Chemical Weathering: A Great Natural Force

Updated on February 7, 2017
Will Apse profile image

The author is a scientist with a passion to describe the natural world in easy to understand ways.

Even the awe-inspiring Rocky Mountains will eventually fall to the effects of erosion and chemical weathering.
Even the awe-inspiring Rocky Mountains will eventually fall to the effects of erosion and chemical weathering.

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.
  • Sinkholes.
  • 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

Mountain Building

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.

The rock cycle: how erosion, heat and pressure transform rocks.
The rock cycle: how erosion, heat and pressure transform rocks.

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.

Carbon dioxide dissolves in water to produce an acid that helps to break down calcite.
Carbon dioxide dissolves in water to produce an acid that helps to break down calcite.

Hydrolysis

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.

Example of the hydrolysis of an igneous rock: alkali feldspar.
Example of the hydrolysis of an igneous rock: alkali feldspar.

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.

Quartz crystals
Quartz crystals

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.

Entrance to a  large limestone cave in Malaysia
Entrance to a large limestone cave in Malaysia | Source

Limestone Caves

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.

Source

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

A sinkhole swallows a house near Montreal. A man died during this incident.
A sinkhole swallows a house near Montreal. A man died during this incident.

Sink Holes

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

Metals

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.

The Eiffel Tower. No real rust!
The Eiffel Tower. No real rust!

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.

A weathered, copper-clad dome.
A weathered, copper-clad dome. | Source

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.

'Concrete cancer'
'Concrete cancer'

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'.

Hadrian's arch. Athens
Hadrian's arch. Athens | Source

Marble Buildings

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.

Questions & Answers

    Comments

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      • profile image

        Sully 

        6 years ago

        Chemical weathering;

        Yes, I would say that this is the way Mother Nature is cleansing herself from man’s dirty footprints.

      • profile image

        James 

        7 years ago

        No, I was wondering because I would like to use it in a photoshop contest...

      • Will Apse profile imageAUTHOR

        Will Apse 

        8 years ago

        Thanks suziecat7. Its the kind of picture you should never look at if you live in a limestone area!

      • suziecat7 profile image

        suziecat7 

        8 years ago from Asheville, NC

        Interesting Hub. What an unbelievable sinkhole you have pictured here.

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