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.
Some of the results of chemical weathering dealt with on this page include:
- Vast underground cave systems.
- Stalactites and stalagmites.
- The rusting of steel and iron structures.
- Patinas on copper clad buildings.
- The impact of acid rain.
- Concrete 'cancer'.
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.
This page explores the two processes as distinct but closely entwined phenomena.
Chemical Weathering of Natural Materials
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.
In areas where magma reaches the surface and cools, igneous rocks like granite and basalt form. Sometimes the land that is raised during these upheavals has sedimentary rocks, like limestone, as a layer.
At the top of Mount Everest, for example you will find limestone that formed beneath an ancient sea, complete with fossils.
The Rock Cycle
Even as mountains are rising they are subjected to chemical weathering and erosion. The rock cycle below illustrates some of the endless interactions.
Some of The Chemical Reactions Involved
Atmospheric gasses and water have the biggest impact when rocks and man-made materials are weathered.
The Role of Carbon Dioxide and Water
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 calcite.
Igneous rocks like granite and basalt are especailly 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.
The Importance of Quartz
Of all the igneous rocks, only quartz is immune to chemical attack by water and atmospheric gasses. When quartz is eroded by physical forces like wind and waves, the result is sand, a very durable material often used in building construction.
Soil Formation as a Result of Erosion and Chemical Weathering
Soils contain many materials which come from the breakdown of rocks:
- When quartz is eroded by wind, or other physical processes, sand is formed.
- The chemical weathering of igneous rocks results in the formation of clay.
The only other significant non-living components of soil are organic constituents, like humus or peat. These 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 acidified by dissolved carbon dioxide (see the chemical equations above). The rushing waters of underground streams cause erosion adding to the speed of the process. 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.
Sandstone Can Be Affected by Chemical Weathering, Too
Although sandstone is predominantly made of chemical-resistant quartz grains, the 'cement' that holds the grains together can be vulnerable to chemical attack. Many sandstone rocks are mixed with feldspar that can be subject to hydrolysis, as described above.
The video below explores the formation of a sandstone sinkhole in Guatemala.
Chemical Weathering of Man-made Structures
Everyone is familiar with the result of the 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.
Only the 'noble' metals are immune to chemical weathering. These include ruthenium, rhodium palladium, silver, osmium, iridium, platinum and gold.
Although most kinds of iron and steel will rust quickly, some kinds of steel like stainless steel are highly resistant to chemical weathering. Cast iron is also resistant to corrosion.
Why Does the Eiffel Tower Not 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.
Verdigris and Other Patinas
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 sea spray, containing sodium chloride.
Cement 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 increased 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 and facilitates the reaction.
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.
You can find other important buildings that are under threat here: endangered-heritage-sites.