# FORM FOUR PHYSICS STUDY NOTES TOPIC 7: GEOPHYSICS

GEOPHYSICS

Geophysics is a branch of science that deals with the physical, chemical, geological, astronomical and other characteristic properties of the earth.It deals with geological phenomena such as the temperature distribution of the earth’s interior, the source, configuration and the geomagnetic field.

Structure and Composition of the Earth

The Structure of the Earth

Describe the structure of the earth

The structure of the earth is composed of three major zones arranged in concentric manner. These are crust, mantle and core.

The crust

Is the outer solid layer o the earth.It is extremely thin(5 to 15km) compared to the radius of the earth (6,371km).There are two types of crust, namely:

1. Continental crust: This is heterogeneous and of relatively low density(2 to 2.8 tonnes per cubic meter).-It is composed mainly of granites and sedimentary rocks.
2. Oceanic crust: This is basaltic and more denser (3.0 to 3.1 tonnes per cubic meter). Both the continental and the oceanic crusts floats on the denser mantle. Because of its low density,the continental crust floats on the mantle at a higher elevation, forming the land masses and mountains. The continental crust is 30 to 70 km thick. The denser oceanic crust floats at a lower elevation forming oceanic basins.It is about 8km thick. The boundary between the crust and the mantle is called Mohorocivic discontinuity or simply Moho. It is a zone between one and several kilometers thick.

The mantle

It begins from the Moho and extends to a depth of 2,900km below the earth’s surface,up to its boundary with the earth’s core.This boundary is called the Gutenberg discontinuity.

The mantle contains about 70% of the earth’s mass.It is composed of rocks,both in solid and mountain states.The upper surface of the mantle has the temperature of about 870°C,and this temperature increases downwards through the mantle to about 2,200°C near the core.

The core

Is the innermost part of the earth.It extends from the Gutenberg discontinuity to the earth’s geometric centre. The core consists of two distinct regions namely:

1. The inner core: It is composed of solid material because the high pressure at this depth.-It is composed of iron-nickel alloys.
2. The outer core: Is composed of liquid of molten nickel and iron known as magma. It extends from the mantle to a depth of about 5,000km below the earth’s surface.

Activity 1

In groups of five, discuss why the outer core is liquid while the inner core is solid.

The Composition of the Layers of the Earth

Describe the composition of the layers of the earth

Continental crust is made of granite and sedimentary rocks forming the lands and the mountains while the oceanic crust forms oceanic basins. Mantle is made of solids and molten rocks. The outer core is made of molten nickel and iron called magma while the inner core is solid because of the high pressure. The crust and the mantle are separated by the mohorovicic discontinuity.

The Importance of the Layers of the Earth

Explain the importance of the layers of the earth

Continental crust forms the land and mountains of the earth on which all human activities are carried out e.g farming, housing etc. Oceanic crust forms the base of the oceans and seas on which oceanic water rests and all aquatic organisms like fishes live. The mantle provides the heat transfer from the core to the outer layers a process which causes the volcanic actions and earthquakes.

Earthquake and Volcanoes

Both of them, volcano and earthquake are caused by the movement of molten rock and heat deep inside the earth. These movements are referred to as subterranean movements. Most earthquakes and volcanic activity happen near tectonic boundaries.

The Origin of Volcanoes

Explain the origin of volcanoes

Volcanoes are places where molten rock called magma leaks out through a hole or a crack in the earth’s crust.Magma originates from the mantle,where high temperature and pressure cause the rock to melt.When a large pool of magma if formed,it rises through the denser rock layer towards the earth’s surface.

Magma that has reached the earth’s surface is called lava. Most volcanoes form along constructive and destructive boundaries between tectonic plates.However a few form plate boundaries.

Types of volcanoes

There are two main types of volcanoes, namely:

1. Fissure volcanoes: These occur along the cracks in and between tectonic plates. They can be many kilometers long. Lava is usually ejected quietly and continuously,forming enormous plains or plateaus of basaltic volcanic rock.
2. Central volcanoes: These have a single vertical main vent through which mzgm reaches the earth’s surface. They usually develop a cone shape that builds up from successive layers of lava and ash.

Classification of volcanoes

Volcanoes are classified into three categories based on their frequency of eruption, namely:

1. Active volcanoes: Are those that either erupt constantly or have erupted in recent times. Eg; Oldonyo Lengai.
2. Dormant volcanoes: Are those that have been inactive for some time(a few thousand years) but can erupt again. Eg; Mt Kilimanjaroc.
3. Extinct volcanoes: They have not erupted in recorded history. They will probably never erupt again.

Effects of Volcanoes

Describe effects of volcanoes

Effects of volcanoes include:

1. Landscape: Most of the earth’s surface is covered with volcanic rocks.Volcanoes are also responsible for the formation of many mountains and islands.
2. Vegetation and wildlife: Volcanic eruption sometimes set the surrounding vegetation into fire.Wild animals are also killed by being buried into the lava or being burnt by the forest fires.
3. Environment: Volcanic eruptions emit harmful gases into the environment. Such gases include sulphur dioxide. Some of the gases contribute to global warming and climate change.
4. Human life and property: Volcanic eruptions sometimes kill people and destroy property.
5. Soil: Volcanoes help in soil formation by bringing important minerals from from deep underground onto the earth’s surface.
6. Minerals: Volcanoes also bring valuable minerals to the earth’s surface.The minerals are important economic resources.

The Origin of Earthquake

Explain the origin of earthquake

An earthquake is a sudden motion or shaking of the earth caused by a sudden relese of energy that has accumulated within or along the edges of the earth’s tectonic plates.

Earthquakes happen when rocks in the earth’s crust move suddenly, shaking the earth.Earthquake also occur as a result of movement of magma at constructive boundaries under volcanoes and where continental plates collide and push mountain ranges.

How earthquake occur

Earthquakes mostly occur on or near the boundaries between tectonic plates.However ,earthquakes can also occur far from plate boundaries. Such earthquakes probably occur as a result of faults formed millions of years ago.

Pressure liquid builds between them until the friction force holding the plates together gives way.The plates move suddenly,releasing the pressure or energy and then holds together again. This sudden jerk is what is felt as an earthquake.

The point within the earth where an earthquake begins is called the hypocenter or the focus of the earthquake. Earthquake rarely occur along constructive plate boundaries.

Seismic waves

This refers to the energy released by an earthquake. They are grouped into three categories:

1. Primary waves or p-waves: Are the first waves released from the hypocenter. They are felt as a sudden jolt.
2. Secondary waves or s-waves: These arrive a few soconds later after p-waves.They are felt as a series of side-to-side tremors.
3. Surface waves.-They radiate outward from the point on the earth’s surface directly above the hypocentre. This point is called the epicentre of the earthquake.

There are two types of surface waves:

1. Reyleigh waves- create a rolling movement that makes the land surface move up and down.
2. Love waves- make the ground shift from side to side. It is the surface waves that demage to surface structure such as buildings and hydroelectric power plants.

The Principle of Measurement

Describe the principle of measurement of earthquake

The nature of an earthquake is usually described by measuring two properties, namely the magnitude and intensity.

The magnitude of an earthquake is a measure of the energy it releases. It is usually measured on the Richter scale.

The Richter scale magnitudes are based on a logarithmic scale(base 10).The intensity of an earthquake is a measure of its strength based on the changes it causes to the landscape.The intensity is usually measured on the Modified Mercalli scale.The scale is calibrated 1 to 12.

Note:An earthquake can have only one magnitude. However its intensity reduces as the seismic waves spread out from the hypocentre,just the same way the loudness of a sound changes as you move away from the source.

The Seismograph:Is an instrument used to record ground movements caused by earthquakes. It measures ground oscillations by recording the relative motion between a pendulum and the ground. It is also possible to use the ratio between the deflection and the of the pendulum and the acceleration of the of the ground to record an earthquake.

Precaution against Earthquake Hazards

Identify precaution against earthquake hazards

Earthquakes hazards

The following are some of the hazards associated with earthquakes:

1. Landslides
2. Tsunamis
3. Collapsing buildings
4. Fire outbreak
5. Backward rivers

Earthquake warning signs

The following are important signs that are observed before an earthquake occurs:

1. Thermal indicator
2. Water indicator
3. Seismo electromagnetic indicator
4. Animal indicator
5. Human indicator

Precautions to be taken during an earthquake

The following are some precautions that can be taken to minimise injuries or death of human beings in the event of an earthquake:

1. If you are indoors during an earthquake ,drop,cover and hold on.Get under a desk,table or a bench.Hold on to one of the legs and cover your eyes.If there is no desk or table nearby, sit down against an interior wall.
2. Pick a safe place where things will not fall on you-away from windows or tall heavy furniture.
3. Do not run outside when the earthquake happens because bricks, roofing and other materials may fall from buildings during and immediately after an earthquake, injuring persons near the building.
4. Wait in your safe place until the shaking stops, then check to see if you are hurt. You will be better able to help others if you take care of yourself first,then check on the people around you.
5. Move carefully and watch out for things that have fallen or broken creating hazards. Be ready for additional earthquakes called after shocks.
6. Be on the lookout for fires. Fire is the most common earthquake-related hazard due to damaged gas and electrical lines.
7. If you must leave a building after the shaking stops, use the stairs and not elevator. Earthquakes can cause fire alarms and fire sprinklers to go off. You will not be certain whether there is a real threat of fire. As a precaution,use the stairs.
8. If you are outside during an earthquake,stay outside. Move away from buildings, trees, streetlights and power lines. Crouch down and cover your head. Bricks,roofing and other materials can fall from buildings, injuring persons nearby.Trees, streetlights and poor lines may also fall, causing damage or injury.

Structure and Composition of the Atmosphere

The Vertical Structure of the Atmosphere

Describe the vertical structure of the atmosphere

The atmosphere is a layer of gases containing numerous small suspended solid and liquid particles surrounding the earth.It has no outer boundary,it just fades into space.The dense part of the atmosphere lies within 30km above the earth’s surface.

The atmosphere is divided into regions based on its thermal characterestics (temperature changes),chemical composition, movement and density. It is divided into five regions, which are:

1. Troposphere
2. Stratosphere
3. Mesosphere
4. Thermosphere
5. Exosphere

The Composition of the Atmosphere

Describe the composition of the atmosphere

Troposphere

This is the region nearest to the earth’s surface which extends to an altitude up to 10 km above the poles and 20km above the equator.It is the most dense part of the atmosphere(80% by mass of the atmosphere) which contains most of the atmosphere’s water vapour.

The temperature in this region decreases with altitude at an average rate of 6°C/km.It encourages the change of weather(most of weather phenomenon occur in the troposphere).Clouds and rain are formed within this region.

The boundary which separates the tropospher from the the stratosphere is called the tropopause. At the tropopause,the temperature stop decreasing with altitude and becomes constant. The tropopause has an average height of about 10km.

Stratosphere

It starts from the tropopause and extends to 50km high.It is more stable,drier and less dense compared to troposphere.

The temperature slowly increases with altitude due to the presence of ozone layer which absorbs ultraviolet rays from the sun. The ozone layer lies in the middle of the stratosphere between 20 and 30km. Ozone is triatomic(three-molecules) form of oxygen.

The stratosphere together with with troposphere are collectively known as the lower atmosphere.The boundary which separates the stratosphere from the outer layer is called the stratopause.

It absorbs the ultraviolet radiations which would otherwise reach the earth’s surface which is harmful to both plants and animals.

It prevents large storms from extending much beyond the troposphere due to its stability. Planes also fly within this layer because it has strong steady horizontal winds which are above the stormy weather of the troposphere.

Mesosphere

It starts just above the stratosphere and extends to 85km high.The temperature at this layer decreases with altitude.The lowest temperature of the atmosphere occurs within this region(-90°C).

Is the layer where most meteors burn while entering the earth’s atmosphere.The boundary which separates the mesosphere from the thermosphere is called the mesopause.

Thermosphere

It is just above the mesopause and extends up to 690km high. The temperature increases with altitude due to the sun’s heat.The temperature in this region can go as high as1727°C and chemical reactions occur faster in this region than on the earth’s surface.

This layer is also known as the upper atmosphere. The lower part of the thermosphere,from 80 to 550km above the earth’s surface, contains the ionosphere. This is a region containing a high concentration of charged particles called ions and free electrons.

Importance of ionosphere

The large number of free electrons in the ionosphere allows the propagation of electromagnetic waves. It absorbs the dangerous radiations like X-rays and extreme ultraviolet(EUV) radiation. It plays an important role in communition of radio waves.

Exosphere

Is the outermost region of the atmosphere.In this region,the atmospheric gas pressure is very low such that light atoms such as hydrogen and helium may acquire sufficient energy to escape the earth’s gravitational pull.

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.

The Importance of Various Layers of the Atmosphere

Explain the importance of various layers of the atmosphere

The importance of the atmosphere include the following:

1. The troposphere controls the climate and ultimately determines the quality of life on the earth.
2. The troposphere is important for life on earth. The layer contains gases which include oxygen which is used for respiration by animals and carbon dioxide which is used by plants in photosynthesis. The nitrogen found in this laye also provides an inactive environment for many chemical processes to take place. The gases also support many important chemical processes such as combustion,weathering and oxidation.
3. The stratosphere prevents harmful ultraviolet radiation from reaching the earth.
4. The mesosphere,thermosphere and exosphere also prevent harmful radiation such as cosmic rays from reaching the earth’s surface.
5. Communication is also made possible by some layers of the atmosphere, specifically the ionosphere.

The Greenhouse Effect and Global Warming

The Greenhouse Effect

Explain the greenhouse effect

Global warming is the increase of the average temeratures near or on the surface of the earth as a result of what is known as the greenhouse effect. The effect is caused by greenhouse effect. These gases are produced from natural and industrial processes.

The greenhouse effect is the process in which the emission of radiation by the atmosphere warms the earth’s surface.

When heat from the sun reaches the earth’s surface in form of sunlight,some of it is absorbed by the earth.The rest is radiated back to the atmosphere at a longer wavelength than the incoming sunlight.Some of these longer wavelengths are absorbed by greenhouse gases in the atmosphere before they are lost to space.The absorption of this long-wave radiant energy warms the atmosphere.

The greenhouse gases act like a mirror, reflecting back to the earth some of the heat energy which would otherwise be lost to space.

Sources of Greenhouse Effect

Identify sources of greenhouse

Sources of greenhouse effect include:

1. Carbondioxide
2. Clearing and burning of vegetation
3. Burning of fossil fuel
4. Methane
5. Dinitrogen oxide
6. Chlorofluorocarbons(CFCs)

The Occurence of Global Warming

Explain the occurrence of global warming

Global warming is primarily a problem of too much carbon dioxide (CO2) in the atmosphere—which acts as a blanket, trapping heat and warming the planet. As we burn fossil fuels like coal, oil and natural gas for energy or cut down and burn forests to create pastures and plantations, carbon accumulates and overloads our atmosphere. Certain waste management and agricultural practices aggravate the problem by releasing other potent global warming gases, such as methane and nitrous oxide.

The Consequences of Global Warming

State the consequences of global warming

Effects of Global Warming

1. Increase in the temperature of the oceans.
2. Rise in sea levels.
3. Change in world’s climatic pattern.
4. Acidification of the oceans.
5. Extreme weather events.
6. Higher or lower agricultural yields.
7. Melting of Arctic ice and snowcaps. This cause landslides, flash floods and glacial lake overflow.
8. Extinction of some animal and plant species.
9. Increase in the range of disease vectors, that is,organisms that transmit diseases.