MT. RUSHMORE NATIONAL MEMORIAL – PreCambrian Granite Batholith
FIELD TRIP STOP - MOUNT RUSHMORE (A 1.6 b.y.o Granite Batholith)
LOCATION: In Keystone, South Dakota off of Highway 244, southwest of Rapid City.
GEOLOGIC FEATURES: PreCambrian Granite; Dikes; Sheeting; Batholith; Pluton
DESCRIPTION: In the Black Hills of South Dakota, the heads of presidents Washington, Jefferson, T, Roosevelt, and Lincoln are carved into 1.6 byo granite batholith that both intruded into and metamorphosed the surrounding preexisting sedimentary rock These rocks were then uplifted during the Laramide Orogeny between about 76 to 50 mya at which time erosion began to strip away the rocks above the granite and metamorphics.
Each head stands 60 ft tall and was carved into the mountainside by Gutzon Borglum between 1927 and the time of his death in 1941. He chose the portion of the outcrop that was as close to a homogeneous fine-grained granite as possible. Note the lighter-colored dikes protruding into weaknesses in the earlier-formed granite.
(1) What is a Batholith? A Pluton?
(2) What is a dike?
(3) What is Sheeting? Look at the photos below and describe the location of a prominent area of sheeting on the presidential sculptures.
(4) Upon close examination coarser-grained (larger crystal size) lighter-colored veins (pegmatite dikes) of igneous rock can be seen in the presidential sculptures. Look at the photos below and draw the pegmatite dikes seen especially well in Lincoln's forehead and Washington's jacket collar.
(5) CHALLENGE: What precautions are currently being done by the National Park Service to monitor and slow down the natural processes of weathering?
-NPS. Mt. Rushmore – Geologic Activity. Accessed on Jan. 4, 2020: https://www.nps.gov/moru/learn/nature/geologicactivity.htm
Figure 1 - The carvings of Gutzon Borglum in the 1.6 billion year old Harney Peak Granite. Distance from chin to top of the head is about 60 feet.
Figure 2 - Note the slope of rocks carved out the main sculptures of the presidents.
Figure 3- Enlarged photo of Figure 1. Note the lighter-colored pegmatite dikes in Wahington's collar and Lincoln's forehead. Many more dikes are present but not readily visible in the present photo.