Chemistry and the Great Sphinx

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The Great Sphinx
The Great Sphinx

Contents

History of the Great Sphinx

'The Great Sphinx is located in Giza, Egypt. The Great Sphinx has the body of a lion and the head of a man and is believed by some to have been built for King Kafra, though there is no certain documentation of this nor is there documentation of exactly when the Great Sphinx was built. There are also three tunnels inside the Great Sphinx, however, no one knows what they were for because they are inaccessible.' (2)

Composition

Limestone which is CaCO3 (1).

Size

65 feet high, 260 feet long, and 20 feet wide (2).
That's 1.37 school buses high if they were standing straight up, one on top of another and 6.5 school buses long if they were lined up front to back.

Age

Though there is dispute on this, the currently generally accepted time when the Great Sphinx was built was approximately 2600 - 2400 BCE which is about 4,410-4,610 years ago (1). Hard to imagine that we would have trouble creating something today that would last that long, isn't it?

Chemistry

While the Great Sphinx is amazing, what is even more amazing is the fact that it is standing after 4,000 years or more. Of course, that doesn't mean that damage hasn't been done and that damage is what we are here to look at: the effects of chemistry on the Great Sphinx. Gauri believed that the main reasons for erosion on the Great Sphinx were weathering and exfoliation (4), we will be looking into these two main categories as we go along.

Limestone

"Limestone is by definition a rock that contains at least 50% calcium carbonate in the form of calcite by weight. All limestones contain at least a few percent other materials. These can be small particles of quartz, feldspar, clay minerals, pyrite, siderite and other minerals. It can also contain large nodules of chert, pyrite or siderite.
The calcium carbonate content of limestone gives it a property that is often used in rock identification - it effervesces in contact with a cold solution of 5% hydrochloric acid." (3)
The chemical composition of limestone is CaCO3 which is calcium, carbon, and oxygen which in this combination is Calcium Carbonate.
This is the chemical structure of Calcium Carbonate. CaCO3.png
This is the crystal structure of Calcium Carbonate. CaCO3crystal.png

Great Sphinx Limestone

The limestone of the Great Sphinx can be divided up into three categories, relatively the: top, middle, and bottom (4). The bottom being the least durable and the top being the most durable with most of the degradation research focusing on the middle section or middle stratum(4).

Chemical Weathering

Definition: "Chemical weathering (also known as decomposition or decay) is the breakdown of rock (weathering) by chemical mechanisms, the most important ones being carbonation, hydration, hydrolysis, oxidation, and ion exchange in solution.
Chemical weathering changes the composition of the rock material toward surface minerals, such as clays. It attacks minerals that are relatively unstable in surface conditions, such as the primary minerals of igneous rocks like basalt, granite or peridotite.
Water is especially effective at introducing chemically active agents by way of fractures and causing rocks to crumble piecemeal or by loosening thin shells of material (in spheroidal weathering). Chemical weathering may include shallow, low-temperature alteration." (5)
The main form of weathering that we will be looking at is acidic rain in the case of the Great Sphinx.

Acidic Rain

Definition: "rain that contains a high concentration of pollutants, chiefly sulphur dioxide and nitrogen oxide, released into the atmosphere by the burning of fossil fuels such as coal or oil." (13)
"A chemical reaction (Equation 9) between calcium carbonate and sulfuric acid (the primary acid component of acid rain) results in the dissolution of CaCO3 to give aqueous ions, which in turn are washed away in the water flow." (14)
CaCO3(s) + H2SO4(aq) ---> Ca2+(aq) + SO42-(aq) + H20 + CO2
So why are limestone buildings and statues still standing?
The reason for this is that this is a surface level reaction, meaning that the acid can only erode the surface of the limestone that it comes in contact with. As the surface is eroded the next level is exposed and can then be eroded. This is a continual process that slowly wears the limestone down.
But the Great Sphinx has been there for more than 4,000 years, why isn't it completely eroded?
The Great Sphinx is located in Giza, Egypt. The average rainfall for Giza is 0 - 1.06 inches (15). Compare this to the average rainfall in Lawrenceville, GA of 4.63 inches per month (16). This relatively low and unsteady rainfall means that the Great Sphinx is not being continuously eroded by acid rain, though there are other physical erosion factors that are on a more continuous basis such as wind erosion.
So, if it's being eroded, wouldn't it be smooth? Why does it look like the Great Sphinx is all cracked?
When ions of calcium and sulfate dry in the pores of the Great Sphinx, salt crystals are formed. Calcium and sulfate combine to make Gypsum which is CaSO4 2H20. These crystals grow and begin placing pressure on the surrounding limestone. If enough pressure is built up then it can cause the limestone to crack or cause chunks of limestone to fall off. (14)
Description Picture
A chunk of limestone showing the pitting from acid rain. Limestone.jpg
A limestone face on a building showing pitting from acid rain. Acidrainlimestone.jpg
This is a picture of gypsum crystals in rock. Gypsumcrystal.jpg

Exfoliation

Definition: the process in which rocks weather by peeling off in sheets rather than eroding grain by grain (12).
"...exfoliation in which dew, forming at night on the exposed limestone, removes souble salts from the surface of the rock. Capillary forces draw this solution into the pores of the limestone matrix, where further salts are dissolved from the internal pore walls. As daytime temperatures rise, the solution begins to evaporate, precipitating salt crystals within the confined neck of the pores. The pressure that these crystals exert as they grow leads to stress-induced exfoliation from the surface of the limestone." (4)
Stress-Induced Exfoliation - this is simply exfoliation due to pressure being exerted from some foreign object within itself that has the same results as general exfoliation.
Exfoliation.jpg Exfoliation2.jpg Exfoliation3.jpg

Terms

1. Carbonation - absorption of or reaction with carbon dioxide (6).
2. Hydration - reaction of a substance with water (7).
3. Hydrolysis - When water reacts with another substance and as a result the oxygen in water makes a bond with the substance (8).
4. Oxidation - A reaction in which the atoms in an element lose electrons and the valence of the element is correspondingly increased (9).
5. Ion Exchange in Solution - A reversible chemical reaction between two substances (usually a relatively insoluble solid and a solution) during which ions of equal charge may be interchanged. Ion exchange is used in water softening and purification, chemical analysis, the separation of radioactive isotopes, and kidney dialysis. Cation exchange is the exchange of positively charged ions. Anion exchange is the exchange of negatively charged ions (10).
6. Igneous - Relating to rocks or minerals formed by the cooling and hardening of magma or molten lava (11).
7. Limestone - Limestone is by definition a rock that contains at least 50% calcium carbonate in the form of calcite by weight. (3)
8. Chemical Weathering - Chemical weathering (also known as decomposition or decay) is the breakdown of rock (weathering) by chemical mechanisms, the most important ones being carbonation, hydration, hydrolysis, oxidation, and ion exchange in solution. (5)
9. Acidic Rain - rain that contains a high concentration of pollutants, chiefly sulphur dioxide and nitrogen oxide, released into the atmosphere by the burning of fossil fuels such as coal or oil. (13)
10. Exfoliation - the process in which rocks weather by peeling off in sheets rather than eroding grain by grain. (12)

References

  • NOTE: All references for images can be found by clicking on the image itself and looking at the summary information.
1. Ryan, Tom. "Information on the Great Sphinx at Giza"
Trails.com. 2010. Web. 06 November 2010
<http://www.trails.com/facts_33385_information-great-sphinx-giza.html>
2. Unknown, Unknown. "The Great Sphinx"
ancient-egypt-online.com. 2008. Web. 06 November 2010
<http://www.ancient-egypt-online.com/the-great-sphinx.html>
3. Unknown, Unknown. "Limestone"
geology.com. 2010. Web. 06 November 2010
<http://geology.com/rocks/limestone.shtml>
4. Reader, C.D. "A Geomorphological Study of the Giza Necropolis, With Implications for the Development of the Site"
Archaeometry 43.1 (2001): 149-165
6 November 2010
5. Alden, Andrew. "Chemical Weathering"
geology.about.com. 2010. Web. 06 November 2010
<http://geology.about.com/od/glossaryofgeology/g/defchemweathering.htm>
6. Unknown, Unknown. "carbonation"
thefreedictionary.com. 2010. Web. 06 November 2010
<http://www.thefreedictionary.com/carbonation>
7. Unknown, Unknown. "Definition of hydration"
chemicool.com. Unknown. Web. 06 November 2010
<http://www.chemicool.com/definition/hydration.html>
8. Unknown, Unknown. "Definition of hydrolysis"
chemicool.com. Unknown. Web. 06 November 2010
<http://www.chemicool.com/definition/hydrolysis.html>
9. Unknown, Unknown. "oxidation"
thefreedictionary.com. 2010. Web. 06 November 2010
<http://www.thefreedictionary.com/oxidation>
10. Unknown, Unknown. "ion exchange"
thefreedictionary.com. 2010. Web. 06 November 2010
<http://www.thefreedictionary.com/ion+exchange>
11. Unknown, Unknown. "igneous"
thefreedictionary.com. 2010. Web. 06 November 2010
<http://www.thefreedictionary.com/igneous>
12. Alden, Andrew. "Exfoliation"
geology.about.com. 2010. Web. 06 November 2010
<http://geology.about.com/od/geoprocesses/ig/mechweathering/exfoliation.htm>
13. Unknown, Unknown. "acid rain"
thefreedictionary.com. 2010. Web. 06 November 2010
<http://www.thefreedictionary.com/acid+rain>
14. Casiday, Rachel. "Acid Rain Inorganic Reactions Experiment"
chemistry.wustl.edu. 1998. Web. 06 November 2010
<http://www.chemistry.wustl.edu/~edudev/LabTutorials/Water/FreshWater/acidrain.html>
15. Unknown, Unknown. "Giza climate and weather"
world66.com. Unknown. Web. 06 November 2010
<http://www.world66.com/africa/egypt/giza/lib/climate>
16. Unknown, Unknown. "Lawrenceville Weather"
idcide.com. 2010. Web. 06 November 2010
<http://www.idcide.com/weather/ga/lawrenceville.htm>

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