Granite is one of the most common stone types we have at Oakland. It’s also one of the most common rocks on earth, making up the continental crust of our planet! Granite is an intrusive igneous rock, meaning that it formed from magma that cooled slowly, deep underground. It is primarily comprised of quartz and small amounts of feldspar and micas. These minerals tend to be pretty evenly distributed through the structure of the stone. Granite is an excellent building material given its strength and acid resistance.
Many of Oakland’s mausoleums are made of granite and parts of the historic Bell Tower are as well. People particularly love the “granite” that has stripes and swirls. I’m here to tell you—that is not granite. It’s an easy mistake to make, but today we are going to talk about what this striped rock really is: gneiss (pronounced “nice”). Gneiss is a metamorphic rock, meaning that it formed when a “parent rock” underwent very high levels of heat and pressure that re-oriented and rearranged the crystals. Gneiss can have many different parent rocks, but granite is one of the most common, resulting in what is called “granitic gneiss.”
Unlike other common metamorphic rock marble, which by definition derives from dolomite or limestone, gneiss does not have a set chemical formula of specific mineral composition. Gneiss is classified based on foliation (the alternating arrangement of its platy/elongated and crystalline grains). Platy and elongated minerals, the easiest example being Micas, are those that naturally orient themselves parallel to or stacked evenly upon one another. Crystalline grains are usually feldspars and quartz, which fit together like jagged puzzle pieces.
Gneiss usually forms through regional metamorphism (as opposed to isolated contact metamorphism), which affects large areas of rock at areas of high tectonic activity. The earth’s continental crust is made of granite, so it is no wonder that granitic gneiss is found near granite and often confused with it. Granitic gneiss can easily be found near Atlanta, notably in Lithonia.