For thousands of years, granite has been one of mankind's first choices for creating buildings and monuments from stone. Granite is the most abundant rock on earth, making up about 75% of the earth's crust. Quarrying and working with granite has never been easy, as it is exceptionally hard, durable, and resistant to the elements...the same qualities that make it a great material for buildings and monuments.
Granite rocks are igneous rocks formed by slowly cooling pockets of magma trapped beneath the earth's surface. Igneous rocks form from molten rock which is either ejected at the surface of the earth or cools underground. Igneous rocks make up 95% of the rocks of the earth's crust. They are also some of the oldest rocks found at the surface of the earth. Igneous rocks are classified on the basis of where they form and the composition of the molten rock. Magma originates from the melting of the earth's crust and upper mantle. This melting occurs around a depth of 20 to 140 miles below the earth's surface. As pockets of magma cool slowly underground, the magma becomes an igneous rock. This molten rock is less dense than the solid rock, so it rises towards the surface. Molten rock that cools before it reaches the surface hardens to become intrusive igneous rock. Because it forms deep beneath the earth's surface, it has more time to cool and develops large crystals. Intrusive rocks include granite.
An intrusive rock means that molten rock cooled within the crust and was never expelled as molten rock. The gradual cooling of molten rock is imperative to create the large crystals of a singular mineral that we see in granite. With time, there is differential lithification or solidifying of molten rock dependent on chemical makeup, this allows for different types of minerals to form at different periods of time and alter the final resulting granite. Therefore, the size of individual grains is proportional to how slowly the molten rock was cooled. Extrusive rocks cool during a volcanic eruption and allow no time for orientation of minerals, creating a homogenous looking rock with no discernible grains.
The minerals which make up granite are composed of silicon and oxygen. Granite is made out of big crystals; you can easily see them with the naked eye. This is because the granite cooled slowly underground, shielded by the rock surrounding it so that its great heat could escape slowly. Granite is mostly made out of feldspar and quartz, plus minor amounts of other minerals. Feldspar and quartz are light minerals. Therefore, when hot molten rock is coming up from the mantle, some minerals grow crystals and then later, others form. If the magma (liquid rock) keeps moving, those early crystals are left behind, until finally what are left are mostly the ingredients for feldspar and quartz. The result: granite.
Granite is a conglomerate of minerals and rocks, primarily quartz, potassium feldspar, mica, amphiboles, and trace other minerals. Granite typically contains 20-60% quartz, 10-65% feldspar, and 5-15% micas (biotite or muscovite). The minerals that make up granite give it the unique colors we see in different types of granite.
The relative proportion of different colored minerals in a granite is largely due to the original source of molten rock that cooled to form the granite. If the molten rock was abundant in potassium feldspar, the granite is more likely to take on a salmon pink color. On the other hand, if the molten rock is abundant in quartz and minerals that make up amphibole, you will likely get a black and white speckled granite.
The combination of the minerals below make up most of the colors we typically see in granite:
- Quartz - typically milky white color
- Feldspar - typically off-white color
- Potassium Feldspar - typically salmon pink color
- Biotite - typically black or dark brown color
- Muscovite -Â typically metallic gold or yellow color
- Amphibole - typically black or dark green color
White granite is a granite that is composed primarily of quartz (milky white) and feldspar (opaque white) minerals.
Pink colored granite is a result of an abundance of potassium feldspar within the granite.
Black and White Granite has equal parts quartz, feldspar, and amphibole, making a speckled black and white granite.
Red granite is a variation of pink potassium feldspar abundant granite, where the k-feldspar takes on a redder than pinker color. Also, you can get red coloring from iron oxide in hematite grains or inclusion within feldspar, essentially the same process that makes rusted metal red colored.
The granite that makes up the base of the Mississippi State Capitol building (as well as the main steps on the south side of the building) was quarried from the huge single deposit of granite near Elberton, Georgia. Geologists estimate that the granite from this deposit is approximately 35 miles long, 6 miles wide, and between 2 and 3 miles deep. Mineralogically the granite mined around Elberton, Georgia, is similar to other granites, except the Elberton granite composition is such that it weathers extremely well, its color does not fade, and it absorbs only 0.23% water.
The majority of the Mississippi State Capitol building that is visible above ground is made out of Bedford limestone, and is a very different color and texture than the Georgia granite that it sits on top of.
To log this EarthCache, you must send me the following information and post a photo of yourself or something personal in front of the Mississippi State Capitol Building (your face is not required to be in the photo).
1. Describe the major color and texture difference of the Georgia granite at the base of the Capitol versus the Bedford limestone above it.
2. Take a close look at the granite base. What color granite is this? Based on the information above, what are the main minerals that make up this granite?
3. Approximately what percentage of the earth's crust does granite account for?
4. Why do you think granite was chosen as the base of the Capitol building?
Information for this EarthCache was created with the help of the Elberton Granite Museum & Exhibit in Elberton, Georgia; from the tour guides at the Mississippi State Capitol Building; and from the Guide to the Architecture of the Mississippi State Capitol Building, a free pamphlet that can be obtained from the tour guides at the Capitol.
This EarthCache was developed on March 5, 2021; the 35th anniversary of the Mississippi State Capitol Building being designated a Mississippi Landmark. It was added to the National Register of Historic Places on November 25, 1969, and it was designated a National Historic Landmark on October 31, 2016. The current State Capitol Building was completed in 1903.
