Mount Rushmore is perhaps one of the most amazing feats of artwork and sculpturing. It's not unusual for sculptures to be made out of granite rock, and this is the case for Mt. Rushmore. But the presence of granite is just a small part of the geologic evolution that created the Black Hills, and the rock face in which the monument is carved.
The geologic processes that took place over billions of years to create the rock formations and topography of this area are:
- Precambrian sedimentary rocks were deposited in the form of shale and sandstone in what is now western South Dakota
- About 1.6 billion years ago, a magma intrusion into the sedimentary crust broke through the surface to cool and from granite
- The sedimentary rock was heated and altered by the hot granite intrusion to from metamorphic rocks of quartzite and schist
- The area was buried by multiple layers of sedimentary rock , including limestone, as seas moved farther inland and then receded repeatedly
- About 70 million years ago, the area was uplifted and the top layer of sedimentary rocks eroded, exposing the metamorphic and granite rocks
- The granite and metamorphic rocks erode much more slowly, so knobs and hills of these rocks persist today
Igneous Intrusive and Metamorphic Rocks of Mt. Rushmore
The first rocks that were deposited in the Mt. Rushmore region, which is the south western part of South Dakota, were sedimentary rocks. Between 1.6 and 2.5 billion years ago the entire part of what is now the western U.S was covered with seas. These seas moved farther inland and receded for millions of years, depositing fine sediment and forming sandstone and shale rock in these areas. This was during the Precambrian period and because there was no life during this time on Earth, there are no fossils in this part of the geologic record. These sandstone and shale deposits were also several miles thick.
Later in the Precambrian, about 1.6 billion years ago, a massive intrusion of magma in this area broke through the crust. The magma at the top cooled quickly, forming fine-grained granite. The magma at the bottom cooled slowly, forming coarse-grained granite, or pegmatite. The Mt. Rushmore monument is carved into the fine-grained granite
Where the still cooling granite came into contact with the overlaying sedimentary rocks, metamorphism occurred. The intense heat and pressure caused by the intrusion of granite partially melted the sandstone and shale. The heated sandstone formed a metamorphic rock called quartzite, named after the dominant mineral in sandstone - quartz. The clay and quartz in the shale were converted into new minerals - mainly muscovite and biotite - that formed a metamorphic rock called schist.
Uplifting and Erosion Formed the Topography
During the Cambrian Period - about 550 to 500 million years ago - this area experienced repeated burial by sandstones, shales and limestones. This process continued throughout most of the Paleozoic Era, and there was life on the planet during this time, so fossils are present in this part of the geologic record. This buried organic life is also the source of hydrocarbon formations in some parts of the western U.S today.
Later in the Paleozoic and for the early part of the Mesozoic - between 500 and 100 million years ago - erosion dominated the area and much of the sedimentary material that had been deposited by inland seas was stripped away by water and wind. About 6,000 feet of sedimentary rock was removed and deposited in areas surrounding the Black Hills and Badlands National Park.
Towards the end of the Mesozoic Period - about 70 million years ago - the entire region was uplifted. This uplift is still occurring today and is related to the uplift of the central Rocky Mountains. As the granite and metamorphic rocks were uplifted, the cracked and broken sedimentary rocks above were further eroded. Left behind were knobs and hills of more sturdy rock; the granite, quartzite and schists that the Black Hills are made of today.
The rock face that Mt. Rushmore is carved into, the Harney Peak Granite, was deemed especially fitting for the location of the sculpture because of its fine-grained, durable, light-colored rock. Of this location, the monuments creator, Gutzon Borglum, says,
“…Let us place there, carved high, as close to heaven as we can, the words of our leaders, their faces, to show posterity what manner of men they were. Then breathe a prayer that these records will endure until the wind and the rain alone shall wear them away.”
Related Articles:
Origin of Fossil Fuels : Hydrocarbons are Created in Sedimentary Environments
How to Identify Rocks: The Differences between Igneous, Sedimentary and Metamorphic Rocks
Sources:
U.S. Geological Survey, America's Volcanic Past - South Dakota
Mount Rushmore National Memorial Website
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