PHYSICS FORM SIX-ENVIRONMENTAL PHYSICS: GEOPHYSICS
PHYSICS FORM SIX-ENVIRONMENTAL PHYSICS: GEOPHYSICS
Geophysics is the branch of physics which deals with the study of seismic waves and the Earth’s magnetic and gravity fields and heat flow.
Because we cannot directly observe the Earth’s interior, geophysical methods allow us to investigate the interior of the Earth by making measurements at the surface. Without studying these things, we would know nothing of the Earth’s internal structure.
STRUCTURE OF THE EARTH
Major zones of the earth
The earth is divided into two major zones, namely;
(a) Outer zone, and
(b) Inner zone.
a) Outer zone: the earth’s outer zone consists of;
(i) The hydrosphere – water bodies,
(ii) The atmosphere – gaseous envelope
(iii) The biosphere – living organisms, plant and animals
b) Inner zone: the earth’s inner zone consists of;
(i) The crust – lithosphere
(ii) The mantle – mesosphere,
(iii) The core – barysphere
Atmosphere is the envelope of gases that surround the Earth (oxygen, nitrogen, carbon dioxide, etc)
Hydrosphere is the water bodies filling the depressions in the Earth. Examples are rivers, oceans, seas, oasis,
Lithosphere is the solid outer most part of the earth.
Layers defined by composition
Layers are defined by composition because of density sorting during an early period of partial melting, Earth’s interiors not homogeneous.
· Crust – the comparatively thin outer skin that ranges from 3 kilometers at the oceanic ridges to 70 kilometers in some mountain belts. It makes up 1% of the Earth’s volume.
Continental crust (SIAL, Silicon and aluminium)
Average rock density about 2.7 g/cm3
Its density varies between 2.0 to 2.8 g/cm3
Composed of silicon and aluminium
Floats higher on the mantle forming the land masses and mountains. It is 30 to 70 km thick.
Oceanic crust (SIMA), silicon and magnesium)
Oceanic crust ranges from 3 to 15 km thick
Density vary between 3.0 to 3.1 g/cm3
Floats lower on the mantle forming the oceanic basins. It is about 8 km thick.
Mantle – a solid rocky (silica-rich) shell that extends to a depth of about 2900 kilometers. It makes up 83% of the Earth’s volume
The mantle can further be dived into:
(i) Upper layer of mantle (Asthenosphere)
(ii) Transition layer and,
(iii) Lower layer of mantle (Mesosphere)
Upper mantle is a rigid layer of rock with average density 3.3kgm-3
Transition layer is the layer that separates upper and lower mantle.
Lower mantle plays an important role in tectonic plate movement which creates earthquakes and volcanoes.
Its density is about 5.7 kgm-3
The mantle rocks are said to be in a plastic state.
The upper part of a mantle has a temperature of about 8700C. The temperature increases downwards through the mantle to about 22000C near the core.
· Core – an iron – rich sphere having a radius of 3486 kilometers making up 16% of the Earth’s volume
The core is divided into two parts:
(i) Outer core
(ii) Inner core
i. Outer core is a liquid of molten iron and nickel alloys. The Earth’s magnetic field is generated within the outer core due to convective. It is 2270 kilometers thick.
ii. Inner core is a solid iron and nickel alloys. The temperature within the inner core is higher than the outer core but the inner core is solid, this is because higher pressure in this region causes the melting point to rise. It is a sphere of radius of 1216 kilometers.
Average density is nearly 11 gcm-3and at Earth’s center.
Layers defined by physical properties
Lithosphere (sphere of rock)
Earth’s outermost layer
Consists of the crust and uppermost mantle
Relatively cook, rigid shell
Averages about 100 kilometers in thickness, but may be 250 kilometers or more thick beneath the older portions of the continents
Asthenosphere (weak sphere partially molten)
Beneath the lithosphere, in the upper mantle to a depth of about 660 kilometers
Small amount of melting in the upper portion mechanically detaches the lithosphere from the layer below allowing the lithosphere to move independently of the asthenosphere i.e. allows tectonic plate movement.
Mesosphere or lower mantle
Rigid layer between the depths of 660 kilometers and 2900 kilometers
Earth’s major boundaries
Discontinuity is the name given to any surface that separates one layer from another layer of the Earth.
The Moho (Mohorovicic discontinuity)
Discovered in 1909 by Andriaja Mohorovicic
Separates crustal materials (crust) from underlying mantle.
· Discovered in 1914 by Beno Gutenberg
· Is the boundary between the outer and inner core.
The Earth’s Structure
TEMPERATURE INSIDE THE EARTH
Earth’s temperature gradually increases with an increase in depth at a rate known as the geothermal gradient.
Temperature varies considerably from place to place
Averages between about 200C and 300C per kilometer in the crust (rate of increase is much less in the mantle and core)
The rate of heat flow within the Earth depends on:
(i) The thermal conductivity of the rock,
(ii) Temperature gradient of the rock
Sources of heat Energy within the Interior of the Earth
Major processes that have contributed to Earth’s internal heat include:
1. Heat emitted by radioactive decay of isotopes of uranium (U), thorium (Th), and potassium (K).
2. Heat released as iron crystallized to form the solid inner core.
3. Heat released by colliding particles during the formation of Earth.
4. Gravitational work done by the Earth due to its rotation through its own axis.
5. Electron motion in the core behaves like an electric current.
Heat Lost by the Earth
Heat in the earth is transferred by the process of;
(i) Convection and
In the solid inner core and in the Earth’s crust heat is transmitted by conduction process. Rates of heat flow in the crust vary.
In the Mantle heat is transmitted by conduction process. Rates of heat flow in the crust vary.
In the Mantle heat is transmitted by convection process. There is not a large change in temperature with depth in the mantle.
Mantle must have an effective method of transmitting heat from the core outward.
Transfer of heat in the Earth by mantle convection
Uses of the Mantle
1. The mantle transfers heat by convection from the earth’s crust to the out regions of the earth and thus help it to regulate its temperature
2. The upper part of the mantle is molten, this allows tectonic plates movements.
An earthquake is a sudden motion or shaking of the earth caused by a sudden release of energy that has accumulated within or along edges of the earth’s tectonic plates.
Earthquakes occur within the Earth’s crust along faults that suddenly release large amounts of energy that have built up over long periods of time.
The shaking during an earthquake is caused by seismic waves.
Seismic waves are propagating vibrations that carry energy from the source of the shaking (earthquake) outward in all directions.
Seismic waves are generated when rock within the crust breaks, producing a tremendous amount of energy. The energy released moves out in all directions as waves, much like ripples radiating outward when you drop a pebble in a pond.
CAUSES OF EARTHQUAKES (SEISMIC WAVES)
The main causes of the Earthquakes and so seismic waves are:
1. Movement of tectonic plate.
2. Volcanic activity.
3. Landslide and avalanches.
4. Rebound of the crust.
5. Human activities.
Movement of tectonic plate
The Earth’s crust is made up of segment (layers) called tectonic plates which are slowly drifting in various directions. Tectonic plates may create a fault.
A boundary is a line where two tectonic plates meet.
A geologic fault is a fracture in the earth’s crust causing loss of cohesion and accompanied by displacement along the fracture.
How an earthquake is formed
Tectonic plates grind past each other, rather than slide past each other smoothly. As the plates move past each other they can become locked together due to friction. For some time, they don’t move and strain energy builds up. Stresses builds between them until fractional force holding the plates together give away. The plates move suddenly, releasing the energy and then held again. This sudden jerk is what is felt as an earthquake.
(a) The Earth’s crusts near tectonic plate edges are forced to bend, compress, and stretch due to the internal forces within the earth, causing earthquakes.
(b) Nearly all earthquakes occur at plate boundaries.
Molten rock “magma” from the mantle is forced through a weak point in the Earth’s crust creating a volcanic eruption. When magma reaches the Earth’s surface it is known as “Lava”. Successive eruptions leads to the buildup of lava on the sides of the vent creating the familiar “cone – shape” of a volcanoes
Earthquakes may be created by the violent explosions which occur if there are sudden movements of the magma.
Landslides and avalanches
A landslide occurs when a large mass of land slips down a slope. An Avalanche occurs when a large mass of snow pours down a mountain side. Both of these effects can start an earthquake
Rebound of the crust
Elastic rebound theory state that “as tectonic plates move relative to each other, elastic strain energy builds up along their edges in the rocks along fault planes”. Since fault planes are not usually very smooth, great amount of energy can be stored (if the rock is strong enough) as movement is restricted due to interlock along the fault. When the shearing stresses induced in the rocks on the fault planes exceed the shear strength of the rock, rupture occurs.
It follows from this that if rocks along the fault are of a certain strength, the fault is a certain length, and the plates are slipping past each other at a defined rate, it is possible to calculate the amount of time it will take to build up enough elastic strain energy to cause an earthquake and its probable magnitude.
When a fault breaks it release elastic strain energy it stored, and hence earthquake.
Human activities such as those caused by nuclear bombs can create earthquake, together with mine actives.
Energy released by an earthquake moves outwards from the origin in the form of concentric waves.
Focus (Hypocenter) is the point in the Earth where seismic waves originate.
Epicenter is the point on the earth’s surface vertically above the focus.
Hypocentral distance is the distance between the focus and the seismic detection station.
Epicentral distance is the distance between the epicentral and the seismic station.
S = Seismic station
E = Epicenter
ES = Epicentral distance
TYPE OF SEISMIC WAVES
i. Seismic waves are elastic waves that propagate within the earth.
There are two type of seismic waves:
1. ii. Body waves, spread outward from the focus in all directions.
2. iii. Surface waves (Long, L – waves) spread outward from the epicenter to the Earth’s surface along the crust, similar to ripples on a pond. These waves can move rock particles in a rolling motion that very few structures can withstand. These waves move slower than body waves.
There are two types of Body Waves
(1) Primary P – wave and
(2) Secondary, S – waves
1. 1. Primary Wave (P – wave): Are longitudinal (compression) wave (travels in the same direction the waves move)
Characteristics of P – waves
1. Are the fastest seismic waves (7 – 14 km/second). Arrives at recording station first, hence the name primary means first.
2. Can pass through solid, gas and liquid, hence can pass through crust, mantle and the cores.
3. Are longitudinal compression waves. The rocks that transmit the P – waves are alternately compressed and expanded.
Velocity of P – waves
The velocity of primary waves depends on the density,bulk modulus B and the shear modulus
In solid, =
In liquid =
A fluid cannot support shear stresses hence
2. Secondary Wave (S – wave): Are transverse (shear) wave (travels perpendicular to the wave movement).
Characteristics of S – waves
1. i. Slower moving (3.5 – 7 km/second) hence are detected after primary waves.
2. ii. Caused by a shearing motion
3. iii. Cannot pass through a fluid (gas or liquid) because they are transverse. Hence are unable to pass through the liquid outer core.
Velocity of S – waves
The velocity of shear waves depends on the density and the shear modulus
In solid, =
In liquid =
Note: Since the density and states of the earth layers varies, the speed of the seismic waves also vary from layer to layer, the solid part showing greater speed and the liquid ones lower speed.
Primary wave and secondary wave
Variation of speed of body waves with depth
SURFACE WAVES/LONG WAVES
Surfaces waves are produced when earthquake energy reaches the Earth’s surface.
These are the slowest moving waves, but are the most destructive for structures on earth
There are two types of L – Waves:
(i) Love long waves
(ii) Rayleigh long waves
i. Love Waves
Love waves are Transverse horizontal motion, perpendicular to the direction of propagation and generally parallel to the Earth’s surface.
They are formed by the interaction of S waves with Earth’s surface and shallow structure and are dispersive waves. The speed at which a dispersive wave travels depends on the wave’s period.
Characteristics of Love Waves
1. i. Love waves are transverse and restricted to horizontal movement (horizontally polarized).
2. ii. The amplitude of ground vibration caused by a Love wave decrease with depth. The rate of amplitude decrease with depth also depends on the period/frequency.
3. iii. Loves wave are dispersive, i.e. wave velocity is dependent on frequency; low frequency – higher velocity.
4. iv. Speed of love waves is between 2.0 and 4.4 km/s
5. v. Love waves travels within the earth’s crust only.
Rayleigh waves are vertically polarized long waves. The slowest of all the seismic wave types and in some ways the most complicated.
Characteristics of Rayleigh Waves
1. Rayleigh waves are transverse and restricted to vertical movements (vertically polarized).
2. The amplitude of Rayleigh wave decreases with depth. The rate of amplitude decrease with depth depends on the period/frequency
3. Rayleigh wave are dispersive, i.e. wave velocity dependent on frequency; low frequency – high velocity
4. Speed of love waves is between 1.0 and 4.2 km/s slowest of all waves.
5. Travels within the earth’s crust only.
6. Depth of penetration of the Rayleigh waves depend frequency, with lower frequencies, penetrating greater depth.
PROPAGATION OF SEISMIC WAVES
Like all other types of waves, seismic waves may undergo,
(i) Reflection, (ii) Refraction, (iii) Dispersion, (iv) Diffraction, (v) Attenuation.
Seismic waves bounce (reflect) rock boundaries of different rock type (density).
Waves change velocity and direct (refract) when they enter a medium of different density it the one they just passed through.
surface waves are dispersive which means that different periods travel at different velocities. The effects of dispersion become more noticeable with increasing distance because the long travel distance spreads the energy out (it disperses to energy).
SEISMIC WAVE PATHS
By comparing the data recorded by many stations all over the world the nature, speed and the paths of the seismic waves can be determined. This information can be used to tell us about the earth’s interior such as density sand state in each layer.
L – Waves travel within the Earth’s crust only
P and S waves travel through the earth in a curve path. The waves are refracted because their speeds a constantly changing with depth due to continue increase in density. Waves are also strongly refracted the Mantle – Core boundary.
Surface waves travels through the Earth crust only
Shadow zone is the region on the Earth’s surface where no S or P waves are present.
This lies between 1050 and 1400. Only surface waves may be detected in this region.
Shadow zone occurs because:
(i) P – Waves are strongly refracted at the liquid outer core.
(ii) S – Waves can’t travel through the liquid outer core.
Seismic waves can also be used to locate the discontinuities in the earth’s crust. A change in density or crack would affect the propagation of the waves.
This alteration in the wave’s path or speed would indicate the discontinuity.
The fact that S waves do not travel through the core provides evidence for the existence of a liquid layer beneath the rocky mantle.
The change in the velocity of P waves at crust – Mantle boundary reveals the presence of Mohorovicic discontinuity
P waves passing through the inner core show increased velocity suggesting that the inner core is solid.
Both P and S – Waves slow down when they reach the asthenosphere. Because of this scientists know that the asthenosphere is partially liquid
MEASUREMENTS OF EARTHQUAKES
i. Seismology is the scientific study of earthquakes (seismic waves) and artificially produced vibrations in the earth.Seismograph is a sensitive instrument that is used to record earthquakes and seismic waves (i.e. ground movements).
ii. Seismogram is the record of ground movement drawn by a seismograph.
The arrival of seismic waves at a station
Seismograph consists of a heavy weight suspended from a frame fixed into the ground. When the earth vibrates the frame moves but the heavy weight remains stationary due to great inertia. A pen attached to weight plots the earth’s movements on a chart recorder to produce a seismogram. To obtain a complete record of the earthquake measurements must be taken in all three planes (x, y and z).
The recording of the motion caused by seismic waves can be done by using;
(a) Mechanical method, as in the drawing above.
( (b) Optical method, where light is used to write the motion on a photosensitive paper instead of using a pen.
(c) Electronic method, where a coil is fixed to the mass of the pendulum and moves in a magnetic field. This induces a voltage which is amplified so that they can be easily interpreted.
Seismometers record both the magnitude and intensity of the earthquake.
LOCATING THE EPICENTRE
Although S – waves, P – waves and surface waves all start out at the same time, they travel at different speeds. The speed of a traveling seismic wave can be used to determine the location of an earthquake epicenter.
A seismograph records the arrival time and the magnitude of horizontal and vertical movements caused by an earthquake. The arrival time between different seismic waves is used to calculate the travel time and the distance from the epicenter.
The difference in arrival time between primary waves and secondary waves is used to calculate the distance from the seismograph station to the epicenter.
It is crucial that seismic waves are recorded by three different seismograph stations in order to estimate the location of the epicenter.
(i) Locate at least 3 stations on a map that recorded the seismic waves.
(ii) Calculate the time difference between arrival of P – waves and arrival of S – waves from a seismogram. The time difference is proportional to the distance from the epicenter. Because the direction to the epicenter is unknown, the distance defines a circle around the receiving station. The radius of each circle equals that station’s distance from the earthquake epicenter.
(iii) The epicenter is where the circles intersect.
SIZE OF AN EARTHQUAKE
The size of an earthquake can be measured in terms of its intensity (Mercalli/Wood Neumann scale) or its magnitude (Richter scale).
Mercalli Intensity Scale
The Mercalli scale measures the intensity of how people and structures are affected by the seismic event. In essence, it measures damage. It is much more subjective and uses numbers ranging from 1 (no damage) to 12 (total destruction).
Detected by a seismograph only
|6||Felt by all, many frightened. Some heavy furniture moved, some fallen plaster, general damage small|
|12||Total damage, large cracks, waves seen moving through the ground, objects are thrown upwards.|
Intensity distribution maps can be drawn up showing the intensities of an earthquake over a region. The earthquake is most intense at the epicenter and decreases with distance.
Isoseismal lines are line joining points of equal intensity.
Richter magnitude scale
The magnitude of an earthquake is measured in terms of energy released by an earthquake. This is determined from the amplitude of the seismic wave recorded on a seismogram 100 km from the epicenter. The magnitude is equal to the logarithm of the amplitude. Therefore each successive number represents a tenfold (x10) increase in the ground motion. The Richter scale starts at 0 but has no upper limit. -However 8 represent an earthquake that causes total destruction within the region.
|Magnitude||Amount of explosives (TNT) needed to release the equivalent energy, in tons|
Intensity of an earthquake is a measure of its strength based on the changes it causes to the landscape.
EARTHQUAKE PREDICTIONS (WARNINGS)
Forecasting (predicting) earthquakes is very difficult, although there are a number of warning signs which occur before an earthquake happens.
(i) Change in the velocity of p – waves.
(ii) Electrical resistivity of the rocks decreases.
(iii) An increase in radon, emission (radon is an inert gas, radon is found to increase in soil and water samples).
(iv) Increase in fore shock (small tumors that occur just before an earthquake).
(v) Local variations in the magnetic field.
(vi) Animals begin to behave strange.
(vii) Water levels rise or fall in wells few days before earthquake.
(viii) Increase in temperature of the area few months before the occurrence of an earthquake
Some of the world’s populations are living in regions where there is a high risk of an earthquake. Most of these regions lie along fault lines. However a few precautions can be taken to reduce the damage caused.
(a)Build structures that can withstand the forces of an earthquake. One method is to include shock absorbers into the buildings foundations.
(b)Scientific research has shown that pumping water out of the earth reduces the stress in the crust hence preventing an earthquake. However this technique is very expensive.
(c) Stay away from tall buildings or structures during an earthquake if you are outside on occurrence.
(d) If you are inside a house, stay in a safe place where things will not fall on you.
Earthquake give rises to a number of hazards which pose a great risk to human life, animals, property and the environment at large. The following are some hazards:
1. Landslides and avalanches: The shaking caused by an earthquake can cause unstable hillsides, mountain slops’ and cliffs to move downwards creating landslides. Earthquakes can also trigger avalanches on snow slopes
2. Tsunamis: If an earthquake occurs under the sea or ocean, the shock waves disturb the water. The ocean floor can rise or fall causing the water to rise and fall too. This movement creates huge water waves called tsunamis that travel across the ocean.
3. Collapsing building: Buildings or structures may collapse during a strong earthquake. The collapse of the building may kill people.
4. Fire outbreak: Earthquakes can cause gas or oil pipes to break and or the collapse of electricity lines. This may set up fire.
5. Backward rivers: Tilting ground due to earthquakes can make rivers change their course.
This is the study of reflection of seismographic waves by different materials inside the earth.
(i) Location of underground oil and water
(ii) Locate discontinuities within the earth
Seismic prospecting is the sending of seismic waves into the deep earth’ crust in order to study the structure of the earth or detecting oils or gases in the interior of the earth by utilizing the property of reflection and refraction of the seismic waves.
THE EARTH’S MAGNETIC FIELD
The earth has a weak magnetic field, 95% of this field is created inside the Earth’s core 5% is the result of atmospheric effects above the Earth’s surface.
Geomagnetism is science of study of the earth magnetic field, its causes and its variations.
Generation of the Earth’s magnetic field within the core
The accepted explanation for the origin of the Earth’s magnetic field within the core is given by Lemoir’s self exciting dynamo theory.
The Earth’s Outer Core consists of molten conducting metals (Iron and Nickel) which are rich in free electrons. The Earth’s rotation causes the molten metal to rotate and hence large convection currents are set up within the outer core. These currents generate a magnetic field.
Eddy currents are now generated due to a conducting material moving in a magnetic field. These Eddy currents modify the position of the Earth’s magnetic field so that it does not lie along the Earth’s axis of rotation. The present magnetic poles are situated 800km from the Earth’s axis.
Generation of magnetic field in the Atmosphere
In the Earth’s atmosphere there is a region know as the ionosphere which consists of free electrons and ions. The movement of these charges creates a magnetic field. This effect provides a small fraction of the Earth’s total magnetic field.
TERMS ASSOCIATED WITH THE EARTH’S MAGNETIC FIELD
Magnetic meridian: A vertical plane passing through the axis of a freely suspended magnetic needle.
Geographic meridian: A vertical plane passing through the geographic axis.
Magnetic equator: Is the locus of points on earth’s surface where the needle (free to rotate in a vertical plane) remains horizontal.
The Earth’s magnetic field pattern is similar to that produced by a giant bar magnet or solenoid.
Note: (i) The magnetic North pole which lies in the Northern Hemisphere behaves like a south pole or a bar magnet, i.e. the field lines are directed towards it.
(ii) The magnetic south pole which lies in the southern hemisphere behaves like a north pole of a bar magnet, i.e. the field lines are directed away from it.
ELEMENTS OF EARTH’S MAGNETISM
Angle of variation of declination, at a place is the angle between the geographic meridian and the magnetic meridian at that place.
Angle of dip or declination, at a place is the angle between the directions of intensity of the earth’s total magnetic field declinationand the horizontal direction, in the magnetic meridian at that place.
Horizontal component of Earth magnetic field It is the component of the Earth’s total magnetic field along the horizontal direction in the magnetic meridian.
By Pythagoras theorem
By trigonometric ratio
Points to note about angle of Dip
(a) At the poles,
Therefore, only horizontal component exists at the poles
(b) At the equator
At the equator only horizontal component exist.
VARIATIONS OF THE EARTH’S MAGNETIC FIELD
The Earth’s magnetic field is not constant but varies continuously with time.
(i) Short term variations (Irregular changes): The magnetic field changes daily due to variations in the magnetic field created in the ionosphere. The charged particles in this region of the atmosphere are affected by the Sun’s gravitational pull (which is stronger when the sun is directly above that area)
Also during periods of high solar activity charged particles from the solar wind are able to penetrate the magneto pause and arrange themselves under the influence of the magnetic field in a formation called Van Allen Belts.
These charged particles cause further Eddy currents within the ionosphere, altering the Earth’s magnetic field strength.
Solar wind is a continuous stream of moving electrons and protons in the atmosphere which are produced from flare (eruptions) from the sun. Normally these charged particles move from west to south at 300 – 500 km/s.
Magnetic storm is a sudden worldwide disturbance of the earth’s magnetic field caused by dynamic interaction of the earth’s magnetic field and the sun. During magnetic storm, the earth’s magnetic field is unusually active.
Effects of Magnetic Storm
(a) Large storms can cause the loss of radio communication
(b) Damage satellite electronics and affect satellite operations.
(c) Increase pipeline corrosion
(d) Induce voltage surges in electric power grids causing blackouts.
(e) Reduce the accuracy of global positioning systems.
(ii) Long term variations (Secular changes): The Earth’s magnetic field position is constantly changing, now the magnetic North pole is moving at 8 km per year, and the magnetic South Pole at 16 km per year.
Evidence from the alignment of magnetized rocks layers in the Earth’s crust show that the Earth’s magnetic field has actually reversed in direction several times during the Earth’s history (i.e. the direction of the fields have reversed causing a north acting pole to become a south acting pole.) The present polarity of the Earth’s magnetic field has not changed for 700,000 years.
VAN ALLEN BELTS
The Van Allen belts consist of two regions of highly charged particles which are trapped within the Earth’s magnetic field:
Inner Belt consists of protons and positive charged particles
Outer Belt consists of electrons and negatively charged particles.
Earth’s atmosphere is divided into five main layers, the exosphere, the thermosphere, the mesosphere, the stratosphere and the troposphere. The atmosphere thins out in each higher layer until the gases dissipate in space. There is no distinct boundary between the atmosphere and space, but an imaginary line about 110 kilometers from the surface, called the Karman line, is usually where scientists say atmosphere meets outer space.
The troposphere is the layer closest to Earth’s surface. It is 10 km thick and contains half of Earth’s atmosphere. Air is warmer near the ground and gets colder higher up. Nearly all of the water vapor and dust in the atmosphere are in this layer and that is why clouds are found here.
Lapse rate is the rate of fall of temperature in degrees per kilometer rise. It has an average value of 6 0C per km in the troposphere.
Tropopause is the upper boundary of the troposphere.
Importance (uses) of troposphere
1. Controls the climate and ultimately determines the quality of life in the atmosphere.
2. It supports life on earth. It contains oxygen which is used to respiration by animals.
The stratosphere is the second layer. It starts above the troposphere and ends about 50 km above ground.
The temperature of the stratosphere slowly increases with altitude. This temperature increase is due to the presence of Ozone layer which absorbs heat from the sun in the form of ultraviolet light.
The Ozone layer occupies the middle of stratosphere between 20 and 30 km it consists of Ozone formed by oxygen molecules dissociated and reforming into 03.
The air here is very dry, and it is about a thousand times thinner here than it is at sea level. Because of that, this is where jet aircraft and weather balloons fly.
Stratopause is the upper boundary of the stratosphere.
Importance (uses) of stratosphere
The stratosphere prevents harmful ultraviolet radiation from reaching the earth. Ozone absorbs harmful radiation from the sun. The Ozone protects plants and shield people from skin cancer and eye cataracts.
The mesosphere starts at 50 km and extends to 80 km high. The top of the mesosphere, called the mesopause, is the coldest part of the Earth’s atmosphere with temperatures averaging about – 900C. The temperature of the mesosphere decreases with altitude (because there is no ozone to absorb heat).
This layer is hard to study. Jets and balloons don’t go high enough, and satellites and space shuttles orbit too high. Scientists do know that meteors burn up in this layer.
Importance of mesosphere
Mesosphere, thermosphere and exosphere prevent harmful radiation such as cosmic rays from reaching the earth surface.
The thermosphere extends from about 80 km to between 500 and 1,000 km. Temperatures increases as it approaches nearer to the sun. The heating effects of the earth no longer exist at these higher altitudes.
The thermosphere is considered part of Earth’s atmosphere (the upper atmosphere), but air density is so low that most of this layer is what is normally thought of as outer space. In fact, this is where the space shuttles flew and where the International Space Station orbits Earth.
This is also the layer where the auroras occur. Charged particles from space collide with atoms and molecules in the thermosphere, exciting them into higher states of energy. The atoms shed this excess energy by emitting photons of light, which we see as the colorful Aurora Borealis and Aurora Australis.
The exosphere, the highest layer, is extremely thin and is where the atmosphere merges into outer space. It is composed of very widely dispersed particles of hydrogen and helium.
The upper part of the exosphere is called Magnetosphere. The motion of ions in this region is strongly constrained by the presence of the earth’s magnetic field. This is the region where satellites orbit the earth
(i)The troposphere, stratosphere, and mesosphere are collectively forms the homosphere. These layers have the same chemical composition; 78% nitrogen, 21% oxygen, 1% argon and other gasses which sum to about 0.05%. The thermosphere is excluded due to different in chemical composition.
(ii) The upper atmosphere above 90 km is called heterosphere. The atmosphere is no longer a mixture of gases but separates into layers heavier ones forming the bottom layer.
VARIATION OF TEMPERATURE WITH HEIGHT
The temperature above the Earth surface varies as shown in the graph below.
The residence time, is the mean lifetime of a gas molecule in the atmosphere
THE IONOSPHERE AND TRANSMISSION OF RADIO WAVES
The ionosphere is the region containing high concentrations of charged particles ions and electrons.
The ionosphere is created by atoms absorbing U.V radiation, gamma and X – rays.
The ionosphere extends from the lower thermosphere 55 km to 550 km above the earth’s surface.
Due to difference in composition of the air in the ionosphere, the ionosphere is divided into layers.
(i) The lower layer, called D layer; this layer exists only in the day time at an altitude of 55 to 90 km above the earth’s surface. Ionization in this region is relatively weak.
(ii) The next layer, E – layer: this layer is between 90 and 145 km above the earth’s surface. It has a maximum density at noon but is only weakly ionized at night.
(iii) The top layer, the F – layer: At night exists as a single layer in a region of about 145 to 400 km above the earth’s surface. During the day it splits into two layers, F1 and F2.
The Ionosphere and Communication
The ionosphere plays an important role in communication. Radio waves can be reflected off the ionosphere allowing radio communications over long distances. However this process is more successful during the night – time.
Why Transmission is better at Night?
During the day: the ionosphere extends into lower atmosphere (D layer). In this layer there is high concentration of particles and so recombination of electrons and ions due to collision is more likely to occur. The leads to the radio waves being absorbed rather than reflected. Hence distant communications are poor during the day.
During the night: The D layer disappears due to decrease in ionization of molecules but recombination of electrons and ions still occurs at a fast rate. The radio waves are then reflected by E and F layers in which recombination of electrons and ions is rare hence there is less absorption of the radio waves.
PHYSICS FORM SIX-ENVIRONMENTAL PHYSICS: GEOPHYSICS
EXAMPLES: SET C
Example 01: Necta 1985 P1
(a) (i) Distinguish between P and S waves, state clearly the difference between their speeds in a medium.
(ii)Draw a schematic diagram showing how one station on the Earth’s surface can receive P or S waves from a distant source and state which waves can be refracted by the Earth’s outer core.
(b) (i) Give a summary of the origin and composition of the ionosphere.
(ii) What is the net electric charge in the ionosphere?
(iii) Show graphically how electron density changes with altitude in the ionosphere.
(a) (i) P – waves are longitudinal compression waves which can pass through solid, gas and liquid, whereas S – waves are transverse shearing waves which cannot pass thorough a fluid (gas or liquid)
The speed of P – waves in a medium is approximately twice that of the S – waves hence P – waves are faster than S – waves.
(ii) Refer the diagram for the seismic wave paths
(b) (i) Ionosphere is the upper part of the atmosphere. The ionosphere is formed due to the ionization of gaseous atoms as they absorb ultraviolet radiation from the sun, gamma and X-rays.
(ii) The net electric charge in the ionosphere is zero.
(iii) Variations of electron density in the ionosphere Electron density increases from D to F layer
Example 02: Necta 1988/1993 P1
(a) What are the factors that influence the velocities of P – and S – waves?
(b) Explain briefly the characteristics property of seismic waves which is used to locate discontinuities in the earth’s crust.
(a) The velocities of both P and S – waves are influenced by;
(i) Density of the rock material (Media),
(ii) Moduli of elasticity.
(b) Speed is the characteristic property of seismic waves that is used to locate discontinuities
Between the crust and mantle there is abrupt change of density, which shows an abrupt change in speed of both P – and S – waves, a Mohorovicic discontinuity exists here. Both P – and S
waves travels across this discontinuity.
Between the mantle and the core there is the Gutenberg discontinuity only P – waves travel this discontinuity.
Example 03: Necta 1989 P1
(a) State three sources of heat energy in the interior of the earth.
(b) (i) How does temperature vary with depth of the Earth?
(ii) What are the factors that influence the flow of heat from the interior of the Earth?
(a) Refer notes
(b) (i) The temperature increases with increasing depth
(ii) The rate of heat flow (conduction) is given by
The heat flow from the interior of the earth depends on:
Thermal conductivity of the rock,
Temperature gradient of the rock
Example 04: Necta 1989 P2
(a) What do you understand by the terms?
(i) Solar wind,
(b) What are the various factors that contribute to the Earth’s magnetic field?
(c) (i) With the aid of a suitable diagram, illustrate the components of the earth’s magnetic field at a given point P in the earth’s atmosphere.
(ii) An electron whose kinetic energy is 10 eV is circulating at right angles to the earth’s magnetic field whose uniform induction is 1.0 x 10 Wbm-2. Calculate the radius of the orbit and its frequency in that orbit.
(a) (i) Solar wind is a continuous stream of fast moving charged particles in the atmosphere which are produced from flare (eruptions) from the sun:
(ii) Magnetopause is the upper boundary of the magnetosphere.
(iii) Magnetosphere is the upper most part of the exosphere consisting mainly of charged ions. These particles move under the influence of the earth’s magnetic field.
(b) Short term variations: Disturbances in the magnetosphere due to solar emissions, these charged ions travel and in the ionosphere they form ring currents which give rise to a magnetic field.
Long term variations: The molten inner core of the earth is partly ionized. The movement of this ionized core causes a magnetic field which contributes to the earth’s magnetic field.
(c) (i) refer notes (ii) refer electromagnetism
Example 05: Necta 1990 P1
(a) Define the term “isoseismal line”.
(b) Write short notes on each of the following regions of the atmosphere.
(i) Troposphere, (ii) Stratosphere, (iii) Exosphere
Answer: Refer notes
Example 06: Necta 1990 P2
(a) Explain clearly how P and S – waves were used to ascertain that the outer core of the earth is in liquid form.
(b) Giving reasons, discuss the temperature variation in atmosphere (above the earth’s surface).
(a) P – waves are longitudinal elastic, waves capable of passing through solids and liquids and S – waves are traverse elastic waves capable of a travelling through solids only.
As both waves are projected towards the surface from interior core only the P – waves are recorded. This shows that the outer core is in liquid form.
(b) From the ground level, the atmospheric temperature decreases steadily as altitude increases steadily as altitude increases up to the troposphere. Thereafter the temperature increases with altitude up to the stratosphere. The ozone of the stratosphere absorbs the incoming sun radiation hence the temperature increases. In the mesosphere there is no ozone thus there is a decrease (cooling) with increasing altitude. The heating effect of the earth ceases in the thermosphere so, the closer to the sun, the higher graph refer notes.
Example 07: Necta 1991 P2
(a) List down four physical changes that took place at a location just before onset of an earthquake at that particular location.
(b) Give brief accounts of the processes that give rise to:
(i) The earth’s magnetic field,
(ii) Volcanic eruptions
(a) Density of rocks, stresses faults and waves
(b) (i) Explain generation of the earth’s field in the atmosphere and the outer core.
(ii) The seismic or earthquakes waves result from a fracture or sudden deformation of the earth’s crust. Vast stresses do occur locally in the rocks being concentrated where the rocks are sliding over one another. In regions where pressure is reduced, pockets of molten rock called magma are formed. Once the rock has melted the pressure may force it into cracks and fissures in the surrounding solid rock. This may emerge above the surface as a lava flow or volcano.
Example 08: Necta 1992 P1
(a) What do you understand by the term ionosphere?
(b) Explain how short wave long distance transmission and reception of radio waves is more effective at night than it is during the day time.
(b) In the day time, the base of the ionosphere (D-layer) is at lower heights where the high concentration of particles allows for ionization and recombination of ions by collision. Because of this, radio waves are absorbed rather than reflected, so distance communication is poor.
During the night time, the D – layer disappear, the base of the ionosphere is higher thus the recombination of ions is rare and so less absorption of waves occurs. Obliquely transmitted waves therefore can be reflected for distant reception.
Example 09: Necta 1993 P2
(a) What is the origin of the earth’s magnetic field?
(b) The diagram below shows the structure of the Earth. Name the parts indicated by the letter A to F.
(b) A represents Mohorovicic discontinuity
B represents Gutenberg discontinuity
C represents core
D represents Mantle
E represents Epicenter
F is not clear to interpret.
Example 10: Necta 1994 P1
(a) Define the terms: angle of inclination (dip) and angle of declination (variation) as used in specifying the earth’s magnetic field at any point.
(b)The earth’s total resultant flux density BR in a certain country is found to be 5.0 x 10-5 T and the horizontal component is BH is 2.0 x 10-5 T. Calculate ;
(i) The vertical component, Bv, and
(ii) The angle of inclination in that country
(b) (i) The vertical component is given by
(ii) Angle of inclination is given by
Example 11: Necta 1994 P1
(a) (i) Name the lowest layer of the atmosphere and the lowest layer of the ionosphere.
(ii) State the importance of each of these layers.
(b) What is the ozone layer?
(a)(i) The lowest layer of the atmosphere is troposphere and the lowest layer of the ionosphere is called the D – layer.
(ii) The t troposphere supports life
The D – layer is important for communication purposes as it reflects radio waves.
(b)The ozone layer is within the stratosphere. In the ozone layer molecular oxygen (O2) is dissociated into atomic oxygen (O) which is then reformed into ozone (O3)
The ozone so formed absorbs ultra violet radiation thus protecting plants and shielding people from skin cancer and eye cataracts.
Example 12: Necta 1994 P2
(a) Illustrate the component of the earth’s magnetic field at a given point P in the earth’s atmosphere by a suitable diagram.
(b) Using a tangent galvanometer, explain how you could determine the earth’s magnetic field.
Example 13: Necta 1995 P1
(a) (i) which region of the solid earth includes the e earth’s centre?
(ii) On which region of the solid earth do the continent rests directly?
(iii) Which region of the ionosphere has the highest electron density?
(b) Briefly explain how earthquake can be detected
(a) (i) inner core (ii) crust (iii) F – region
(b) Detection of earthquake is done by recording or measuring the seismic waves generated by the earthquakes. These waves are recorded by instrument called seismograph.
Example 14: Necta 1995 P2
(a) Draw a well labeled diagram which shows the interior structure of the earth. Indicate also which part of the interior are in solid form and which are in liquid form.
(b) Name and distinguish the type of waves that are produced by an earthquake.
(c) Briefly describe the three ways in which signal form ground based transmitter can reach the receiver.
(a) There are four types of seismic waves:
Body waves – divided into P and S – waves
Surface waves – divided into love and Rayleigh
(b) A telecommunication problem.
Ground wave, sky wave and space waves
Example 15: Necta 1998 P1
(a) State any three magnetic components of the earth’s magnetic field
(b) The horizontal and vertical components of the earth’s magnetic field at a certain location are; 2.73 x 10-5 and 2.1 x 10-5T respectively. Determine the earth’s magnetic field at
the location and its angle of inclination θ
(a) Components of the earth magnetic field are:
Vertical component (which point vertically downward)
Horizontal component which comprise lf:
Eastly component (towards geographic north pole)
Northly component (towards magnetic north pole)
Example 16: Necta 1998 P1 B
(a) What is the origin of the earth’s magnetic field?
(b) The following diagram shows the main layers forming the interior of the earth name the layers indicated by letters A to G.
(a) Refer notes
(b) A = Earth’s surface, B = Crust, C = Moho discontinuity, D = Gutenberg discontinuity, E = outer core, F = Mantle and G = inner core.
Example 17: Necta 1998 B
(a) Explain the following terms; Earthquake, Earthquake focus, Epicenter and body waves.
(b) List down three (3) sources of earthquakes,
(c) (i) Define ionosphere
(ii) Mention the ionosphere layers that exist during the day time
(iii) Give the reason for better reception of radio waves for high frequency signal of night than during day time.
(d) Explain briefly three different types of radio waves traveling from a transmitting station to a receiving antenna.
(a) Refer notes
(b) Refer notes
(c) (i) During the day time all the layers D,E,F1, and F2 – layers exists.
(ii) Refer Necta 1992 (b)
(d) Ground (surface wave)
Sky waves) (refer telecommunication notes)
Example 18: nectar 2000 P1
(a) With reference to an earthquake on a certain point of the earth explain the terms ‘focus’ and ‘Epicenter’
(b) What is importance of the following layer of the atmosphere?
(i) The lowest layer
(ii) The ionosphere
(c) (i) Describe two ways by which seismic waves may be produced.
(ii) Describe briefly the meaning and application of “seismic prospecting”.
(a) Refer notes
(b) (i) Importance of troposphere is supports life on earth
(ii) Ionosphere enhances communication over long distances.
(c) (i) Describe any two causes of earth quake
(ii) Seismic prospecting is an artificial production of seismic waves purposely for searching underground fuels and oils or gases
Example 19: Necta 2001 P1
(a) (i) Define the terms “angle of declination” as used in the specification of the earth’s magnetic field at a point
(ii) The horizontal component of the earth’s magnetic field at a location was found to be 26.0 while the angle of inclination was Find the magnitude of the field and the vertical component of the field at the location
(b) (i) Define an earthquake
(ii) Distinguish between P and S waves. What factors influence their velocities?
(a) (i) Refer notes
(b) The velocities of P and S waves are influenced by;
Density, of the media
Shear modulus, of the media, and
Bulk modulus, B of the media.
Example 20: Necta 2002 P1
(a) (i) What is the importance of ionosphere to mankind?
(ii) Explain why transmission of radio waves is better at night than at day time.
(b) (i) What is an earthquake?
(ii) Explain briefly any four (4) causes of earthquake
Example 21: Necta 2003 P2
(a) Explain the following:
(i) Earthquake (ii) Earthquake focus (iii) The epicenter.
(b) List down three sources of earthquake
(c) (i) Define the ionosphere
(ii) State the ionosphere layer that exists during day time.
(iii) Give the reason for better waves reception for light frequencies signal at night than during the day time
Example 22: Necta 2004 P1
(a) (i) Explain the terms epicenter and focus as applied to earthquake.
(ii) State any four (4) indications that may predict the occurrence of an earthquake.
(iii) State and explain two variations of the earth magnetic field.
(iv) State one necessary precaution to be taken to people living in a region with a high risk of occurrence of earthquakes.
(b) Explain the following
(i) Solar wind (ii) Magnetopause (iii) Ionosphere.
Example 23: Necta 2005 P1
(a) Define the following terms
(i) Epicentral distance (ii) Body wave (iii) Seismograph
(b) (i) explain the meaning of reflection seismology state its application
(ii) Show how the magnetic field within the atmosphere is generated?
(c) (i) Name the lowest layers of the atmosphere and the ionosphere
(ii) State their importance
(a) (i) Lowest layer of atmosphere is troposphere and that of the ionosphere is the D – layer.
Example 24: Necta 2006 P1
(a) (i) State two (2) ways by which seismic wave may be produced
(ii) What is seismic prospecting?
(b) (i) Discuss briefly the importance of the lowest layer of the atmosphere and the ionosphere.
(ii) Sketch the temperature against altitude curve for the atmosphere indicating the important atmospheric layers.
(iii)The average velocity of P – waves through the earth’s solid core is 8kms-1. If the average density of the earth’s rock is 5.5 x 103kgm-3 find the average bulk modulus of the earth’s rock.
(a) (i) Causes of an earthquake
(b) (ii) using the formula
Example 25: Necta 2007 P1
(a) (i) What are the differences between P and S waves?
(ii) Explain how the two terms of waves (P and S) can be used in studying the internal structure of the earth.
(b) Write short notes on the following terms in relation to the changes in the earth’s magnetic field; long term (secular) changes, short – period (regular) changes, and short – term (irregular) changes.
(c) (i) What is geomagnetic micro pulsation?
(ii) Give a summary of location, constitution and practical uses of the stratosphere, ionosphere and mesosphere.
(c) (i) Geomagnetic micro pulsation are small rapid changes in the earth’s magnetic field. They have periods between 0.2 second and 10 minutes and intensities less than 0.01% of the minimum field.
Example 26: Necta 2008 P1
(a) Define the following terms:
(i) Earthquake (ii) atmosphere
(b) Distinguish between body waves and surface waves that are produced by an earthquake.
(c) (i) Define the terms epicenter and focus as applied to earthquake.
(ii) Draw a well labeled diagram which shows the interior structure of the earth.
Example 27: Necta 2009 P1
(a) (i) What is meant by the shadow zone?
(ii) Why does the shadow zone occur?
(b) (i) Name the lowest layer of the atmosphere and the lowest layer of the ionosphere.
(ii) State the importance of each of these layers in b (i) above
(iii) Explain briefly the reason for better reception of radio waves for high frequency signals at night times than during day times.
(c) State the sources of heat energy in the interior of the earth.
Example 28: Necta 2010 P1
(a) (i) Explain the terms: earthquake, earthquake focus and epicenter.
(ii) Describe clearly how P and S waves are used to ascertain that the outer core of the Earth is in liquid form.
(b) (i) Define the ionosphere and give one basic use of it.
(ii) Why is the ionosphere obstacle to radio astronomy?
Example 29: Necta 2011 P1
(a) (i) Define the following terms: Geophysics, Atmosphere and Epicenter
(ii) Write down brief notes on the location, composition and importance of the following:
Troposphere, Stratosphere, Mesosphere and Thermosphere
(b) (i) Draw sketch diagram showing the working part of a Seismometer.
(ii) Explain how temperature varies with both altitude and depth of the Earth.
(iii) Write down two factors that governs heat flow from the interior of the Earth.
Example 30: Necta 2012 P1
(a) (i) Name three layers of the atmosphere
(ii) Describe any two major zones of the earth.
(b) (i) What are the factors that influence the velocities of P and S waves?
(ii) The P and S waves from an earthquake with a focus near the earth’s surface travel through the earth at nearly a constant speed of 8 km/s and 6 km/s respectively. If there is no reflection and refraction of waves how long is the delay between the arrivals of successive waves at a seismic monitoring station at 900 in the latitude from the epicenter of the earthquake?
(a) (ii) any two of core, mantle, crust, hydrosphere, atmosphere
(b) (i) the density of rock, moduli of elasticity of rock material.
(ii) Illustration (R = earth radius)
Distance travelled by the waves (distance between focus and seismic station) is
Time taken by P – waves to arrive at the station is
Time taken by the waves to arrive at the station is
The time interval between the arrival of the two waves is t = t2 – t1 = 25.1 = 18.9 = 6.2 minutes.
Example 31: Necta 2012 P1
(a) (i) What do you understand by the word environmental physics?
(ii) Briefly explain three effects of seismic waves.
(b) (i) Mention three types of environmental pollution
(ii) Explain on the following climatic factors which influence plant growth: Temperature, Relative humidity and wind.
Example 32: Necta 2013 P1
(a) (i) The main interior of the earth core is believed to be in molten form. What seismic evidence supports this belief?
(ii) Explain why the small ozone layer on the top of the stratosphere is crucial for human survival
(b) Electrical properties of the atmosphere are significantly exhibited in the ionosphere.
(i) What is the layer composed of and what you think is the origin of such constituents
(ii) Mentioned two uses of the ionosphere
(c) Briefly explain why long distance radio broadcasts make use of short wave
(a) (i) When P and S seismic waves are sent from one side of earth to the other, only P waves can be detected on the other side. The fact that S waves do not travel through the core provides evidence for the existence of a liquid core.
(ii) Ozone absorbs harmful radiation from the sun. The Ozone projects plant and shield people from skin cancer and eye cataracts.
(b) (i) The layer is composed of free electrons and positive ions. The ionosphere is created by atoms absorbing UV radiation, gamma and x-rays.
(ii) Uses of the ionosphere
Ionosphere supports radio communication over long distances
Particles in the ionosphere absorbs U.V radiation gamma and X-rays, thus protecting people from harmful effects of these radiations
(c) Refer telecommunication notes.
Example 33: Necta 2013 P1
(a) Briefly explain on the following types of environmental pollution:
(i) Thermal pollution
(ii) Water pollution
(b) Describe the soil temperature with regard to agriculture, physics which causes lower crop growth at a particular area
(b) High soil temperature causes the crop roots to rot, this leads to insufficient water supply to plant leaves and hence lower the growth of crop.
Lower soil temperature inactivates soil organisms. Decomposition of organic matter is lowered and hence the supply of nutrients to crop which in turn lead to lower crop growth.
(a) (i) What are auroras?
(ii) Define the homosphere
(b) (i) What are the factors which contribute toward volcanic eruptions?
(ii) What are the effects of volcanic eruptions?
(iii) What are lahars?
Lahars are rapidly flowing mixtures of rock debris and water that originate on the slopes of a volcano. They are also referred to as volcanic mudflows or debris flow. Volcanic eruptions may directly trigger one of more lahars by quickly melting snow and on a volcano or eject water from a crater lake. The form in a variety of at always including through intense rainfall on loose volcano rock deposits and as a consequence of debris of debris avalanches