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Geophysics
- 1. GEOPHYSICS Introduction Geophysics isthebranch ofsciencethat is concerned with the physical, chemical, geological, astronomical -and other characteristic propertiesof the earth. It deals with geologicalphenomena such as the temperaturedistribution of the earth'sinterior, thesource, configurationand thegeomagnetic field. Interior structure ofthe earth The structureofthe earth is composed of threemajor zones arranged ina concentric manner. Theseare crust, mantle and core; Internal structure of the earth The crust
- 2. The crust isthe outer solid layer of the earth. It is extremelythin (5 to 15km) compared totheradiusof the earth (6371km). There aretwo types of crust, namely continental crust and oceanic crust. Continentalcrust is heterogeneousand of relatively low density(2 to 2.8 tonnes per cubic meter). It is composed mainlyof granites and sedimentary rock. Oceanic crust isbasaltic and hasa. higher density (3.0 to 3.1 tonnes per cubic meter). Both the continentaland oceanic crustsfloat on the denser mantle. Because of its low density, the continentalcrust floatson the mantleat a higher elevation, forming the land massesand mountains. The continentalcrust is 30 to 70km thick. The dense oceanic crust floats at a lower elevation forming oceanic basins. It is about 8km thick. The boundarybetweenthe crust and the mantle is called Mohorovicic discontinuityor simply Moho. It is a zone betweenone and several kilometersthick. The mantle The mantlebeginsfrom the Moho and extends to a depth of 2900km below the earth'ssurface, up to itsboundarywith the earth'score. This boundaryis called the Gutenberg discontinuity. The mantlecontainsabout 70% of the earth'smass. It is madeup of rocks, both in solid and in molten states. These rocks are said to be in a plastic state. The upper part of the mantle hasa temperatureofabout 870°C. The temperatureincreasesdownwardsthrough themantletoabout 2200°C near thecore. Circulationofmaterialsinthe mantleis the mainmechanism ofheat transfer from thecore of the earth to theouter regionsof the earth. It is the mainforce that drivesthe movement of continentsaswell as volcanism and earthquakes. The core
- 3. The core isthe innermost part of the earth. It extendsfrom the Gutenberg discontinuitytothe geometric center. Thecore consistsof two distinct regions, namely the outer core and the inner core. The outer core is composed of a liquid of molten nickel and iron known as magma. It extendsfrom the mantleto a depth of about 5000km below the earth'ssurface. The inner coreis solid; it is composed of iron-nickelalloys. The materialofthe inner core is solid becauseof the high pressure at this depth. Tectonic plates The earth’scrust and part of the mantleare cracked intohugepiecescalled tectonic plates. Theseplatesfloat on top of the semi-moltenrock underneath. They move about at a very slow speed. The movements of the tectonic platesmeanthat some continentsaremoving apart and some are moving towardseach other. Thisprocess is referred to as continental drift and hasbeen going on for hundredsof millionsof years. Tectonic platemovements have split the continentsas- we, know them today. Tectonic plate
- 4. The line wheretwo tectonic platesmeet is called a boundary. Thereare three maintypesof boundaries. Theseare destructiveboundaries, constructiveboundaries and conservativeboundaries. Destructive boundary (convergent)- isone found at the edges of two platesmoving towardseach other. Constructive (boundariesdivergent)- areformed at the edges of two platesmoving awayfrom each other. Conservative boundaries- areformed when two platesslide past each other without moving apart or towardseach other. Diagram constructive and destructiveboundary Volcanoes and earthquake Volcanoesand earthquakesareclosely related. They areboth caused by the movement of molten rock and heat deep inside the earth. These movements are referred to as subterranean movements. Most earthquakesand volcanic activityhappennear tectonic boundaries. Volcanoes Volcanoesare places wheremolten rock called magma leaks out through a hole or a crackinthe earth'scrust. Magma originatesfrom themantle where high temperaturesand pressurecausetherock to melt. When a large pool of magma isformed, it rises through thedenser rock layer towardsthe earth'ssurface. Magma that hasreadied theearth'ssurfaceis called lava.
- 5. Volcanic eruption Most volcanoesform along constructiveand destructiveboundaries betweentectonic plates. However, few form far from plateboundaries. Volcanoes at destructiveboundaries When an oceanplate plungesunder another plate, theocean platerubs against theplateabove it and gets hot. The rock melts resulting in magma under the upper plate. This pool of magma forcesitsway through weak pointsin the crust. This createsa line of volcanoes parallel to the boundary but off to one sidein the upper plate. Most of the world’s volcanoes occur at destructiveboundaries. Nearlyall the way around the Pacific Oceanisa line of destructiveplateboundarieswhew ocean platesslide under continental platesand other oceanplates. These boundariesharecreated a circleof volcanoes around the rim of the Pacific. Thiscircleiscalled the Ring of Fire. It runs along the west coastsof South America, and North America, through Japan, thePhilippinesand New Zealand.
- 6. The ring offire Volcanoescan also form along constructiveboundaries. Theseareformed where two platesmove apart at the boundary. Magma movesup from underneath tofill the gap left by the separatingplates. TheMid-Atlantic Ridgethat runs along the floor of the Atlantic Oceanisa continuous volcano that is thousandsof kilometers long. It formed at a constructive boundarybetweenthe plates.
- 7. The mid Atlanticridge Hot-spot volcanoes Volcanoesalso erupt thousandsof kilometers awayfrom tectonic plate boundaries. It is thought that these eruptionsoccur over placesin the mantlethat arehotter thannormal. Magma from these hot-spotsforces its way through thecrust above and onto the earth'ssurface. Nyamulagira Volcanoin Congo is a hot-spot volcano. Types of volcanoes There aretwo maintypes of volcanoes, namely fissure and central volcanoes; Fissure volcanoes These occur along cracksinand betweentectonic plates. They canbe many kilometerslong. Lava is usually ejected quietlyand continuously, forming enormous plainsor plateausof basaltic volcanic rock. Fissurevolcano Central volcanoes
- 8. These hare asingle verticalmainvent through which magma reachesthe earth'ssurface. They usually develop a cone shape that buildsup from successivelayers of lava and ash. Central volcano Classificationof volcanoes Volcanoesare classified intothree categoriesbased ontheir frequencyof eruption. These are active, dormant and extinct volcanoes. Active volcanoes-Arethosethat either erupt constantlyor have erupted in recent times. They include the Ol Donyo Lengai Volcano. Dormant volcanoes-Arethosethat have been inactivefor some time(a few thousand years) but can erupt again. Anexampleis Mt. Kilimanjaro. Extinct volcanoes-Arethosethat havenot erupted in recorded history. They will probablynever erupt again. Effects of volcanoes 1.Landscape - Volcanoeshave a great effect on the landscape. Much of the earth'ssurfaceis covered with volcanic rocks. Volcanoesare also responsiblefor the formationofmany mountainsand islands. 2.Vegetation and wildlife - Volcanic eruptionsdestroyvegetation. The eruptionssometimesset the surrounding vegetationonfire. Such fires consumehuge tractsof vegetationthat includeforests, woodlands and grasslands.
- 9. 3.Wild animals-are alsokilled by being buried inthe lava or being burnt by the forest fires. 4.Environment - Besides the destructionofvegetation, volcanic eruptions emit harmfulgases intothe environment. Such gases includesulphur dioxide. Someof thegases contributetoglobalwarming and climate change. 5.Human life and property - Volcanic eruptionssometimeskillpeople and destroyproperty. People who monitor volcanic activitiesusuallywarn people of an impending eruptionsothat people life can vacatesuch areas. However, some eruptionshappenunexpectedly. Such eruptionsbury people, animalsand buildingsinmountainsof lava. 6.Soil- Volcanoes help in soil formationby bringing importantsoil mineralsfrom deep underground ontothe earth'ssurface. 7.Minerals=Volcanic eruptionsalsobring valuablemineralsto the earth's surface. The mineralsare importanteconomic resources. Earthquakes Earthquakeshappenwhenrocks in the earth'scrust move suddenly, shaking the earth. About 10,000 earthquakeshappen, everyyear but most are so small that they can only be detected byvery sensitiveinstruments. Earthquakesalsooccur as a result of movement of magma at constructive boundaries, under volcanoes and where continentalplatescollideand push mountainranges. How earthquakesoccur Earthquakesmostlyoccur on or near the boundariesbetweentectonic plates. However, earthquakescanalso occur far from plateboundaries. Such earthquakesprobablyoccur asa result of faultsformed millions of years ago. Actually, most earthquakesoccur onor near destructiveand conservative boundariesof tectonic plates.
- 10. Tectonic platesgrind pasteach other, rather thanslide past each other smoothly. As the platesmove past each other they canbecomelocked together due to friction. For some tune, they don't move and energy builds up. Earthquakebuild-up Pressurebuildsbetween them until thefrictionalforceholding the plates together givesway. The plates move suddenly, releasing thepressure (energy) and then hold together again. Thissuddenjerk is what is felt as an earthquake. An earthquakeisa suddenmotionor shaking of the earth caused by a sudden release of energythat has accumulatedwithinor along the edges of the earth'stectonic plates. The point withinthe earth wherean earthquakebeginsiscalled the hypocentre or thefocus of the earthquake. Earthquakesrarelyoccur along constructiveplateboundaries. Seismic waves Earthquakesreleasetheir energy in threewaves of energy called seismic waves. These are primary waves, secondary waves and surface waves.
- 11. Primary waves orp-waves arethe-first waves released from the hypocenter. Primarywavesarefelt as a sudden jolt. Secondary waves or s-waves arrivea few seconds later. They are felt as a series of side-to-sidetremors. Seismic waves Surfacewaresradiateoutwardfrom a point on the earth'ssurfacedirectly above the hypocenter. Thispoint is called the epicenter ofthe earthquake. There aretwo types of surfacewaves. These are Rayleigh waves and love waves. Rayleigh waves createa rolling movement that makesthe land surfacemove up and down. Love waves make theground shift from side to side.
- 12. It is thesurfacewaves that do damagetosurfacestructuresuch as buildings and hydroelectric power plants. EARTHQUAKE SCALES The natureof an earthquakeisusually described by measuring two properties. Theseare the magnitudeand intensity. The magnitudeofan earthquakeisa. measureof the energy it releases. It is usually measured on theRichter scale. The Richter scaleis based on the amplitudeofthe largest seismic wave recorded for an. earthquake, no matter what typeof wave was the strongest. The Richter scalemagnitudesarebased on a logarithmicscale(base30). Thismeans that for every whole number you go up on the Richter scale, the amplitudeofthe ground motiongoes up ten times. Using the Richter scale, art earthquake.ofmagnitude 7.0would result in ten' timesthelevel of ground shaking as an earthquakeofmagnitude 6·0. The Richter scalecan be used to describeearthquakesso small that theyare expressed in negativenumbers. The scale has no upper limit. The intensityof an earthquakeisa measureof its strength based on the changesit causes to the landscape. Theintensityof anearthquakeisusually measured on the Modified Mercalli scale. The scale is calibrated from 1 to 12. On this scale, level 1 is a minor tremor that causesno damagewhereas level 12 causes totaldevastation. Thetablebelow shows a descriptionofthe 12 levels onthe Modified Mercalli scale. Intensity level Effect I No felt, except by very few people under especially favorable condition. ii Felt only by o few people at rest, especially on upper floors of buildings. Delicatelysuspended objectsmayswing. Iii Felt quitenoticeablybypersons indoors, especially on upper floors of buildings. Manypeople do not recognizeit as an earthquake. Standing motor carsmayrockslightly. Vibration
- 13. similar tothat ofa passing truck. Durationcanbeestimated. iv Felt indoorsby many, outdoorsby few during theday. At night, some awakened .Dishes, windows, .doors disturbed. Walls makecracking sound. Sensationsimilar tothat of a heavy truckstriking a building. Stationarycarsrocked noticeably. V Felt nearly by everyone. Manyare awakened. Somedishes, windowsbroken. Unstable objectsoverturned. Pendulum clocks may stop. Vi Felt by everyone. Manyare frightened. Someheavy furniture moved. A few instancesof fallen plaster. Damageslight. Vii Damagenegligiblein buildingsof good designand construction. Slight tomoderatedamageinwell-built ordinarystructures. Considerabledamageinpoorly built or badly designed structures. Somechimneysarebroken. Viii Damageslight in speciallydesigned structures. Considerable damageinordinarysubstantialbuildingswith partial collapse. Damagegreat inpoorly built structures. Fall of chimneys, factorystacks, columns, monumentsand walls. Heavy furnitureoverturned. ix Damageconsiderableinspecially-designed structures. Well- designed framestructuresthrownout of plumb. Damage great in substantialbuildings, with partialcollapse. Buildings shifted off foundations. x Some well-built wooden structuresdestroyed. Most masonry and framestructuresdestroyed with foundations. Railbent. xi Few, if any structures(masonry) remainstanding. Bridges destroyed. Rails bent greatly. xii Totaldamage. Lines of sight and level are distorted. Objects throwninto the air. Note that: An earthquakecanonly have one magnitude. However, its intensityreducesas the seismic wavesspread out from the hypocenter, just the sameway the loudness of a sound changesas you move awayfrom the source.
- 14. The seismograph The seismographisan instrument used to record ground movements caused by earthquakes. It measuresground oscillations-byrecording the relativemotionbetween a pendulum and the ground. It is also possible to use the ratiobetweenthe deflectionof the pendulum and theacceleration of theground to record an earthquake. Thetimeof Initiationofground oscillationsis recorded and marked and areincluded on the graphsevery minuteand hour on the seismograph paper. Seismograph In order to measureground motions, the seismograph must remainsteady when the ground moves. Varioustypesof pendulumshave been used to obtainthissteadystate. The simplest type of pendulum is a heavy mass suspended by a wireor rod from a fixed point. , Other forms are the inverted and horizontalpendulum. The inverted pendulum hasa heavy mass fixed to the upper end of a verticalrod pointed at itslower end, while the horizontalpendulum has a rod with a masson its end which is suspended at two points so that it swingson a horizontalplane. Vertical a pendulumseismograph
- 15. Recording the pendulummotion Therecording of the motionof the pendulum can be done, in variousways. The most common ones are the mechanicalmethod, the opticalmethod and the electronic method. Mechanical method In the mechanicalmethod, a sheet of smoked paper is wrapped around a rotating drum and mounted tomove with the earth. A moving pen connected to the pendulum presses lightly on the paper. As time passes, the drum rotatesso that the recorded lines are not superimposed on each other. Deflectionof the pendulum is commonlymagnified mechanicallyby single or double multiplying levers so that thegraph is easier to see. Thismethod is simpleand economical. However, the seismograph must have a heavy mass to overcomethe frictionbetweenthe pen and the paper. Consequently, some mechanicalseismographsweigh onetonne or more. Optical method The opticalmethod still uses a pendulum motionto record the ground movements'. However, to overcome friction, mirrorsareused to reflect the light onto photosensitivepaper wrapped on a drum. Electronic method Technologicaldevelopmentshave given riseto high-precision seismometersand sensors of ground motion. In these electromagnetic instruments, a coil is fixed to the massof a pendulum and moves in a magnetic field. Theelectric current, generated inthe coil, operatesa galvanometer In the .sameway a dynamo operatesa motor. Thevoltages .produced by motionsof thependulum is passed through electronic circuits to amplifythe ground motionfor moreexact readings. The seismograph recordsboth themagnitudeand theintensityof the earthquake. Earthquake hazards
- 16. Earthquakesgiveriseto a numberof hazardswhich pose a great riskto humanlife, animals, propertyand the environment at large. The following are some of the hazardsassociated with earthquakes: (a) Landslides - The shaking caused by earthquakescancause unstablehillsides, mountainslopes and cliffs to move downwards, creating landslides. In massivelandslides created byearthquakes, soiland rock acceleratedownthe slope, sweeping awayeverything in their path. Landslides canfill valleys, creating temporarydams. Thesedamsrelease floodwater when they collapse. Earthquakescan also trigger avalanches on snow slopes Land slide caused by an earth quake (b)Tsunamis –If an earthquakehappensinrocks under the sea or ocean, theshock waves disturb thewater. The ocean floor can also rise or fall causing thewater to riseor fall too. These movements createhugewater waves called tsunamisthat travelacrossthe ocean. When a tsunami reaches shallow water, it slows down, itswavelength reduces and its height grows. When the tsunami hitstheshore it crashes inland carrying everything inits way, including destroyed building.
- 17. (c)Collapsing buildings -Earthquakesdonot actuallykill people. It is the hazardsthat areassociated with earthquakesthat killpeople. The majorityofpeople killed or injured in earthquakesaretrapped in buildingsthat collapsebecauseof the ground, underneath shaking. A strong earthquakecanflattena whole city. An example is the Japanesecityof Kobe which wascompletely flattened by an earthquake measuring 7.2 on-the Richter scale. The earthquakeoccurredin January 1995. More than6 000 people died and about 200,000 buildingscollapsed or were damaged.
- 18. Collapsebuilding (d) Fire outbreak- An earthquakecantrigger a fireoutbreak. This happenswhen the earthquakecausesgas or oil pipes to break. It canalso occur as a result of the collapse of electricitylines. (e) Backward rivers - Tilting ground canalso make riverschangetheir course. Thiscanresult in the creationof earthquakelakesthat cover huge tractsof previously settled land. Earthquake warning signs The following are important signs that are observed before an earthquake occurs: 1. Thermal indicator - A few months before theoccurrenceof an earthquake, theaveragetemperatureofthe area keeps increasing. Onthe day of the earthquake, thetemperatureofa place is about 5 to 9 degrees Celsius abovetheaveragenormaltemperaturefor that day. 2. Water indicator - About one or threedays before an earthquake, there is- a sudden rise or fall in water levels in wells. The rise can be as high as one meter. The well water mayturnmuddy/. At timesa fountainappears insidethe well. Sometimesa fountainmayappear in the ground. This normally happens; a few hours before the quake. There is also a sudden and rapid increaseor decreaseof water flow in the rivers. Thishappens about one to twodays before the quake.
- 19. 3. Seismo electromagneticindicator -Beforethe occurrenceof an earthquakethesub-surfacetemperaturerises. As a result of this, the geomagneticfield is reduced. The reductioningeomagnetic field adversely affectsthe propagationofelectromagneticwaves. Thisis experienced abundantlyon the radio, televisionand telephone. This is a very reliable indicator. It is usually recorded about 10 to 20 hours before thequake. If a particular radiostationisreceived at a frequency of 1000kHz, thesame stationwill be received in the potentialepicenter area at higher frequencies, about 10 to 30 hours before theearthquake. Similarly, receptionof television signalsis affected. The mobiletelephone is one of themost reliableindicatorsofan impending earthquake. About 100 to150 minutesbeforethe occurrenceof an earthquake, mobiletelephonesstop functioning or becomemalfunction. Note that all the above indicatorsarevalid only when seen and manifested extensively. Failureof one or two instrumentsshould not be takenas an earthquakeindicator. 4. Animal indicator - Between 10 and 20 hours before the occurrenceof an earthquake, theentireanimalkingdom becomeshighlydisturbed and restless. They move in a directionlessmanner and in fear. Birdsdo not perch on trees but move about at a low height, emittinga shrill noise. Rodents like ratsand mongooses go into a panic. Domestic animalslike cows, dogs and catsstruggleagainst being tied up and mayeven turn on the owner. 5. Human indicator - Doctorsand nurses have observed that some sensitivepatientsinhospitalsbecomehighly disturbed beforean earthquake. Theyexhibit a suddenrise in blood pressure, heart trouble, headache, migraineand respiratorydisorders. Indeed, the number of outpatientsin< hospitalsincreasesbyfive to seven times, some 10 to 20 hours before the quake. The best humanearthquakeindicator' isthenumber of child deliveries in any hospital. On the penultimatedayof the earthquake, thenumber of deliveries goes up about threeto five times, while on the day of the earthquakeit isas high as seven to eight timesthe average. Precaution to be taken during an earthquake
- 20. The following aresome precautionsthat canbetakento minimizeinjuries to or death of humanbeings, in the event of an earthquake: 1. If you are indoorsduring an earthquake, drop, cover and hold on. Get under a desk, tableor bench. Hold on to one of thelegs and cover your eyes. If thereis no tableor desk nearby, sit down against aninterior wall. An interior wall is less likely to collapse thana wall on the outsideshell of the building. 2. Picka safe placewhere thingswill not fall on you away from windows or tall, heavy furniture. 3. Do not run outsidewhen an earthquakehappensbecausebricks, roofing and other materialsmayfall from buildings during and immediately after an earthquake, injuring personsnear the building. 4. Wait inyour safe placeuntil the shaking stops, then check to see if you are hurt. You will be better ableto help othersif you takecareof yourself first, and then checkon the people around you. 5. Move carefullyand watch out for thingsthat havefallen or broken. Creating hazards. Be readyfor additionalearthquakescalled aftershocks. 6. Be on the lookout for fires. Fire is themost commonearthquake-related hazard due to damaged gasand electricallines.If you must leave a building after the shaking stops, use the stairsand not the elevator.Earthquakescan causefire alarmsand fire sprinklers to go off. You will not be certain whether there is a real threat of fire. As a precaution, usethe stairs. 7. If you areoutsideduring an earthquakestayoutside. Moveaway from buildings, trees, streetlightsand power lines. Crouch down and cover .your head. Bricks, roofing and other materialscanfall from buildings, injuring persons nearby. Trees, streetlightsand power lines may also fall, causing damageor injury. Structure and composition of the atmosphere The earth is surrounded above it by a layer of gasescontaining numerous small suspended solid and liquid particles. Thislayer is called the atmosphere. Theatmosphereconsistslargely of, a mixtureof gases,
- 21. extending toa heightof many kilometersabove the earth. It has no outer boundary. It just fades into space. The dense part of atmospherelies within 30 km above the earth'ssurface. Structure of the atmosphere The atmosphereisdivided into regionsbased on itsthermalcharacteristics (temperaturechanges), chemicalcomposition, movement and density. The five regions arethe troposphere, stratosphere, mesosphere, thermosphere and exosphere. Verticalstructureofatmosphere Troposphere Thisis theregion nearest to the earth. It extendsto an altitudeof up to 10 km above the poles and 20 km above the equator. Thisregion is the densest part of the atmosphere. It contains80% by mass of theatmosphere. It containsmost of the atmosphere'swater vapor. The
- 22. temperatureinthisregiondecreaseswith altitudeat anaveragerateof 6°C/km. Thetroposphereis well mixed. Air molecules can travel to the top of the troposphereand backdown againin just a few days. This mixing encourageschanging weather. Most weather phenomena occur in the troposphere. Clouds and rainare formed withinthisregion. The boundarywhich separatesthetroposphereand the stratosphereis called the tropopause. At thetropopause, the temperaturesstop decreasing with height and becomeconstant. The tropopausehasan averageheight of 10 km. Stratosphere The stratospherestartsfrom thetropopauseand extendsto 50 km high. Thislayer is morestable, drier and less dense compared tothe troposphere. The temperatureinthe stratosphereslowly increaseswith altitude. The temperatureincreaseisdue to the presenceof the ozone layer which absorbsultraviolet rays from the sun. The ozone layer lies in the middleof the stratospherebetween20 and 30 km. Ozone is a triatomic(three- molecule) form of oxygen. Thislayer plays the importantroleof absorbing ultraviolet radiationswhich would otherwisereach the earth's surface. Ultraviolet radiationisharmful to both animaland plant life on earth. The stableair of the stratosphere also prevents large stormsfrom extending much beyond the tropopause. Planes also fly in the stratosphere. Thisis becauseit has strong steady horizontalwinds which areabove the stormyweather of thetroposphere. The troposphereand stratospherearecollectively known as the lower atmosphere. The boundarywhich separatesthestratosphereand theother layers is called stratopause. Mesosphere
- 23. The mesospherestartsjust abovethe stratosphereand extendsto 85 km high. The temperatureat thislayer decreaseswith altitude. Thelowest temperatureofthe atmosphere(-90°C) occurswithinthisregion. The mesosphereis the layer in which most meteorsburn while entering the earth'satmosphere. The boundarywhich separatesthemesosphereand the thermosphereis called the mesopause. Thermosphere Thislayer startsjust abovethe mesosphereand extendsup to 690 km high. The temperatureincreases with increasing altitudedue to thesun’s heat. The temperatureinthisregion cango as high as 1 727°C. Chemical reactionsoccur much faster herethan on the surfaceof the earth. Thislayer is also known as the upper atmosphere. The lower part of thethermosphere, from 80 to 550 km above the earth's surface, containstheionosphere. Thisis a region containing a high concentrationofcharged particlescalled ions and free electrons. The large number of free electronsin the ionosphere allows the propagationof electromagnetic waves. The ionospherealso absorbsdangerousradiation. Theradiationabsorbed in the ionosphereincludes hard and soft X-raysand extremeultraviolet (EUV) radiation. The ionosphereplays an importantrolein communications. Radiowaves canbe reflected off the ionosphere allowing radiocommunicationsover long distances. Exosphere The exosphereis the outermost regionof theatmosphere. In this region, the atmosphericgaspressureis very low. Light atomssuch as hydrogen and helium may acquiresufficient energytoescapethe earth'sgravitationalpull The upper part of theexosphere is called magnetosphere. Themotionof ions in this regionis strongly constrained bythe presenceof the earth's magnetic field.
- 24. Thisis theregion wheresatellitesorbit theearth. Importance of the atmosphere The following are some ways in which the layers of the atmosphereare important: 1. The tropospherecontrolsthe climateand ultimatelydeterminesthe qualityof life on the earth. 2. The troposphereis importantfor life on the earth. The layer contains gases which includeoxygen which is used for respirationbyanimalsand carbondioxidewhich isused by plants in photosynthesis. The nitrogenfound in thislayer also providesan inactiveenvironment for many chemicalprocessestotake place. The gases also support many important chemicalprocessessuch ascombustion, weathering and oxidation. 3. The “Stratosphereprevents harmful ultraviolet radiationfrom reaching the earth. 4.The mesosphere, thermosphere and exospherealso prevent harmful radiationsuch ascosmic raysfrom reaching theearth'ssurface. 5.Communicationisalso madepossible by some layers of the atmosphere, specificallytheionosphere. Global warming Global warming istheincreaseof the averagetemperaturesnear or on the surfaceof theearth as a result of what is known as the greenhouse effect. The effect is caused by greenhousegases. These gases areproduced from naturaland industrialprocesses. The greenhouse effect 1. The greenhouseeffect is the process in which theemission of radiationbytheatmospherewarmstheearth’ssurface.
- 25. When heat fromthe sun reachesthe earth'ssurfacein form of sunlight, some of it is absorbed by the earth. The rest is radiated back to the atmosphereat a longer wavelength thanthe incoming sunlight. Some of these longer wavelengths areabsorbed by greenhousegases in the atmospherebeforethey are lost to space. The absorptionof this long-wave radiant energywarmstheatmosphere. Thegreenhouse gases act like a mirror, reflecting backtotheearth some of the heat energy which would otherwisebe lost to space. Sources of greenhouse effect The maingreenhousegases are carbondioxide, methane, chlorofluorocarbonsand Dinitrogenoxide. •Carbon dioxide Carbondioxideisthe maingreenhousegas. The gascontributes over 50% of the greenhouse effect. The following are some of the sources of carbondioxideinthe atmosphere: Clearing and burning of vegetation-Greenplants, especiallywoody plants, absorb carbondioxidefrom theatmosphereasthey grow. When the wood dies, the carbondioxideisreleased backinto the atmosphere. Clearing of forests (deforestation) and burning vegetationresultsin the release of carbondioxidetothe atmosphere. The loss of the forestsalso meansthat there arefewer trees to absorb carbondioxide Deforestationcontributeglobalwarming Deforestationremainshigh inthe world.
- 26. •Burning of fossilfuels Carbondioxideis a by-product inthecombustionof fossil fuels such as coaland petroleum. These fossil fuels areburnt in cars, power stationsand industries. •Methane The mainsourceof methaneis- agriculturalactivities. It is released from wetlands, such as ricefields and from animals, particularlycud-chewinganimalslikecows. The emissionof methane gas, therefore, increaseswith, increaseinagriculturalactivities. Methaneis also produced during themining of coal and oil and when vegetationisburnt. Sincethe 1960sthe amount of methanein the air has increased by1% per year, twiceasfast as the build-up of carbondioxide. Methanemolecules have a lifetimeof 10 years in the atmosphere. •Dinitrogen oxide Dinitrogenoxideis produced from both naturaland human- madeprocesses. Humanactivitieswhich produce, Dinitrogenoxide includecombustionof fossil fuels in vehicles and power stations, use of nitrogenousfertilizers, and the burning of vegetationand animal waste. •Chlorofluorocarbons
- 27. Chlorofluorocarbons(CFCs) are organiccompoundsmadeup of chlorine, fluorine and carbon. Thesources of CFCs in the atmosphereincludefridges, air conditionersand aerosols. CFCs are extremelyeffective greenhousegases. A CFC molecule is 10 000 times more effectivein trapping heat thana carbondioxidemolecule. Effects of the global warming The following are some of the effects of global warming: •Increase in the temperature of the oceans This causesthe bleaching ofcorals. Bleaching of coralreefs is the loss of pigmentsand microscopic plant cellsfrom coral tissues. Thisresults in the whitening of the coralreefs. •Rise in sea levels Sea levels are rising due to thermalexpansionof the oceans and the melting of land ice. Thismay eventually lead to flooding of coastal lands. •Change in world's climatic patterns The climatic patternsinmost partsof the world have changed. It is becoming hard toforecast the weather accurately. Rain no longer falls when expected. Sometimestherainsareheavier thanexpected, leading to flooding. Other times, the rainsare far less thanexpected, leading to drought. The extent of theearth'ssurfaceunder desert conditionis also increasing. •Acidification of the oceans The world's oceans soak up much of the carbondioxide produced by living organismseither in. theform of dissolved gas, or from the skeletons of tinymarinecreaturesthat fallto the bottom to becomechalk or limestone. '
- 28. The carbondioxidedissolves inthewater and forms a weakcarbonic acid, therebylowering the pH of the oceanwaters. Increased acidityand temperaturesofocean waterseventually lead to the bleaching and death of coral reefs. •Extreme weather events These includefloods, droughts,' heal waves, hurricanesand tornadoes. Hurricanewinds Other effects of global warming include: higher or lower agriculturalyields, Melting of Arctic iceand snow caps. Thiscauseslandslides, flash floods and glaciallakeoverflow, extinctionofsome animaland plant species, Increasein the rangeof diseasevectors, that is, organismsthat transmit diseases Scientistspredictthat theearth'saveragetemperaturewillincrease by between1.4 and 5.8°C by the year 2100.
- 29. They also forecastthat thesea level will rise by at least 25 m, leading to coastalflooding that will displacemillionsof people. Small islands (such as Zanzibar and Pemba) and low-lying areaswill be totally covered by oceanwaters. About 25% of all plant and animalspecieswill becomeextinct. Solutions to global warming The effects of the greenhouse gasesin the atmospherewill continue to be felt for a long time. Thisis becausethegreenhouse gasesremainin the atmospherefor long periodsof time. For example, carbondioxide molecules have a lifetimeof between5O and -100-yearsin -the atmosphere, whilethat of CFC molecules is about 110 years. This meansthat global warming willcontinue even if we were to cut down on theemission of the greenhouse gases. However, it is important toreduceon the amount of the emissions before they reach alarming levels. The following are some of the measures that canbe takento reducegreenhousegas emissionsinto the atmosphere: Put in place energy-conservationmeasurestoreducetheuse of fossil fuels. These measuresincludeuse of public transport tominimizethe number of vehicles on the roadsand the use of fuel-efficient cars. Use of cleaner alternativesourcesof energy, such as solar and wind. Check deforestationand replant trees(afforestation) that would absorb carbondioxide. Afforestation
- 30. Countriesshould commit themselvestominimizing theemissionof greenhouse gasesinto the atmosphere. Thisisbeing done through agreementssuch as the Kyoto Protocol. Chapter summary 1. Geophysicsis the branch of scienceconcerned with the physical, chemical, geological, astronomicaland other characteristic properties of theearth. 2. The interior of the earth is composed of three mainlayers, namely the crust, mantle, outer coreand the inner core. 3. The earth'scrust and part of the mantleare cracked intohugepieces called tectonic plates. Thetectonic platesmove about at very low speeds. 4. The line where two tectonic platesmeet is called a boundary. 5. There arethree maintypes of boundaries.Thesearedestructive boundaries, constructiveboundariesand conservativeboundaries. 6. Volcanoesmainly form along destructiveand constructiveplate boundaries. 7. Hot-spot volcanoes form awayfrom tectonic plateboundaries 8. There aretwo maintypes of volcanoes, namely centraland fissure volcanoes. 9. Volcanic eruptionsaffect thelandscape, vegetation, wildlife, environment, humanlife and property, soil formationand the availabilityofmineralson the earth'ssurface. 10. Earthquakesoccur whenrocks withintheearth'scrust move suddenly, shaking the earth. 11. Earthquakesoccur on or near destructiveplateboundaries 12. The point withinthe earth'ssurfacewherean earthquakebeginsis called the hypocentreor the focus of the earthquake. 13. Earthquakesreleasetheir energy in threewaves called seismic waves. These are primarywaves, secondarywaves and surfacewaves. 14. Earthquakesareusually described intermsof their magnitudeand intensity. 15. Themagnitudeofan earthquakeisusually measured, on theRichter scale. The scale is .based on theamplitudeof the largest seismic wave recorded, no matter what typeof wave was the strongest. 16. The intensityof an earthquakeisa measureof its strength based on the changesit causesto thelandscape. The intensityof an earthquake is measured on the Modified Mercalli scale.
- 31. 17. A seismographis an instrument used to record ground-movements caused by an earthquake. 18. There aremany hazardsassociated with earthquakes. Theyinclude landslides, tsunamis, collapsing buildings, fireoutbreaksand backward rivers. 19. Earthquakes areusuallypreceded by a number of signs. They include temperaturechanges, riseor fall of water levels in wells, animals becoming highlydisturbed and restless, and an increasein the number of patientsand child deliveriesin hospitals. 20. The earth'satmosphereisdivided into several verticallayers. They are thetroposphere, the stratosphere, themesosphere, the thermosphereand the exosphere. 21. Global warming istheincreasein the averagetemperatureonor near the earth'ssurfaceas a result of the greenhouse effect. 22. The greenhouseeffect is caused by greenhouse gases, which include carbondioxide, methane, dinitrogenoxide, and chlorofluorocarbons (CFCs). 23. Global warming resultsinincreasein the temperatureof oceans, rise in sea level, changeinthe earth’sclimatic patterns, acidificationof the oceans, and extremeweather events such as heat waves, hurricanesand drought. 24. Some of the measuresthat should be put in-placeto check global warming include implementationof energy conservationmeasures, use of dean alternativesourcesof energy, checking deforestation, and minimizing emissionofgreenhouse gases.