic/ultramafic magma was emplaced at a shallower, cooler level in Earth’s crust due to divergent tectonics 10.1.3 Sudbury Ig
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