Geophysics is the study of the physical properties of the Earth. The Earth is composed of three main zones - the crust, mantle, and core. The crust makes up the outer solid layer and exists in two types, continental and oceanic. Below the crust is the mantle, which takes up 70% of the Earth's mass and is solid but plastic. The innermost part is the core, which has a liquid outer core and solid inner core. The Earth's crust and upper mantle are broken into tectonic plates that move slowly over time. Volcanoes and earthquakes occur near plate boundaries as a result of this movement. Earthquakes release energy in seismic waves that can be measured on scales such as the Richter

1. GEOPHYSICS
Introduction
Geophysics isthebranch of sciencethat 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 is the 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 is the 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 where two 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 volcanoes form 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 of fire
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 Atlantic ridge
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 a single 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 also killed 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 past each 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 or p-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 the surfacewaves 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 of a 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 seismograph isan 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
The recording 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 number of 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 electromagnetic indicator -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 are some 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 height of 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.
The troposphereis 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 above the 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 wheresatellites orbit 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 from the 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 fossil fuels
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 organic compoundsmadeup
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 in thewater 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 forecast that 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 themselvesto minimizing 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 seismograph is 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.