Download Getting to Know: Chemical Weathering

Getting to Know: Chemical Weathering
Have you ever taken the opportunity to observe a stone statue in a park or other public area? It is quite likely that the coloring of the statue has changed significantly over time as a result of chemicals in the air. The air you breathe contains a multitude of chemical elements and compounds. Chemical reactions occur when these elements and compounds combine to form new substances. Sometimes, these chemical reactions are responsible for changing the landscape of Earth—
including the coloring of local landmarks.
Chemical weathering occurs when chemical reactions cause permanent changes to rocks and other physical features. Rocks are made primarily of minerals, and the chemical composition of some minerals makes them prone to chemical changes. Chemicals in the air, in water, and from living things can cause chemical weathering.
How do air, water, and organisms
contribute to chemical weathering?
Atoms can bond together to form molecules and
compounds. Some of these chemical compounds
Chemical compounds released into
come from the air, whereas others are in water.
the atmosphere from factories can
Water itself is a molecule that can cause chemical
contribute to chemical weathering.
weathering. For example, iron-bearing rocks
exposed to air and water will rust. Rusting causes a
permanent change to the iron compounds in the rock. Some of the sources of the chemicals
in the atmosphere and in water are natural. However, human activities have added other
pollutants like in the case of acid rain. Factories, automobiles, and other human-made systems
pump chemical compounds such as sulfur dioxide and nitrogen oxides into the environment.
When these chemical compounds combine with precipitation in the atmosphere, the result is
rainwater with an increase in acidity known as “acid rain.” When acid rain falls on buildings and
monuments made of limestone or marble, chemical weathering takes place.
Living organisms also produce chemicals that react with minerals on Earth’s surface. Lichens
are plants and funguses that grow on rocks. Chemicals released from the lichen can slowly
dissolve away rock material through chemical reactions. Regardless of the source, chemical
weathering is not reversible.
How is chemical weathering different from mechanical weathering?
Chemical weathering involves a chemical change to a substance on an atomic level.
Mechanical weathering is the physical breakdown of rock material. Rocks break apart when
water seeps into cracks and freezes. The rock is physically broken into two or more parts, but
the type of rock is still the same. Erosion occurs when any broken or dissolved rock material
moves to a new location. Water, wind, animals, and gravity all cause erosion. Chemical and
mechanical weathering can work in tandem with erosion to reshape Earth’s surface. For
example, a large rock broken down by the mechanical motion of the wind becomes many
smaller rocks. The smaller rocks are more susceptible to chemical weathering.
Concept: Chemical Weathering
Getting to Know
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Misconception 1: Mechanical weathering and chemical weathering work
independently.
The two processes actually work together. Mechanical weathering can speed up the rate
that chemical weathering happens. As mechanical weathering breaks down a rock, more
surface area is available on which chemical reactions may occur.
Do all rocks weather equally?
Some rocks combine more readily with chemicals than others. For example, limestone
dissolves easily, but granite does not. Iron rusts easily, but copper does not. In addition,
changes in climate can speed the process of weathering. Increases in temperature cause
increases in the chemical reaction rate of substances. Glacial melting also leads to changes in
water distribution over Earth’s surface. As some areas are exposed to more water, there may
be an increase in the rate of chemical and mechanical weathering.
Misconception 2: Chemical weathering only happens on a microscopic scale.
Chemical weathering also causes large-scale changes to rocks. When combined with
physical weathering, chemical weathering can wear large mountains down into hills
over long periods of time.
Does chemical weathering have any benefits?
Chemical changes to rocks help create new minerals. Some rocks dissolve in water, creating clays that are useful in art and building projects. Beautiful caves form when water seeps through the spaces in limestone rock formations. The water dissolves some of the minerals in the rock and, over millions of years, enlarges a tiny crack to form a large cavern. Chemical weathering also helps break down rocks into small pieces to form soil, which is essential for plant growth.
The compounds that cause chemical
weathering come from both natural and
human-made sources. In this lesson, find
out how chemical weathering has shaped
some of the most interesting features on
Earth.
Concept: Chemical Weathering
Getting to Know
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Water carrying dissolved minerals drips through
limestone rocks.The dissolved limestone is
deposited on the ceiling of a cave as a stalactite.
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© Discovery Education. All rights reserved.
Discovery Education is a subsidiary
of Discovery Communications, LLC.