Chemical Weathering: a Great Natural Force
Landscapes, especially dramatic mountain landscapes, can seem unchanging. The huge bulk of rock that constitutes the Rocky Mountains, for example, seems destined to remain forever. Yet there are powerful forces at work that will cause these mountains to gradually disappear.
Wind, rain and water are constantly eroding material from every exposed surface. To add to the forces of erosion, are the effects of chemical weathering.
Chemical weathering is sometimes simply referred to as corrosion. It affects man-made structures and materials as well as natural or geological material. Rusting is one example of chemical weathering that everyone is familiar with. The patinas that you find on copper and aluminum are other examples of chemical weathering.
Pictured above is the copper dome of St. Augustine's Seminary, Toronto. The beautiful, green verdigris coating is mostly copper carbonate (from carbon dioxide in the air).
Sometimes, near the sea, the verdigris will be copper chloride as a result of salt-laden sea spray.
Is Chemical Weathering one of the Forces of Erosion or is it Distinct?
Some authorities include chemical weathering as one of the many forces involved in erosion. Others say chemical weathering is a distinct process because it does not involve transportation of material as happens with wind, river or glacial erosion, for example.
Chemical Weathering of Natural Materials
Mountain Building and Disperal
Land rises to form mountains when there is pressure from molten rock in the earth's core, seeping upwards. The biggest mountain ranges are found in places where tectonic plates meet.
Sometimes the land raised has sedimentary rocks, like limestone, as a layer. In areas where magma reaches the surface and cools, igneous rocks like granite and basalt form.
All rock formations are subject to erosion and weathering, even the largest mountains. Weathered and eroded materials from mountains are carried across the surface of our planet by wind and water. This material forms soils and gives rise to new sedimentary rocks.
Atmospheric gasses and water have the biggest impact when rocks and man made materials are weathered.
The most reactive gas in the atmosphere, in any abundance, is oxygen. Oxygen eats into metals especially vigorously causing rust.
Carbon dioxide is not an especially reactive gas, but when it dissolves in water it produces a weak acid which, over time, will dissolve many kinds of rock especially limestone.
Water will also attack and break down igneous rocks, like granite, over a long period of time.
Igneous rocks like granite and basalt are hard to cut and carve. They can seem indestructible, but water can attack even the hardest granite until it is easy to crush in your hand.
The main process involved is hydrolysis. Hydrogen from water reacts with minerals in the rocks and undermines the rock's structure.
Soil Formation as a Result of Erosion and Chemical Weathering
Soils contain many materials which come from the breakdown of rocks.
When sedimentary rocks are eroded by wind, or other physical processes, sand is often formed.
The chemical weathering of igneous rocks results in the formation of clays.
Organic constituents of soil, like peat or humus, are the result of biological processes.
Examples of Chemical Weathering
Chemical weathering almost never happens in isolation. The forces of physical erosion like wind or the effects of freezing and heating, are also involved.
Some examples of large-scale changes brought about predominantly by chemical weathering are illustrated below.
Caves are often formed by the action of water on limestone rocks.
Most limestone rocks form in seas and oceans. When marine life dies, the calcium rich shells of creatures like diatoms and crustaceans settle on the sea bed and are compacted over time to form limestone.
The calcites in limestone dissolve in rainwater. The rushing waters of underground streams cause erosion. Spectacular cave systems can result.
Stalactites and Stalagmites
Stalactites and stalagmites are formed by chemical weathering. Water dissolves the calcites in the rock of a cave roof and the calcite is deposited as strange and wonderful structures below.
Pictured above, are stalactites in Gosu Cave, Korea
Sink holes are most commonly formed when an underground cavern collapses. They are most widespread in areas where the underlying rocks are carbonates like limestone. Water erodes and dissolves the softer rocks, carrying them away. The rocks above may then collapse, sometimes with catastrophic consequences.
In the US, Florida is notorious for sinkholes as is Wisconsin.
Sink holes can form in Sandstone Areas, too.
As you can see in the video below.
Chemical Weathering of Man-made Structures
Everyone is familiar with the result of chemical weathering of steel. Rust is the great enemy of cars and many other important machines and structures in our lives..
The majority of pure metals will react with oxygen and water in the atmosphere. Some metals like copper and aluminium develop a thin protective patina of oxidized material as they weather. The patina will protect the metal from further corrosion by blocking the path of atmospheric gases.
Although most kinds of iron and steel will rust quickly, some kinds of steel like stainless steel are highly resistant to chemical weathering.
Why Doesn't the Eiffel Tower Rust?
The Eiffel Tower is made of cast iron. The high carbon content of cast iron makes it highly resistant to rusting. The Eiffel tower should last for many centuries.
Cements and Concretes
Any material made largely from calcite, like the cement in concrete, will dissolve slowly in rainwater. Acid rain of the kind found in polluted industrial areas and cities can eat into concrete even more quickly and is an example of chemical weathering that human activity influences.
Where concrete structures rely on steel reinforcement, the process of decay is speeded by rusting.
Concrete can weaken and collapse as a result of these kinds of chemical weathering.
An additional process, is the reaction between the silicates in sand and the alkali in cement as water penetrates the concrete.
Damage of the kind seen in the picture above is called spalling by engineers or, sometimes, 'concrete cancer'.
Marble statues and facades are susceptible to acid rain too. The Acropolis in Athens is one irreplaceable building that has been damaged by rainwater acidified by pollution from car exhausts and industry.
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