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CHEMICAL
WEATHERING
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Done by :
- Majed Abdelrahim Alsmadi / 0188539
- Omar Sami Alkouz / 0206775
- Omar Ahmad Almhareb / 0195206
https://www.youtube.com/watch
?v=uSi6DWbWL1U

Introduction:
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▸ Chemical weathering is a natural process whereby rocks and minerals are
transformed through chemical reactions with various elements in the environment
such as water, air, and organisms. This process occurs as a result of multiple factors
including temperature, pressure, and the presence of acidic or alkaline substances.
▸ The effects of chemical weathering can lead to a range of alterations in rocks and
minerals including the formation of new minerals, the breakdown of minerals into
smaller particles, and changes in their chemical composition.
▸ This natural process has significant impacts on the environment, contributing to the
formation of geological features like canyons, mountain ranges, and caves while also
influencing soil quality and water sources.

Introduction:
3
▸ Chemical weathering is primarily caused by slightly acidic water. As raindrops fall
through the air, they dissolve atmospheric carbon dioxide:
▸ H2O + CO2 S H2CO3
▸ Rainwater thus is a weak solution of carbonic acid (H2CO3)—like Perrier water! When
weakly acidified rainwater becomes soil water or groundwater, it may dissolve
additional carbon dioxide from decaying organic matter, becoming more strongly
acidified. Another way that rainwater can become acidified is by interacting with
anthropogenic (human-generated) sulfur and nitrogen compounds released into the
atmosphere. This produces a phenomenon called acid rain. Human-caused acid rain
is stronger than natural acid rain and causes accelerated weathering.

Introduction:
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▸ Through dissolution, minerals can be completely removed without leaving a residue.
Some common rock-forming minerals, such as calcite (calcium carbonate) and
dolomite (calcium magnesium carbonate), dissolve in slightly acidified water.

Introduction:
5
▸ Water can also alter the mineral content of rock without dissolving all of it. One reaction of
special importance in chemical weathering is ion exchange. Ions, which have a positive or
negative electric charge, exist both in solution and in minerals. They form when atoms give
up or accept electrons . Ions in minerals are tightly bonded and fixed in a crystal lattice, but
ions in solutions can move about randomly and cause chemical reactions. Ion exchange is
important in a common chemical weathering process known as hydrolysis, in which
hydrogen ions (H+) released from acidic water enter and alter a mineral by displacing larger
positively charged ions such as potassium (K+), sodium (Na+), and magnesium (Mg2+)
(Figure 1). This type of weathering alters the composition of both the minerals and the water
solutions that fill the pore spaces and fractures in the rock. Where do the potassium and
other ions go after they are replaced by hydrogenions? Some remain in the groundwater,
accounting for the taste of “mineral water” that some people find pleasant and others do
not, and some flow out to sea and form part of the ocean’s reserve of dissolved salts. On the
other hand, if the water evaporates, the dissolved materials can precipitate out again as
solid evaporites, such as halite and gypsum.

Introduction:
6 Ion exchange . Figer-1

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Chemical weathering is, basically, a
surface phenomenon; therefore,
the greater the surface area exposed,
the more intense the reaction

Surface area :
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▸ The extent and pace of chemical weathering are significantly influenced by the
surface area of rocks and minerals. Generally, the higher the surface area, the
quicker the weathering process occurs. This is due to the fact that chemical
reactions take place at the surface of the rock or mineral. If a rock is broken into
smaller pieces, the total surface area of the rock will increase, which provides more
space for chemical reactions, causing it to weather faster.
▸ The surface area effect is also relevant to many practical applications, such as the
use of limestone in construction or the mining of ore deposits. In these cases, the
amount of exposed surface area can affect the efficiency and effectiveness of
industrial processes that involve chemical reactions.
▸ Additionally, the shape of the rock also affects the weathering rate; rocks with
uneven surfaces, such as those with cracks and crevices, weather more quickly than
those with smoother surfaces. Understanding the surface area effect is crucial in
predicting and comprehending the extent and speed of chemical weathering.

Factor of water in chemical weathering:
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▸ Water is one of the most important factors in chemical weathering. It plays a crucial
role in facilitating chemical reactions that break down rocks and minerals. Water is a
universal solvent, meaning it can dissolve a wide range of substances, and it can
penetrate deep into rocks and minerals to promote chemical reactions.
▸ One of the most common forms of chemical weathering that occurs in the presence
of water is hydrolysis. Hydrolysis occurs when water reacts with minerals in rocks to
break them down into new compounds. For example, feldspar minerals can undergo
hydrolysis in the presence of water to form clay minerals and dissolved ions.
▸ Another way that water can contribute to chemical weathering is through oxidation
reactions. Water is necessary for the oxidation of iron-bearing minerals, such as
pyrite and magnetite, which react with oxygen to form iron oxides.

Factor of water in chemical weathering:
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▸ Water can also increase the rate of chemical weathering by promoting the growth
of microorganisms that aid in the breakdown of rocks and minerals. In some cases,
microorganisms can produce acids that dissolve minerals, leading to increased
weathering rates.
▸ The impact of water on chemical weathering can be seen in many natural
landscapes, such as canyons and river valleys, where water has played a significant
role in shaping the landscape over time. Understanding the role of water in chemical
weathering is crucial for predicting and managing the impact of weathering on
natural environments and human-made structures.

Factor of plants in chemical weathering:
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▸ Plants can also play an important role in chemical weathering, particularly through
their ability to produce organic acids that can dissolve minerals in rocks. These
acids are produced by the roots of plants as they extract nutrients from the soil. The
organic acids can dissolve minerals such as calcium carbonate, which is found in
limestone, and contribute to the weathering of rocks and minerals.
▸ In addition to producing organic acids, plants can also contribute to weathering
through their physical action on rocks. As roots grow and expand, they can exert
pressure on rocks, causing them to crack and break. This process, known as root
pry, can expose more surface area of rocks to chemical weathering agents such as
water and oxygen.
▸ Furthermore, plants can help to create and sustain microenvironments that support
the growth of microorganisms that contribute to chemical weathering. For example,
the roots of plants can create spaces within rocks that provide a suitable habitat for
microorganisms to live and thrive.

Factor of plants in chemical weathering:
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▸ The role of plants in chemical weathering is especially important in areas with
significant vegetation cover, such as forests and grasslands. In these ecosystems,
the combination of organic acid production, root pry, and microorganism activity
can lead to significant weathering of rocks and minerals over time.
▸ In summary, plants can contribute to chemical weathering through the production of
organic acids, root pry, and the creation of microenvironments that support the
growth of microorganisms. Understanding the role of plants in chemical weathering
is important for predicting and managing the impact of weathering on natural
environments.

Effect of climate :
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▸ Climate is an important factor in chemical weathering. High levels of rainfall, warm
and humid temperatures, and the abundance of water can increase the rate of
chemical weathering. In contrast, arid regions may experience more physical
weathering processes.
▸ The type of climate also influences the type of chemical weathering that occurs.
Understanding the impact of climate on weathering processes is crucial for
predicting the long-term effects of weathering on natural environments and human-
made structures.

Effect of climate :
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▸ Climate affects the rate of weathering in two different ways. As
noted earlier, chemical weathering is more intense and extends to
greater depths in a warm, wet, tropical climate than in a cold, dry,
arctic climate. In cold, dry climates, such as in Greenland or
Antarctica, chemical weathering proceeds very slowly. On the
other hand, mechanical weathering is fairly rapid in these harsh
environments. The only environment where both kinds of
weathering proceed very slowly is a hot, dry climate (Figure 2).
-
2

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Classification of Weathering Grade

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▸ Weathering leads to a general disintegration of rock through changes in mineral
composition, increase in void space, and weakening of interparticle bonds . The
degree of disintegration depends on the original composition and texture of the rock,
as well as the processes and rates of weathering, Depending upon the degree of
weathering, the engineering properties of weathered rock can be significantly
different compared to fresh rock. Therefore, the engineering description of a rock
must include a description of its degree or grade of weathering in both qualitative and
quantitative terms.

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▸ Based on visual observations, the Geological Society of London (1995) proposed six grades of weathering
(Table-1) :
Table-1

In conclusion, chemical weathering is an important geological process that occurs over long
periods of time, resulting in the breakdown of rocks and minerals. Factors such as surface
area, water, plants, and climate all play crucial roles in promoting chemical weathering.
Understanding these factors and their impacts is important for predicting and managing the
effects of weathering on natural environments and human-made structures. Overall,
chemical weathering is a complex and dynamic process that contributes to the shaping of
the Earth's surface over time.
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References:
- Visualizing Geology by Barbara W. Murck (Book)
- https://www.accessscience.com/content/article/a743100 (Article in site)
- Geology: A Very Short Introduction by Jan Zalasiewicz (Book)
- PRINCIPLES OF GEOLOGY BY CHARLES · LYELL, ESQ. F. R. S (Book)
- https://education.nationalgeographic.org/resource/weathering/ (Article in site)

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CHEMICAL WEATHERING

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