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 Earthmdash 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

Chemical compounds released into the atmosphere from factories can contribute to 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 come from the air whereas others are in water Water itself is a molecule that can cause chemical 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 ldquoacid rainrdquo 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 Earthrsquos 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 Earthrsquos 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 wwwdiscoveryeducationcom

1 copy Discovery Education All rights reserved

Discovery Education is a subsidiary of Discovery Communications LLC

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 Earthrsquos 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

Water carrying dissolved minerals drips through limestone rocksThe dissolved limestone is deposited on the ceiling of a cave as a stalactite

Concept Chemical Weathering Getting to Know wwwdiscoveryeducationcom

2 copy Discovery Education All rights reserved

Discovery Education is a subsidiary of Discovery Communications LLC