Chemical Weathering: a Great Natural Force
81
Landscapes, especially dramatic mountain landscapes can seen unchanging. The huge bulk of rock that constitutes the Rocky mountains, for instance, seems destined to remain forever. Yet there are powerful forces at work that will cause these mountains- and every other- to gradually shrink to nothing.
Wind, rain and the effects of water are constantly eroding material from every surface that is exposed to them. 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, Rust is one effect of chemical weathering everyone is familiar with. The patinas you find on copper and aluminium are other examples of chemical weathering.
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
Rocks exposed to the air are subject to the gases in the atmosphere and to water.
The most reactive gas in the atmosphere in any abundance is oxygen. Oxygen eats into metals especially vigorously.
Carbon dioxide is not reactive but when it dissolves in water it produces a weak acid which, over time, will dissolve many kinds of rock especially limestones.
Water will also attack and break down igneous rocks like granite- as explained in the video below.
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 the water reacts with minerals in the granular rocks and undermines the rocks structure.
Soil Formation as a Result of Erosion and Chemical Weathering
Soils contain many things 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 silicate rich igneous rocks by low concentrations of carbonic acid and other diluted solvents results in the formation of clays.
http://soils.usda.gov/education/facts/formation.html
Organic constituents of soil like peat or humus come from biological processes.
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, Sometimes the land raised has sedimentary rocks, like limestone, as a layer. Sometimes it is just igneous rock from a magma outflow that is pushed skyward.
Other Examples of Chemical Weathering
Chemical weathering almost never happens in isolation- the forces of erosion are also involved. Some examples of large scale changes brought about predominantly by chemical weathering are illustrated below.
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, The rushing waters of underground streams cause erosion. Spectacular cave systems can result.
Limestone Caves
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.
In the US, Florida is notorious for sinkholes as is Wisconsin.
Sink hole formation in Wisconsin: geology/geo101/weather.htm
SInk holes can form in Sandstone Areas too.
Chemical Weathering of Man Made Materials
Metals
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 atmoshpere. Some metals like copper and aluminium develop a thin protective patina of oxidised 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. Cast iron is also notable for its very slow rate of corrosion.
Cements and Concretes
Any material made largely from calcites 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.
Marble
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,
Erosion and the Different kinds of Weathering
Wikipedia article on erosion: http://en.wikipedia.org/wiki/Erosion
This page has a good overview of the various processes that break down geological material: http://facstaff.gpc.edu/~pgore/geology/geo101/weather.htm
How can structures be protected from weathering? weathering+and+conservation
CommentsLoading...
Thanks suziecat7. Its the kind of picture you should never look at if you live in a limestone area!
Do you have a copyright on your lime stone photo?
Is that a rhetorical question, James?
No, I was wondering because I would like to use it in a photoshop contest...
I almost always use US government pictures because they are copyright free but I can't find the attribution. I would check out this page for fantastic pictures of caves of all kinds:
http://www.darkroastedblend.com/2008/10/caves-worl
All the pictures are properly attributed.
Chemical weathering;
Yes, I would say that this is the way Mother Nature is cleansing herself from man’s dirty footprints.
suziecat7 Level 5 Commenter 21 months ago
Interesting Hub. What an unbelievable sinkhole you have pictured here.