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UNIT 5:THE ORIGIN OF THE EARTH
Key unit competence: The student teacher should be able to discuss thetheories of the origin of the Earth.
Introductory activity
Using previous knowledge gained in Geography and other disciplines:
a) Explain how the Earth came into existence.
b) Identify the major characteristics of the earth.
c) Describe the internal structure of the Earth.
d) Find out and describe the various geological eras and periods recorded in history.
5.1. Theories of the origin of the earth: Big bang theory
and creation theory
Activity 5.1
Do the exercise shown below:
a) Collect the balloons provided and blow air in each of them.
b) Make sure the balloons are inflated to their maximum sizes.
c) Tie the ends of the balloons containing air to prevent it from escaping.
d) Press hard on the balloons to make sure that they burst.
e) Discuss most generally accepted hypothesis for the formation of theuniverse related to the practical exercise performed in (a-d) above.
In order to understand how the earth came into existence, we need to first
be aware of how the entire universe was formed since the earth is part of the
universe.
Scientists have tried to explain the origin of the earth. It is believed that the
earth and the universe were formed about 15 billion years ago. Some believe
that the earth was formed in a massive explosion known as the big bang and
others believe that the earth and the entire universe were created by God.
There are several theories advanced to explain the origin of the earth. However,
only two theories are going to be discussed: Big Bang theory and Creation
theory because they seem to be the most accepted.
5.1.1. The Big Bang Theory
According to this theory, the entire universe was at one time confined to a
heavy, hot, massive ball that disastrously exploded about 15 billion years ago.
The explosion propelled the materials in all directions away from the centre of
the explosion. This explosion was called the big bang and marked the beginning
of the universe.
The masses of gas from the explosion cooled and condensed to form the stellar
systems like galaxies that are always moving away from the centre of theexplosion in a centrifugal motion.
5.1.2. The Biblical or creation theory
The biblical theory is based on the God’s creation story that is found in the first
chapter of the bible (Genesis). This chapter states that the earth was created by
God and draws the calendar of the whole creation process. The creation process
took place in six days.
On the first day, God created the day and the night.
• On the second day, He created the sky.
• On the third day, He created the land, water and the plants.
• On the fourth day, He created the sun, moon and stars.
• On the fifth day, He created the sea creatures and the birds.
• On the sixth day, He created animals of all kinds and human beings.
Therefore, according to this theory, the Earth and universe were all togethercreated by God.
Application Activity 5.1
From the theories that you have learnt above;
a) Explain the difference between the Big bang theory and the Biblical
theory about the origin of the earth.b) Make a research on other theories of the origin of the earth.
5.2. Characteristics of the EarthActivity 5.2
Read the passage below and answer the questions asked
Far back in ancient times, everybody thought the earth was flat. If you are
in a boat way out in the middle of the ocean, the top of the water looks flat
in every direction and the sky seems to fit over it like an upside-down bowl.
The line where the sky and water meet is called the “horizon”. The horizon
looks like a circle with yourself at the center. If you are on land, the land
stretches out to a horizon also. The horizon on land, however, is not even.
It goes up and down because of houses, trees, hills, and other things. Some
ancient people suspected that the earth went on forever. They thought it
might be a huge flat piece of land and sea with no end at all.
a) What does the story above talk about?
b) How do you find the shape of the earth?
c) Find out the evidences advanced to support the most agreed shape of our planet.d) How big is the planet Earth?
5.2.1. The shape of the earth is spherical
1) The shape of the earth
Studies based on modern technology have proved that though the earth is a
sphere, it is not a perfect sphere but a geoid. This is because the observation
of the polar areas reveals that they have a flat landscape, at the same point theequatorial areas have a bulging appearance as shown below:
2) Evidences showing that the earth is spherical
There are several evidences to prove the spherical form of the earth:
─ The circumnavigation: If one travelled across the world along the
straight path without stopping would come back to the point of origin
where the journey started from.
─ Images from the space: Photographs taken from space by satellites
show that the earth’s surface is curved (spherical shape).
─ The view of other planets: All observations from telescopes reveal that
the planetary bodies are spherical from whichever angle. Therefore,
since the earth is one of them, its shape is also spherical in nature.
─ The shadow of the earth during eclipses: The shadow resulting from
the eclipse of the moon (Lunar eclipse) shows that the earth is round
and only spherical objects can cast a round image.
─ Day-night and seasonal change: The earth’s tilted axis produces
seasonal climatic conditions and gives days and nights of varying length.
If the earth was at right angles to the sun, the day and the night would
always be of equal length, and there would only be one season throughout
the year.
─ The size and diameter of the earth: The earth’s equatorial diameter
and circumference are much bigger than those of the polar areas.
─ The sun rise and sun set: If the earth was flat, the sun would rise and set
at the same time in all countries. But, the sun rises and sets at different
times in different places.
─ The ship sailing away from the coast: The lower parts of a ship that
is moving away from the coast disappears before the upper parts. Thefigure below helps to understand this evidence:
─ The view of circular horizon: The distant horizon viewed from the top
of a hill or deck of a ship at a sea is always circular in shape.
5.2.2. The Earth’s size: radius, diameter, circumference, volume and mass
• The average radius of the earth is 6,371 km.
• The Earth’s equatorial diameter is 12,756 km while the polar diameter
is 12,714 km.
• The Earth’s circumference at the equator is 40,075 km. From pole topole, its circumference is 40,008 km.
The size of the Earth
• The Earth’s volume is 1.08321×1012 km3
• The Earth’s mass is 5.9736 x 1024 kg.
• The Earth’s density is 5.52 g/cm3.
• Gravity of the earth is approximately 9.8 m/s2
Application Activity 5.2
Study the illustration provided below and use it to answer the questions
that follow:
─ Use an egg, a piece of thread, a ruler and markers.─ Mark that egg following the marks in the illustration below.
a) With reference to the divisional lines drawn on the illustration above,
use the egg provided to measure the diameter and the circumference.
b) From your findings above, describe the shape of the earth.
c) Assume the above illustration stands for our planet earth, name X, E
and Y, and indicate the size of E:
i) X: …………………………………………………………
ii) E: ……………………………………………………………...iii) Y: …………………………………………………………….
5.3. The internal structure and mineral composition of the earth
Activity 5.3
1. The study of the earth’s environment cannot be complete without
understanding the nature of the earth’s interior structure. What do
you know about the interior of the earth?
2. Using the knowledge acquired so far, study the illustration given belowand give names of layers represented by figures.
a) Name the parts of the earth corresponding to numbers, 1,2, 3a,
3b,4,5,6 and 7.
b) Describe the characteristics of the parts you have named.
3. Carry out a research to find the main mineralogical composition of
the earth.
5.3.1. The internal structure of the earth
The layers of the Earth’s interior include the crust, mantle, liquid outer core,
and solid inner core. The interior structure of the earth is layered in concentric
shells:
i) An outer silicate solid crust,
ii) A highly viscous mantle,
iii) A viscous outer core,
iv) A solid inner core.
The figure below depicts the main layers of the Earth’s interior part and theirrespective properties.
1) The crust
The crust is the outermost layer of the Earth. It is also referred to as the
lithosphere. The crust is divided into two types, the oceanic crust and the
continental crust:
• Oceanic crust (SIMA) is a layer consisting mainly of basalt, averaging
6-10 km. in thickness.
• Continental crust (SIAL) is layer consisting mainly of granite; it can
be up to 70 km thick. The crust is separated from the mantle by the
Mohorovicic discontinuity.
2) The Mantle
The mantle is composed mainly of silicate rocks, rich in iron and magnesium.
The mantle is under the crust. It is composed of rocks that are in a semi-molten
state. They are mainly composed of Ferro magnesium silicate (iron, magnesium)
minerals.
The mantle is divided into two main parts, namely the upper mantle and the
lower mantle:
• The upper mantle: It is a layer of less rigid and more plastic rocks. It
extends from Repeti discontinuity to the depth of 1,000 km.
• The lower mantle: It goes from 1,000 km to 2,900 km in depth. At this
depth the lower mantle is separated from the outer core by Gutenberg
discontinuity.
3) The core
The core is composed of outer core and inner core.
• The outer core is liquid and kept in a semi-molten state
• The inner core is solid and made mostly of nickel iron and with some (NIFE).
The figure below shows the relative depth of different internal layers of the
earth. Inner core and outer core are separated by Lehman discontinuity.
In geology, the term “discontinuity” is used for a surface at which seismicwaves change velocity in interior of the earth.
5.3.2. Chemical composition of the Earth
Earth’s crust
The most common chemical elements in the crust are oxygen (gas) (46.6%),
with mineral such as silicon (27.7 %), aluminum (8.1 %), iron (5.0 %),
calcium (3.6 %), potassium (2.8 %), sodium (2.6 %), and magnesium (2.1%).
Continental crust is dominated by Silicon and Aluminum minerals (SIAL),
whereas the oceanic crust has Silicon and Magnesium is large proportion.
Earth’s mantle
In terms of its constituent elements, the mantle is made up of 44.8% oxygen,
21.5% silicon, and 22.8% magnesium. There’s also iron, aluminum, calcium,
sodium, and potassium. These elements are all bound together in the form of
silicate rocks, all of which take the form of oxides.
Earth’s core
The Inner core has 20 % Nickel and 80 % Iron whereas proportions ofminerals in the outer core amount, for Nickel: 2 %, Iron: 86 %, Sulfur: 12 %
Application Activity 5.3
Apply the knowledge you have acquired in this lesson to answer the
following questions:
1) Suppose you are asked to describe how Mount Muhabura is related to
the internal structure of the earth, what would be your response?
2) The earth’s crust is composed of many minerals. Indicate its mostcommon chemical elements.
Skills lab
Visit one of the neighbouring schools in your area and discuss with the studentsaround about the shape of the earth using the knowledge gained from this unit.
End unit Assessment
1. Read the following discussion between two students and answer
the questions asked:
Two students Mugisha and Uwamahoro were discussing about the origin
of the earth. Mugisha said that the earth and other celestial bodies came
into existence in different ways: some were formed due to the collision
of stars, others by explosion. With confidence, Uwamahoro said that
everything in the universe was created by God.a) Who is right, who is wrong?2. The physical features of the earth are changing day by day due to
b) Explain the biblical theory of the origin of the earth.
natural reasons and human activities. Suggest ways which people
can use to conserve the nature for its sustainability.
3. Geologists and archaeologist are directly involved in carrying
out excavations (digging) that make their research successful in
discovering new fossils in different places on the Earth. Suggest the
ways in which humans should protect the environment for the sakeof promoting such archaeological research.