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Roger Weller, geology instructor

wellerr@cochise.edu

tombstones
by Valerie Valdez
Physical Geology
Fall 2010
       
 

                                                         Granite-Tombstones and Monuments

 

Description: http://skywalker.cochise.edu/wellerr/rocks/igrx/6irx-granite1a.jpg

R. Weller/ Cochise College

 

          The earth is in abundance with igneous rock granite. As stocks and batholiths, granite is revealed in mountain ranges because of erosion. A batholith is very large igneous intrusion- plutons where magma has combined with country rocks. Two of the most common uses for granite are the use of granite in tombstones and historical monuments.

Description: Granite Tombstone

Brand: Kadsen, Made in China

 

Question: If we use granite for tombstones and historical monuments, why don’t architects and engineers use granite for buildings? 

The answer is because granite is not fire or heat resistant. In fact, granite cracks and crumbles under heat.
 

Formation

          Granite is an igneous rock formed by slow cooling magma. Because the magma is being slowly cooled, granite has a phaneritic texture meaning it is coarse grained. Due to the coarse grained texture, granite receives its name in which it is “granular”. Below: Table of Igneous Rocks.

 

 

Dark (High Iron)

Intermediate

Pastel (Low Iron)

Volcanic

Fine-grained

(Cooled Fast)

 

BASALT

 

ANDESITE

 

RHYOLITE

Plutonic

Coarse-grained

(Cooled Slow)

 

GABBRO

 

DIORITE

 

GRANITE

 

Low Silica

(Fluid Magma)

 

High Silica

(Viscous Magma)

R. Weller
 

Granite Composition

         Granites are composed of feldspar, quartz, and mica minerals. In granites, feldspar is the dominating mineral which is represented by its appearance and cleavage. Quartz does not have a crystal shape in granite. Actually, quartz acts like a glue and keeps the other minerals in the granite together. Quartz is represented by its luster, which is glassy, hardness of seven on the Moh’s scale, no cleavage, and conchoidal (shell-like) fracture.

Granite’s Color

         In granites, feldspar occurs in two types. These two types of feldspar affect the color of the granite which is orthoclase and plagioclase. The mica in some granite can be either black because it is biotite, or silver because it is muscovite. In granite, both types of mica can occur. As a whole, granites come in a variety of different colors such as pink, gray, and black. In granites, there are large amounts of magnesium and iron. However, large amounts of iron and magnesium indicate there will be lower amounts of silica in granites.

Granite Tombstones and Monuments vs. Granite Buildings

          Which brings us back to why granite is not a good material to use for buildings? At high temperatures, the quartz expands making the granite not fire resistant. Another reason, granite has a poor thermal conductivity. For both these reasons, granite is best used in tombstones and historical monuments. A famous monument made out of granite is Mount Rushmore located in South Dakota.

Description: Full view of Mount Rushmore

Photo: National Park Service U.S. Department of the Interior

Granite: Hard and Tough

          Two qualities, in which granite is a good material for tombstones and monuments, are because granite is hard and tough. Toughness defined, by Roger Weller, is a materials ability to have forces acted upon it and the material does not break. Granite is resistant to scratching and abrasion because it is hard. On the Moh’s Scale of hardness, granite would be between a five, which is apatite, and a seven, which is quartz; Six being feldspar.

Weathering

          Tombstones and historical monuments made of granite are meant to stand the test of time. Granite is hard and tough, but how does weathering play a role? Some varieties of granite are rich in iron, ferrous, and when these varieties are in contact with moisture, the granite will rust. In darker varieties of granite, they have a tendency to fade.

 

Description: http://lh5.ggpht.com/_m3YgDs2HadQ/SLWBAPjXsxI/AAAAAAAADT8/hI6-dNiw70Y/s512/DSCN0967.jpgDescription: mayors monument
faded granite and rusted granite
 

With India in the Lead 

          In the United States, common areas where granite is quarried are in Vermont, North Carolina, and Minnesota. However, India has the lead in the granite industry. India especially exports granite used in tombstones and historical monuments. The United States is one of India’s clients purchasing the granite. Two historical monuments, in the United States, made from the granite purchased from India include the Vietnam War Memorial and the Holocaust Memorial Building. The main reason, why India’s granite industry has the lead, is because they have a wider variety of colors of granite. The eighty percent of granite tombstones being exported to the United States are of the color black. The granite industry of India, not only makes tombstones and monuments, they make statues as well, such as The Mother & Child statue in Christ Church in New Zealand.

Description: http://www.militaryfactory.com/vietnam/imgs/vietnam-war-memorial-4.jpgDescription: http://www.dcphototour.com/images/best_sellers/Holocaust%20Memorial%20Museum.jpg
                  Holocaust Memorial and Vietnam War Memorial

 Porphyritic Granite and Granite Pegmatite

          Since granite has many minerals already in it, the minerals can make porphyritic granites and granite pegmatites. Porphyritic granite is a reference to the feldspar in the granite. The feldspar occurs in large crystals, also known as phenocryst, scattered sparsely through the granite. For this reason, it is porphyritic granite rather than a granite porphyry. However, it is different with granite pegmatites. In granite pegmatites, the quartz and the feldspar minerals are large and the minerals occur separately.

 

Description: ig15Description: GOLDEN RIDGE

porphyritic granite and granite-pegmatite

  

References

National Park Service

                http://www.nps.gov/moru/index.htm 

Granite

                http://www.stonecaretechniques.com/19_m.htm

Weller, Roger

                http://skywalker.cochise.edu/wellerr/aawellerweb.htm
 

Indian Express

                http://www.indianexpress.com/oldStory/17896

Pirsson, Louis V. Rocks and Rock Minerals: Third Edition. New York: John Wiley & Sons, Inc.,  1947.