The two prominent components of granite are quartz and feldspar. The feldspar in the granite reacts with rain to cause weathering, although this is usually a very slow process. The two air pollutants that create acid rain are sulphur dioxide and nitrogen oxide, and when the rain is acidic, a chemical weathering occurs more quickly in granite. The dissolution of the granite is extremely slow since granite is one of the hardest rocks on Earth; but granite does show the effects of acid rain over time.
What Is Acid Rain?
Acid rain is man-made. The amount of acid in a substance is measured on a pH scale. A low pH number means high acidity. The opposite measure of acid is alkalinity. Lye has a pH level of 13 -- a high alkaline content. Pure water's pH is 7 and normal rainfall has a pH of 6. Acid rain ranges from 4 to 5.5 on a pH scale, meaning sulphur dioxide has permeated the rain and caused it to become acidic. Car emissions and the burning of fossil fuels cause most acid rain.
What Is the Effect of Acid Rain on Stone?
The once towering Appalachian Mountains are now reduced to pleasant hills because of acid rain. This transition took millions of years but was expedited by the man-made acidity in the rain. Marble statues show the effects of acid rain; while marble has strong composition, it yields to the acid in acid rain nonetheless. Limestone deteriorates even faster than marble. Granite, the hardest of the building and statuary stones, is now being affected by acid rain. Oporto, Portugal, which is almost entirely built on grey granite, is showing signs of deterioration because of acid rain. Gravestones made of granite are also beginning to show effects of acid rain.
The Power of Granite
While acid rain reduces marble, limestone and cement, granite stands strong in spite of the pockets of deterioration seen in Portugal, Brazil and graveyards. Granite's defence is its combination of feldspar and quartz, which formed the granite when it was in its liquid volcanic state millions of years ago. The liquid crystals mixed together instead of forming layers. It is layering that weakens the composition of stone. Granite is one of the strongest stones on earth and therefore is difficult to penetrate, even by acid rain.
Is It Safe to Build With Granite?
Granite's weathering by acid rain is a slow process. In Brazil, where granite buildings are showing effects from acid rain, the buildings are built tightly around each other in the centre of a high-traffic area. The destructive thin black crust created by exhaust, salt air, the lack of sun reaching the facades and industrialisation is slowly causing the granite underneath to weather faster than in normal conditions. This is a rare case. Buildings made with granite are still the most solid in the industry.
- Environment Canada; Acid Rain and the Facts; December 8, 2000
- Annals of the Brazilian Academy of Sciences; Surface Modification of a Granite Building Stone in Central Rio de Janeiro; Batisto-Neto, Smith, et al.; July 22, 2004
- BBC/KS3 Bitesize: The Rock Cycle: Weathering
- Internet Archive; Granite Deterioration in the Graveyard of Saint James the Less, Philadelphia; Kathryn Marit Sather; 1990
- University of Minho; Granite Weathering and Stone Deterioration in Monuments and Buildings in Oporto: Arlindo Begonha; 1997