This past weekend, Ricardo and I were in South Dakota. While there, hiking the highpoint (Black Elk Peak – 7,242 ft./2207 m), we were also able to visit nearby Mt. Rushmore – my first time seeing this national memorial. The size of the faces carved into the rock is impressive – 60 feet tall (18 meters). No noses (20 feet/6 meters tall) have fallen off since completion in 1941 – 80 years ago! So, what is the rock that has allowed these carvings to endure?
There are two main rock types at Mt. Rushmore – very old granite and much older metamorphic rock. The metamorphic rocks we see (as folds in the rock to the lower left of President Washington) actually began as sediments at the bottom of a sea during the Precambrian period, some 1.6 – 2.5 billion years ago, back when what is now western South Dakota was near the edge of a shallow sea. There was no plant life back then, so nothing could hold the forces of erosion in check, washing large quantities of sand and clay into the sea. Eventually, the pressure of its own weight turned the sand and clay into sandstone and shale, reaching a thickness of several miles!
Meanwhile, a great mass of molten rock was growing and beginning to rise from deep within the Earth’s crust. Rather than erupting as lava, it cooled underground, forming the granite that is now the core of the Black Hills of South Dakota. As the magma came into contact with the old sandstone and shale layers, the intense heat and pressure melted some of the rock and turned the rest into metamorphosed stone – the sandstone became quartzite and the shale became mica schist. In fact, all along this region and at the highpoint, we saw an abundance of mica, a shiny elastic mineral, in the rock and along the trails. A large block of fine-grained granite that cooled quickly is what formed the upper portion of Mt. Rushmore, providing excellent rock for carving.
About 70 million years ago, around the time of the uplift of the central Rocky Mountains, the Black Hills also started to rise, eroding away overlying sedimentary rocks and revealing the crystalline core of granite below. At the surface, granite erodes very slowly at only one inch every 10,000 years and water is the main erosive force that can still threaten the rock. Any cracks in the rock can allow water in, which can then freeze and thaw over the years, eventually wedging blocks of rock apart. Mt. Rushmore has a series of these cracks and without proper restoration, the cracks would continue to widen over time, crumbling the faces. The National Park Service keeps these cracks under close observation using a network of tiny fiber optic cables. Read more about that here: https://www.travelandleisure.com/culture-design/mount-rushmores-new-preservation-technology. When large cracks open, they are filled with Kevlar, while smaller cracks are routinely filled with silicon caulk to slow the effects of weathering and prevent further erosion.
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