igneous
Granite (Mount Rushmore)
Phaneritic Igneous Rock (Harney Peak Granite)
Hardness: 6-7 Mohs; Color: Light gray to pinkish; Luster: Vitreous to dull; Structure: Phaneritic (large visible crystals), mainly orthoclase feldspar, quartz, and mica; Fracture: Granular.
- Hardness
- 6-7 Mohs
- Color
- Light gray to pinkish
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Light gray to pinkish; Luster: Vitreous to dull; Structure: Phaneritic (large visible crystals), mainly orthoclase feldspar, quartz, and mica; Fracture: Granular.
Formation & geological history
Formed approximately 1.6 to 1.7 billion years ago during the Precambrian. It originated from a slow-cooling magma intrusion deep within the Earth's crust that eventually uplifted to form the Black Hills.
Uses & applications
The specific specimen at Mount Rushmore is a National Memorial and historical monument. In general, this granite is used for construction, memorials, countertops, and dimension stone due to its durability.
Geological facts
Mount Rushmore features the faces of George Washington, Thomas Jefferson, Theodore Roosevelt, and Abraham Lincoln. 90% of the mountain was carved using dynamite. The granite erodes at a rate of only one inch every 10,000 years.
Field identification & locations
Identified by its salt-and-pepper appearance and coarse grain. Found extensively in the Black Hills of South Dakota. Collectors look for pegmatite veins within the granite which can contain tourmaline or garnet.
