FORM FOUR PHYSICS STUDY NOTES TOPIC 7: GEOPHYSICS

TOPIC 7: GEOPHYSICS

Geophysics is a branch of science that deals with the physical, chemical, geological, astronomical, and other characteristic properties of the Earth. It studies 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 concentrically: the crust, mantle, and core.

The Crust

The crust is the outer solid layer of the Earth. It is extremely thin (5 to 15 km) compared to the Earth’s radius (6,371 km). There are two types of crust:

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 denser (3.0 to 3.1 tonnes per cubic meter). Both the continental and oceanic crusts float on the denser mantle. Because of its low density, the continental crust floats on the mantle at a higher elevation, forming 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 8 km thick. The boundary between the crust and the mantle is called the Mohorovicic discontinuity or simply Moho. It is a zone between one and several kilometers thick.

The Mantle

The mantle begins from the Moho and extends to a depth of 2,900 km 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 in both solid and molten states. The upper surface of the mantle has a temperature of about 870°C, increasing downwards through the mantle to about 2,200°C near the core.

The Core

The core is the innermost part of the Earth. It extends from the Gutenberg discontinuity to the Earth’s geometric center. The core consists of two distinct regions:

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

The continental crust is made of granite and sedimentary rocks forming the lands and mountains, while the oceanic crust forms oceanic basins. The mantle is made of solid 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

The continental crust forms the land and mountains of the Earth on which all human activities are carried out, such as farming and housing. The oceanic crust forms the base of the oceans and seas on which oceanic water rests and where aquatic organisms like fish live. The mantle provides heat transfer from the core to the outer layers, a process that causes volcanic activity and earthquakes.

Earthquakes and Volcanoes

Both volcanoes and earthquakes 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 occur 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 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 forms, it rises through the denser rock layer towards the Earth’s surface.

Magma that reaches the Earth’s surface is called lava. Most volcanoes form along constructive and destructive boundaries between tectonic plates. However, a few form away from plate boundaries.

Types of Volcanoes

There are two main types of volcanoes:

1. Fissure volcanoes: These occur along 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 magma reaches the Earth’s surface. They usually develop a cone shape built up from successive layers of lava and ash.

Classification of Volcanoes

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

1. Active volcanoes: Those that either erupt constantly or have erupted in recent times, e.g., Oldonyo Lengai.
2. Dormant volcanoes: Those that have been inactive for some time (a few thousand years) but can erupt again, e.g., Mt Kilimanjaro.
3. Extinct volcanoes: Those that have not erupted in recorded history and 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 eruptions sometimes set surrounding vegetation on fire. Wild animals may be killed by being buried in lava or burnt by forest fires.
3. Environment: Volcanic eruptions emit harmful gases into the environment, such as sulphur dioxide. Some of these 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 deep underground to the Earth’s surface.
6. Minerals: Volcanoes also bring valuable minerals to the Earth’s surface. These minerals are important economic resources.

The Origin of Earthquakes

Explain the origin of earthquakes

An earthquake is a sudden motion or shaking of the Earth caused by a sudden release 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. Earthquakes also occur as a result of magma movement at constructive boundaries under volcanoes and where continental plates collide and push mountain ranges.

How Earthquakes 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 due to faults formed millions of years ago.

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

The point within the Earth where an earthquake begins is called the hypocenter or the focus of the earthquake. Earthquakes 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: The first waves released from the hypocenter. They are felt as a sudden jolt.
2. Secondary waves or s-waves: Arrive a few seconds after p-waves. They are felt as a series of side-to-side tremors.
3. Surface waves: Radiate outward from the point on the Earth’s surface directly above the hypocenter. This point is called the epicenter of the earthquake.

There are two types of surface waves:

1. Rayleigh 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. These surface waves damage structures such as buildings and hydroelectric power plants.

The Principle of Measurement

Describe the principle of measurement of earthquakes

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

The magnitude of an earthquake measures the energy it releases. It is usually measured on the Richter scale, which is logarithmic (base 10). The intensity measures the strength based on the changes it causes to the landscape. Intensity is usually measured on the Modified Mercalli scale, calibrated from 1 to 12.

Note: An earthquake can have only one magnitude. However, its intensity reduces as seismic waves spread from the hypocenter, similar to how sound loudness decreases with distance from the source.

The Seismograph

A 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. The ratio between the pendulum’s deflection and the ground’s acceleration can also be used to record an earthquake.

Precautions Against Earthquake Hazards

Identify precautions against earthquake hazards

Earthquake Hazards

Some hazards associated with earthquakes include:

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

Earthquake Warning Signs

Important signs observed before an earthquake include:

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

Precautions to Take During an Earthquake

To minimize injuries or death during an earthquake:

1. If indoors, drop, cover, and hold on. Get under a desk, table, or bench. Hold onto one leg and cover your eyes. If no desk or table is nearby, sit against an interior wall.
2. Pick a safe place where objects will not fall on you—away from windows or tall heavy furniture.
3. Do not run outside during the earthquake because bricks, roofing, and other materials may fall from buildings, injuring people nearby.
4. Wait in your safe place until shaking stops, then check if you are hurt. Take care of yourself first, then help others.
5. Move carefully and watch for fallen or broken objects creating hazards. Be ready for aftershocks.
6. Be alert for fires, the most common earthquake-related hazard due to damaged gas and electrical lines.
7. If you must leave a building after shaking stops, use stairs, not elevators. Earthquakes can trigger fire alarms and sprinklers, so use stairs as a precaution.
8. If outside during an earthquake, stay outside. Move away from buildings, trees, streetlights, and power lines. Crouch down and cover your head. Falling materials and trees may cause 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 gradually fades into space. The dense part of the atmosphere lies within 30 km above the Earth’s surface.

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

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, extending up to 10 km above the poles and 20 km above the equator. It is the densest part of the atmosphere (80% by mass) and contains most of the atmosphere’s water vapor.

The temperature in this region decreases with altitude at an average rate of 6°C/km. This encourages weather changes; most weather phenomena occur in the troposphere. Clouds and rain form within this region.

The boundary separating the troposphere from the stratosphere is called the tropopause. At the tropopause, the temperature stops decreasing with altitude and becomes constant. The tropopause has an average height of about 10 km.

Stratosphere

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

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

The stratosphere and troposphere together are known as the lower atmosphere. The boundary separating the stratosphere from the outer layer is called the stratopause.

Advantages of the Stratosphere

It absorbs ultraviolet radiation that would otherwise reach the Earth’s surface, which is harmful to both plants and animals.

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

Mesosphere

The mesosphere starts just above the stratosphere and extends to 85 km high. The temperature decreases with altitude in this layer. The lowest temperature in the atmosphere occurs here (-90°C).

This is the layer where most meteors burn upon entering the Earth’s atmosphere. The boundary separating the mesosphere from the thermosphere is called the mesopause.

Thermosphere

The thermosphere lies just above the mesopause and extends up to 690 km high. The temperature increases with altitude due to the sun’s heat, reaching as high as 1727°C. Chemical reactions occur faster here than on the Earth’s surface.

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

Importance of the Ionosphere

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

Exosphere

The exosphere is the outermost region of the atmosphere. Here, atmospheric gas pressure is very low, allowing light atoms such as hydrogen and helium to acquire enough energy to escape the Earth’s gravitational pull.

The upper part of the exosphere is called the magnetosphere. The motion of ions here is strongly constrained by 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 includes:

1. The troposphere controls the climate and ultimately determines the quality of life on Earth.
2. The troposphere is important for life on Earth. It contains gases such as oxygen, used for respiration by animals, and carbon dioxide, used by plants in photosynthesis. Nitrogen provides an inactive environment for many chemical processes. These gases support 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 prevent harmful radiation such as cosmic rays from reaching the Earth’s surface.
5. Communication is 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 average temperatures near or on the Earth’s surface due to the greenhouse effect. This effect is caused by greenhouse gases produced from natural and industrial processes.

The greenhouse effect is the process by which radiation emitted by the atmosphere warms the Earth’s surface.

When heat from the sun reaches the Earth’s surface as sunlight, some 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 escaping to space. This absorption warms the atmosphere.

Greenhouse gases act like a mirror, reflecting some heat energy back to the Earth that would otherwise be lost to space.

Sources of Greenhouse Gases

Identify sources of greenhouse gases

Sources include:

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

The Occurrence of Global Warming

Explain the occurrence of global warming

Global warming is primarily caused by excess carbon dioxide (CO₂) in the atmosphere, which acts like a blanket, trapping heat and warming the planet. Burning fossil fuels like coal, oil, and natural gas for energy, or cutting and burning forests for pastures and plantations, increases carbon in the atmosphere. Certain waste management and agricultural practices release other potent greenhouse gases, such as methane and nitrous oxide, worsening the problem.

The Consequences of Global Warming

State the consequences of global warming

Effects of global warming include:

1. Increase in ocean temperatures.
2. Rise in sea levels.
3. Changes in global climate patterns.
4. Acidification of the oceans.
5. Extreme weather events.
6. Higher or lower agricultural yields.
7. Melting of Arctic ice and snowcaps, causing landslides, flash floods, and glacial lake overflow.
8. Extinction of some animal and plant species.
9. Increase in the range of disease vectors, organisms that transmit diseases.