Earth and Life Science

Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

LESSON 5.1
Weathering
Table of Contents

Introduction 1

Learning Competency 2

Learning Objectives 2

Warm-Up 2

Learn about It 3
Weathering 3
Mechanical Weathering 3
Frost Wedging 4
Heating and Cooling 5
Salt Crystal Growth 5
Biological Activity 6
Unloading 7
Chemical Weathering 8
Dissolution 9
Hydrolysis 10
Oxidation 10
Factors of Weathering of Rocks 11
Climate 11
Surface Area and Topographic Relief 11
Rock Composition 12

Key Points 13

Check Your Understanding 14

Photo Credits 15

Bibliography 16
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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

Lesson 5.1
Weathering

The Puerto Princesa Subterranean River is considered one of the world’s longest
underground rivers that outflows directly to the sea.

Introduction
Have you ever been to Puerto Princesa in Palawan? If not, how about knowing this place
from television or from travel vloggers? This city in Southern Luzon is not only known for its
majestic coastlines and captivating beauty, but also for the Puerto Princesa Subterranean
River. The underground river system is a popular tourist spot that boasts of wonderful
sights such as small waterfalls, cave domes, and rock formations. The stunning rock
formations were formed through a geologic process called weathering. Rocks exposed at

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

Earth’s surface undergo weathering. What are the types of weathering? What are the
factors that contribute to weathering of rocks?

Learning Competency
At the end of this lesson, the given DepEd learning competency should be met
by the students.
Explain how the products of weathering are carried away by erosion and
deposited elsewhere (S11/12ES-Ib-12).

Learning Objectives
In this lesson, you should be able to do the following:
● Describe how rocks undergo weathering.
● Differentiate between mechanical weathering and chemical weathering.
● Enumerate the factors that contribute to weathering.

Warm-Up

Weathering by Water

Materials
● glass jar with cap
● small pieces of shale (or any rock that breaks easily)
● water

Procedure
1. Observe the appearance and texture of the pieces of shale.

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

2. Place the pieces of shale inside the glass jar.

3. Pour water into the jar until it is half full. Screw the cap of the jar tightly.
4. Shake the jar vigorously for about 1-2 minutes.
5. Remove the pieces of shale from the jar, leaving the water behind.
6. Answer the guide questions below.

Guide Questions
1. How would you describe the appearance of the pieces of shale afterwards?
2. After removing the pieces of shale, how would you describe the materials left in the
jar?
3. How does water affect the appearance of rocks?

Learn about It

Weathering
Most caves form due to limestone being dissolved. Limestone is a sedimentary rock
composed primarily of calcium carbonate (CaCO3). When limestone embedded in hard
rocks is washed away by water from rain or rivers, spaces within the hard rocks are left, thus
creating a cave. This process is an example of weathering. There are two types of
weathering: mechanical weathering and chemical weathering.

Essential Question
What is the difference between mechanical and chemical weathering?

Mechanical Weathering
Mechanical weathering is a process whereby rocks are broken down into smaller pieces
without changing their chemical composition due to several factors like fluctuating
temperatures and pressure, and biological activity.

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

Fig. 1. Rocks by the sea change shape due to the force of water during high tide

Frost Wedging
This process involves repeated cycles of freezing and thawing of ice. One of the accepted
explanations about this process is the expansion of water as it freezes. Ice lenses start to
form in the cracks as more liquid water is attracted to them from the surrounding pores.
The expansion force of water slowly weakens the rock and eventually causes it to break.

Fig. 2. Weathering of rocks through frost wedging

Frost wedging happens under these three conditions: (1) when there is sufficient moisture;
(2) when there are pre-existing cracks or pore spaces and fractures by which water can

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

penetrate; and (3) when temperature frequently rises and falls past above or below the
freezing point.

Heating and Cooling

Rocks are composed of different minerals which expand and contract when subjected to
sudden changes in temperature. The constant cycle of expansion (due to heating) and
contraction (due to cooling) eventually causes rocks to fracture and break down into
smaller pieces. Rocks in highways develop cracks and small fractures because of too much
exposure to heat.

Fig. 3. Rocks exposed to repeating heating and cooling

Salt Crystal Growth

This process happens when seawater penetrates crevices in rocks which are found
mostly in rocky shorelines and arid regions. Salt crystals are left in pore spaces of the rocks
when seawater evaporates. These salt crystals continue to grow larger as more saline water

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

enters and evaporates. In addition to that, salt crystals also expand when subjected to an
increase in temperature that causes the widening of cracks and eventually breaking rocks.

Fig. 4. Salt crystal growth in rocks

Biological Activity
The action of organisms including plants and animals reduces the size of rocks and
minerals which make rocks more susceptible to chemical weathering. Plant roots, for
example, cause rocks to wedge as it penetrates the cracks in search of water and nutrients.
Burrowing animals help in breaking the rocks by moving fresh materials onto the surface
where different processes can easily degrade them. It may seem trivial at first but these
activities that happen over an extended period contribute significantly to the disintegration
of rocks.

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

Fig. 5. Plant roots wedging a rock

Unloading
Through erosion or uplift, thick layers of sediments overlying deeply buried in rocks are
removed. This process is termed unloading. When rocks are exposed due to unloading, the
stress caused by the overlying material is lowered causing it to expand. Slabs of rocks began
to separate as weathering continued creating exfoliation domes (onion-like layers). The
same process happens in mines and tunnels where rocks suddenly burst due to released
confining pressure during tunneling operations.

Tips
Use the mnemonics Frost BUSH to remember the types of mechanical
weathering: Frost wedging, Biological activity, Unloading, Salt crystal
growth, and Heating and cooling.

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

Fig. 6. Formation of exfoliation dome due to unloading

Chemical Weathering
Chemical weathering is a process wherein rock materials are changed into other
substances that have different physical and chemical compositions. Some agents of
chemical weathering include water, strong acids, and oxygen.

Fig. 7. Coffee-maker analogy of chemical weathering

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

Dissolution
Dissolution is a process wherein a solid (i.e., halite) dissolves in a liquid (i.e., water). Small
proportions of water molecules break apart to form an equal number of hydrogen ions (H+)
and hydroxide ions (OH-). Some minerals are quickly dissolved in water, and the ions are
simply flushed away or leached. Dissolution happens because of the solvent properties of
water. It is the most common type of chemical weathering and significantly contributes to
areas such as karst topography and sinkholes.

A landscape formed by the dissolution of carbonate rocks such as limestone and dolomite is
called karst topography. The presence of caves and sinkholes usually characterizes this
landscape. Sinkholes are produced by several processes, most commonly when a cave
grows large and close enough to the surface and later on collapses.

Fig. 8. The karst topography of Kami-shima island, Japan is a famous tourist attraction.

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

Hydrolysis
In this process, water reacts with a mineral to form a new mineral. Small proportions of
water molecules break apart to create an equal number of hydrogen ions (H+) and
hydroxide ions (OH-). This process happens when water is incorporated into the original
mineral’s crystal structure. During hydrolysis, the water interacts with the original mineral
and splits its weak bonds. It creates softer and more soluble material than the original.

Many minerals weather through hydrolysis, feldspar for example, which disintegrates to
clay. During the weathering of feldspar, water carries off soluble cations like potassium and
combines with less soluble ions ultimately forming clay minerals.

Oxidation
It is a chemical combination of oxygen with a mineral to form an entirely different mineral
in which at least one of the elements has a higher ionic charge. Iron is an essential element
in chemical weathering, abundant in mafic, and ultramafic minerals. Minerals such as
olivine, pyroxene, and amphibole decompose once their iron component oxidizes. Oxidation
also gives the red color for desert landscapes.

Fig. 9. The Red Rock Canyon in Las Vegas

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

Essential Question
What are the factors that contribute to weathering of rocks? How can these
factors influence the rate of weathering?

Factors of Weathering of Rocks

Weathering of rocks is influenced by numerous factors such as climate in the area,
surface area and topographic relief, mineral composition, and texture of rocks.

Climate
It mainly includes the amount of moisture in the air and temperature where weathering
takes place. Weathering occurs faster in hot, wet climates while slower in very cold, dry
areas. One example of the effect of climate on the rate of weathering is the frequency of
freeze-thaw cycles, which causes frost wedging to happen more rapidly.

Surface Area and Topographic Relief

Weathering rapidly occurs when there is a large surface area exposed to surface
processes. When a block is broken into smaller pieces, it has a larger total surface area
which makes it more susceptible to weathering. The topography of the area also affects the
rate of weathering by influencing the amount of rock exposed to weathering. The higher
the elevation of an area, the more susceptible it is to weathering since it is more
exposed to the wind, sudden changes in temperature, and other environmental factors
like gravity.

Fig. 10. As rocks break into smaller pieces, surface area increases

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

Rock Composition
This includes all the chemical traits of rock such as mineral composition and solubility.
Variation in the rates of weathering happens due to the resistance of some minerals to
disintegration. Quartz, for example, weathers slowly compared to feldspar, calcite, and iron.

Goldich stability series illustrates the relative stability or weathering rate of numerous
minerals. This series is roughly the inverse order of original crystallization for minerals
found in Bowen’s reaction series. Olivine, for example, is the first to crystallize and therefore
is the least stable— that is, it weathers quickly. On the other hand, quartz is the last to
crystallize making it more resistant to weathering due to its stability.

Fig. 11. Goldich Stability Series

Did You Know?

The Goldich stability series is developed by a geologist named Samuel
Stephen Goldich. He discovered that minerals formed at higher
temperatures and pressures are less stable on the surface.

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

Key Points

● Weathering is the breaking down of rocks either mechanically or chemically.

● Mechanical weathering is the process where rocks physically break down into
smaller pieces.
○ Mechanical weathering occurs due to several factors such as fluctuating
temperatures and pressure and biological activity. The process that involves
the repeated cycles of freezing and thawing of ice is called frost wedging.
○ When different minerals in rocks expand and contract due to sudden changes
in the temperature, heating and cooling occur.
○ When seawater penetrates crevices in rocks and evaporates, salt crystals
tend to grow. Biological activity is the process wherein activities of
organisms such as plants and animals may break rocks apart.
○ Unloading is the process in which thick layers of sediments overlying deeply
buried rocks are removed through erosion or uplift.
● Chemical weathering is the process wherein rocks break down due to chemical
reactions.
○ Dissolution occurs when a solid rock or mineral dissolves in a liquid such as
water. The process wherein water reacts with a mineral to form a new mineral
is called hydrolysis.
○ Oxidation is a chemical combination of oxygen with a mineral to form a
completely different mineral. At least one of these elements has a higher ionic
charge.

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Grade 11/12 • Unit 5: Geologic Processes on Earth's Surface

Check Your Understanding

A. Identify the term being described in each item.
1. It is a process that involves repeated cycles of ice freezing and thawing.
2. It is the constant expansion and contraction of rocks which eventually causes
rocks to fracture and break down into smaller pieces.
3. It refers to the process wherein a solid rock or mineral dissolves in water.
4. It is the combination of oxygen with a mineral to form a completely different
mineral.
5. It is the process in which thick layers of sediments overlying deeply buried rocks
are removed through erosion or uplift.
6. It is when activities of organisms physically reduce the size of rocks and
minerals.
7. It refers to the method of predicting the relative stability or weathering rate of
various minerals.
8. It is the process wherein water reacts with a mineral to form a new mineral.
9. It is a process of breaking down rocks into smaller pieces through physical
means.
10. It refers to the breaking down of rocks due to chemical reactions.

B. Write true if the statement is correct, and false if the statement is incorrect.
1. Frost wedging is an example of mechanical weathering.
2. Karst topography is a landscape formed by the dissolution of carbonate rocks.
3. Weathering slowly occurs when there is a large surface area exposed to surface
processes.
4. Oxidation is a process wherein water reacts with a mineral to form a new
mineral.
5. Dissolution refers to a process where erosion removes the thick layers of
sediments overlying deeply buried rocks.
6. Water, strong acids, and oxygen are examples of agents of chemical
weathering.

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7. Weathering occurs faster in very cold, dry places.

8. Rocks are composed of different minerals which expand and contract when
subjected to changes in temperature.
9. The relative stability of various minerals is illustrated in the Goldich stability
series.
10. A mineral that is first to crystallize weathers the quickest.

Photo Credits
Introduction Puerto Princesa Underground River by Glennjarz is licensed under CC-BY-SA
4.0 via Wikimedia Commons.

Figure 3 Weathering freeze thaw action iceland by Till Niermann is licensed under CC-BY-SA
3.0 migrated via Wikimedia Commons.

Figure 4 Tafoni 03 by Dawn Endico is licensed under CC-BY-SA 2.0 via Wikimedia Commons.

Figure 5 High rocks on top tree roots 4126 by Clem Rutter is licensed under CC-BY-SA 3.0 via
Wikimedia Commons.

Figure 6 Granite exfoliation- Flickr- S. Rae by S. Rae is licensed under CC-BY-SA 2.0 via
Wikimedia Commons.

Fig. 8 Kami-shima Island- Karst topography by Yanajin33 is licensed under CC-BY-SA 4.0 via
Wikimedia Commons.

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Bibliography

Carlson, Diane H. and Charles C. Plummer. 2008. Physical Geology: Earth Revealed. New York:
McGraw-Hill Companies, Inc.

Coenraads, Robert R. 2005. Rocks & Fossils: A Visual Guide. Australia: Weldon Owen Inc.

Hamblin, W. Kenneth and Eric H. Christiansen. 2003. Earth’s Dynamic Systems, Tenth Edition.
New Jersey: Pearson Prentice Hall

Hewitt, Paul, Suchocki, J. and Hewitt, L. Conceptual Physical Science Fifth Edition. Pearson
Education, Inc.

Holt, Rinehart, and Winston. Earth Science: Interactive Textbook.Harcourt Education Company

Stephen J. Reynolds, et al. 2013. Exploring Geology, 3rd Edition. United States: McGraw-Hill
Companies, Inc.

Tarbuck, Edward J. and Frederick J. Lutgens. 2015. Earth Science. New Jersey: Pearson
Prentice Hall.

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