Earth Science

Grade 11/12 • Unit 2: Why Life on Earth is Possible

LESSON 2.1
The Origin of Planet Earth
Table of Contents

Introduction 1

Learning Competency 2

Learning Objectives 2

Warm-Up 2

Learn about It 4
Accretion 4
Homogeneous Accretion Hypothesis 5
Heterogeneous Accretion Hypothesis 7
Evidence and Loopholes of the Two Hypotheses 8

Key Points 9

Check Your Understanding 10

Bibliography 12
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Grade 11/12 • Unit 2: Why Life on Earth is Possible

Lesson 2.1
The Origin of Planet Earth

A snowman

Introduction
Have you ever built a snowman? Whether you’ve tried it yourself or watched someone make
it in a movie, you would know that it’s a simple process. You just get a handful of snow and
mold it into a spherical shape. Then, you put it back on the ground and roll it around. Since
the snow is likely to stick to itself, your small sphere can grow as big as you are by
accumulating snow from the ground. This is also the same thing that happens in the
formation of planets. Before the solar system was formed, stars and planets merely existed
in a massive cloud of dust and gas. These fragments of dust and gas started to bump into

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each other, forming huge pieces of matter. The process of collision started the process of
accretion. How did accretion form the planets?

Learning Competency
At the end of this lesson, the given DepEd learning competency should be met
by the students.
● Describe the characteristics of Earth that are necessary to support life.
(S11/12ES-Ia-b-3)

Learning Objectives
At the end of this lesson, you should be able to do the following:
● Explain how Earth was formed according to accretion hypotheses.
● Differentiate between homogenous and heterogenous accretion
hypotheses.

Warm-Up

Formation of Earth

Materials
● ball
● string
● color-coded cards with these numbers written on them:
○ black = 6
○ red = 5
○ yellow = 3

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○ brown = 1
○ blue = ½

Procedure
1. Use a string to hang the ball from the ceiling. Make sure that the ball is around three
feet above the ground. This ball represents the particle with the most density and will
draw another particle toward it.
2. Assign one student to take a video from a bird's eye view. Students will watch this
video after the activity.
3. Each student should receive one color card. Then, students will move to random
positions surrounding the ball. Each student represents a particle that has its own
gravity. The arrangement of the particles from heaviest to lightest is as follows: black
- red - yellow - brown - blue. Remember that the ball is the heaviest and densest. It
tends to pull the particles towards it and towards each other.
4. Students will now move towards the ball depending on the number of steps written
on their cards. Make sure that each step will be from heel to toe. If they hit another
student while taking steps toward the ball, they will combine to create a larger
particle by getting the sum of the number written on their card. For example, if a
student with a yellow card (3) bumps into a student with a brown card (1), together
they will move 3 + 1 = 4 steps.
5. Note: The ball's gravity is so strong that each student representing a particle wants to
put one hand on the ball. If a student cannot reach the ball, that student can just put
their hand on the shoulder of the person touching the ball. Take note of the students
touching the ball directly from the first layer. The second layer is formed by the
students touching the shoulders of the person touching the ball directly. If the
shoulders of the students in the first layer are full, other less massive students can
just touch the shoulders of those students in the second layer and so on.
6. Stop when you are attached to the ball already.
7. Watch the video recorded to get a better view of all the steps that occurred.

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Guide Questions
1. What was the shape formed after the particles, represented by students, were pulled
towards the ball?
2. What is the composition of the formed inner layer? How about the outer layers?
3. What is the relationship between particle size and gravitational pull?

Learn about It
In a carnival or fair, there is usually a man with a cotton candy machine. If you’ve seen how a
cotton candy machine works, you would know that sugar is spun on a stick, and the
cotton-like structure expands as more sugar sticks. The process of planetary formation is
similar to this. The accumulation of small pieces of dust forming huge lumps of matter is a
process known as accretion.

Essential Question
What processes were involved in the formation of Earth?

Accretion
Accretion happens when gravity attracts tiny bits of matter towards an object. This results
in a gradual increase in the object’s size. In relation to the solar system’s formation, the
objects increased in size until they turned into planets. As the objects grew bigger, they
pulled more fragments of matter due to a stronger gravitational pull.

As shown in Fig. 1, accretion happens in four steps. First, clumps of dust grains collide,
forming planetesimals and eventually turn into a protoplanet as more planetesimals are
attracted. A protoplanet is a planetary embryo that consists of a collection of matter from
which a planet is formed.

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Fig. 1. Steps in accretion

There are two hypotheses on how the structure of Earth was formed, which both involve
accretion: homogeneous and heterogeneous accretion hypotheses.

Homogeneous Accretion Hypothesis

The homogeneous accretion hypothesis states that the formation of Earth began after the
condensation of fine particles of the primitive nebula about 4.6 billion years ago. When
these particles accreted, they formed a homogeneous primordial Earth. Thus, early Earth
had a uniform solid composition. Its primary components were iron, magnesium, nickel,
silicates, and some radioactive elements such as uranium and thorium. Due to the
gravitational contraction and decay of radioactive elements, the temperature of early Earth

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increased. Iron and nickel melted, and they sank towards the center because of their high
density. On the other hand, less dense silicates were displaced and moved upwards.

In this hypothesis, it took many years for iron and nickel to accumulate and reach the center
of about 4000 miles deep. During this time, Earth’s surface experienced turmoil, violent
earthquakes, continual volcanic eruptions, and flowing lava covering the surface. Eventually,
iron and nickel accumulated and formed Earth’s core. After cooling down, a thin layer of
solid rock formed the crust, including the continental and ocean basins. In between the core
and the crust is the mantle, which is made up of semi-molten silicate rocks and other
minerals.

Fig. 2. Steps in homogeneous accretion

Fig. 2 shows a summary of the homogeneous accretion hypothesis. First, similar elements
are attached to each other, forming a solid mass. Second, particles were melted due to the
heat produced in the process. Lastly, heavier elements descended to the center due to
gravity, forming the solid core of Earth.

Remember
Condensation refers to the accumulation and attachment of materials to
an object at a time. Accretion, on the other hand, refers to the sticking
together of the huge particles to an object.

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Heterogeneous Accretion Hypothesis

The heterogeneous accretion hypothesis states that the core has formed at the same
time as Earth. Therefore, early Earth had its basic layered structure with a core, mantle, and
crust.

Did You Know?

In 2007, researchers at the University of California-Davis identified how old
the stony materials from the asteroid belt are. As a result, they have
identified that the solar system was fully created 4.568 billion years ago.

According to this theory, as the nebula cooled down, its particles condensed depending on
their condensation points. Oxides of aluminum and calcium condensed first, followed by
iron and nickel. When the nebula cooled further, the silicates condensed. The condensed
particles collided with each other and accreted. The formed particles during the initial stage
of condensation accreted first. Following this, aluminum and calcium oxides accreted first,
then followed by iron and nickel to form Earth’s core. The outermost layer is composed of
silicates, as well as volatile particles, including water.

Fig. 3. Steps in heterogeneous accretion

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Fig. 3 shows a summary of the heterogeneous accretion hypothesis. Particles of metal

attached with each other first, forming Earth’s core. As it cooled further, lighter elements
attached to this core.

Remember
In homogeneous accretion, Earth accreted from particles with the same
composition after condensation. Differentiation happened afterward.

In heterogeneous accretion, Earth accreted during condensation. As it

grew, a differentiated planet was created.

Evidence and Loopholes of the Two Hypotheses

The more commonly accepted postulate is the homogeneous accretion hypothesis.
Most materials that formed early Earth homogeneously accreted after their complete
condensation. After the formation of early Earth, collisions with meteorites and comets
resulted in the presence of volatile elements on the surface.

Earth is considered a dynamic planet. It still continuously changes ever since its formation
4.6 billion years ago. Through time, several changes happened in the geographic
distribution of continents and the composition of the atmosphere.

Table 2.1.1. Difference of Homogenous and Heterogenous Accretion Hypothesis

Homogeneous Accretion Heterogeneous Accretion

Hypothesis Hypothesis

Main Point Earth accreted from materials of Earth accreted during

the same composition after condensation, forming a
condensation. Accretion was differentiated planet as it grew in
followed by differentiation. size.

Supporting The homogeneous accretion The heterogeneous accretion

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Statements hypothesis provides a mechanism hypothesis qualitatively explains

that explains the presence of the density differences among
volatile elements in the core. It terrestrial planets (Mercury,
also provides an explanation of Venus, Earth, and Mars). Also, it
the heat source for early mantle can explain the abundance of
melting and the formation of early elements such as osmium,
continents. iridium, ruthenium, and rhodium
in the mantle.

Loopholes The hypothesis cannot explain the Accretion must be very fast (103
abundance of osmium, iridium, to 104 years for completion).
ruthenium, and rhodium in the This rate does not coincide with
mantle. the occurrence of large impact
craters. Also, the abundances of
iron, calcium, titanium, and
aluminum do not coincide with
what was predicted by the
theory.

Key Points

● Accretion happens when gravity attracts tiny bits of matter towards an object. This
will result in a gradual increase in the object’s size.
● In homogeneous accretion, Earth accreted from particles with the same
composition after condensation. Differentiation happened afterward.
● In heterogeneous accretion, Earth accreted during condensation. As it grew, a
differentiated planet was created.

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Check Your Understanding

A. Arrange the following events in order. Write numbers 1 to 5 where 1 indicates the
first event that occurs, 2 is the second, and so on.

Homogeneous accretion

_______________ Iron and nickel melted, and they sank towards the center because
of their high density.

_______________ Fine particles of the primitive nebula condensed.

_______________ Due to the gravitational contraction and decay of radioactive

elements, the temperature of early Earth increased.

_______________ Less dense silicates were displaced, and they moved upward.

_______________ Particles accreted, forming a homogeneous primordial Earth.

Heterogeneous accretion

_______________ The condensed particles collided with each other.

_______________ Oxides of aluminum and calcium condensed, followed by iron and

nickel.

_______________ The nebula cooled further.

_______________ Silicates condensed.

_______________ Aluminum and calcium oxides accreted, followed by iron and nickel
forming the Earth’s center core.

B. Write true if the statement is correct and false if otherwise.

1. Accumulation and attachment of particles to an object is known as

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condensation.
2. Accretion is a term describing the sticking together of huge particles to an object.
3. According to the homogeneous accretion hypothesis, early Earth had its basic
layered structure.
4. In homogeneous accretion, the early Earth’s temperature increased because of
gravity and the radioactive decay of elements.
5. Elements with lower density sink toward the center of Earth.
6. According to the heterogeneous accretion hypothesis, the core has formed at
the same time as Earth.
7. The outermost layer of Earth is composed of iron and nickel.
8. Earth is considered a dynamic planet.
9. The presence of volatile elements in the core is explained by the homogeneous
accretion hypothesis.
10. As the object increases in size, the gravitational pull decreases.

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Bibliography

Anand, Renu. The Story of Planet Earth. New Delhi: The Energy and Resources Institute,
2016.

Martin, Ronald E. Earth's Evolving Systems: The History of Planet Earth. Massachusetts:
Jones and Bartlett Publishers, 2012.

Pidwirny, Michael. Understanding Physical Geography. 1st ed. University of British Columbia
Okanagan, Chapter 4, 2016.
http://www.physicalgeography.net/understanding/contents.html.

Ravizza, Greg. “Growth and Differentiation of Planet Earth – Formation of the Core and
Moon.” University of Hawaii at Manoa, 2016.
https://www.soest.hawaii.edu/krubin/GG325/lect33.pdf.

Shikazono, Naotatsu. Introduction to Earth and Planetary System Science: New View of
Earth, Planets and Humans. Germany: Springer Science & Business Media, 2012.

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