South Dakota
Type |
Symbol |
Year Est. |
|---|---|---|
State Mineral Stone |
Rose Quartz |
1966 |
State Gemstone |
Fairburn Agate |
1966 |
State Fossil |
Triceratops |
1988 |
State Jewelry |
Black Hills Gold |
1988 |
State Mineral Stone: Rose Quartz
Rose Quartz is the pink variety of quartz, which is one of the most common minerals on Earth, primarily due to its simple structure and chemical formula, SiO2. Quartz also has an extremely high hardness, 7 on Mohs hardness scale, meaning that it doesn't scratch very easily and therefore does not break down easily. As the rocks on Earth are slowly eroded over time, most of the other minerals will break down into clay while quartz grains will generally just gets smaller and smaller. The result is that most beach sand is composed of quartz that has a slight hematite (rust) stain to it to give the sand grains their slight yellowish color. Although quartz is a simple mineral, it can come in a variety of colors depending on what type of impurities are present in the crystal structure; pure quartz crystal is clear, milky quartz is white, smoky quartz is grey, amethyst is purple quartz, citrine is yellow quartz, rose quartz is pink, as well as some other colors and varieties. Quartz does not have any cleavage, meaning that when it breaks it doesn't form along perfect surfaces. Instead as the quartz crystals grow, individual mineral molecules of quartz are added to the outside of the crystal from water rich in dissolved SiO2 or mineral melt (liquid rock like lava or magma).
Related: Alabama State Gemstone - Star Blue Quartz; Arkansas State Mineral - Quartz Crystal; Georgia State Gemstone - Quartz; New Hampshire State Gem - Smokey Quartz; South Carolina State Gem Stone - Amethyst
State Gemstone: Fairburn Agate
State Fossil: Triceratops
Triceratops horridus skeleton and cast from the Field Museum in Chicago, IL.
The first discovery of material attributed to Triceratops, was in 1887 by George Lyman Cannon near Denver, Colorado, who had found a set of the brow horns attached to a skull roof. He sent the material to O.C. Marsh who, assuming the rock dates were Pliocene, determined that it was from a prehistoric bison, which he named Bison alticornis. However after a more complete specimen was discovered in 1888 from Wyoming's Lance Formation by John Bell Hatcher, as well as a couple of other discoveries, Marsh reevaluated the initial find and eventually added all the finds under a new species, Triceratops, which he named in 1889, meaning "three-horned face".
Ceratopsian wall at the Natural History Museum of Utah. Triceratops is the one located in the far right at the top.
Triceratops belongs to a group of animals known as ceratopsians. Ceratopsians were prolific during the Cretaceous, with many different varieties evolving with various numbers of horns and frill adornments. Triceratops was one of the last ceratopsians to have evolved with remains having been found in rocks dating from ~69 million years ago to the end of the dinosaurs ~66 million years ago. It is estimated that Triceratops could grow up to 30 feet in length and weigh 12,000-16,000 pounds. There are currently two recognized species of Triceratops, T. horridus (pictured above in Chicago) and T. prorsus. The skull of Triceratops is one of the largest skulls ever discovered, approaching 10 feet in length in some individuals and not only had the three primary horns (one above each eye and one on the snout), it also had a series of spikes along the edge of the frill known as epoccipitals, and hornlike projects on the jugals (cheekbones). The horns are thought to serve multiple functions, such as defense from predators as well mating display structures.
Related: Wyoming State Dinosaur - Triceratops
State Jewelry: Black Hills Gold
The chemical symbol for gold is Au, and gold is one of the unique minerals that, in its pure form, is composed entirely of one element. It has a hardness of 2.5 to 3 on the Mohs hardness scale meaning that it actually is very soft (your fingernail is 2.5). For this reason most gold jewelry is mixed with another metal to prevent scratching and bending easily. The karat rating of the gold represents it's purity, where 24 karat is 99.9% pure, 22 karat 91.7%, 18 karat 75%, and so on. Gold naturally does not corrode or tarnish, so even when it is mixed with other metals it usually has a resistance to tarnishing, enhancing its value for jewelry. When gold is found in place, the highest grade of gold is often found in association with quartz veins. Currently gold is considered one of the most valuable metals on Earth, being used as the standard for most money (gold standard). Gold is often formed initially in relation to volcanic regions, where fluids associated with volcanoes carry the heavy metals up towards the surface and deposit them in rocks. These are found in areas of current or former subduction zones, places where two plates came together forcing one place down and melting it, while the other plate is forced upwards into mountains. Afterwards, erosion will take the gold out of the mountains and carry them down stream. However, since gold is so dense it does not travel easily down rivers and will often settle to the bottom of the river within the rocks and mud within the river sediment. These gold deposits are known as placer deposits and are the primary place where gold panners find gold. They can then use the locations of these placer deposits to backtrack to the original sources of the gold within the streams.
Related: Alaska State Mineral - Gold; California State Mineral - Gold; North Carolina State Mineral - Gold
References
https://statesymbolsusa.org/states/united-states/south-dakota
https://ninkasibrewing.com/
https://www.gbif.org/species/113684498
https://www.britannica.com/animal/Triceratops
Geology of South Dakota's National Parks
Through Pictures
(at least the one's I have been to)
Mount Rushmore National Memorial
Visited in 1997 and 2021
Mount Rushmore National Memorial
Visited in 1997 and 2021
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Obligatory entrance sign photo.
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Geological map of the Black Hills, including Mount Rushmore (noted just below the slice through the map). Image courtesy of the NPS.
Mount Rushmore is set within a large geologic dome. This is a region where all of the land is bowed upwards, like an overturned bowl. After erosion, the result is a bullseye pattern of rocks, where the oldest rocks are in the center of the bullseye and progressively get younger towards the outside.
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Thuŋkášila Šákpe, AKA Mount Rushmore, before being carved. Image courtesy of nativehope.org.
Although this could be considered outside the normal realm of geology, I do want to note that before being known as Mount Rushmore, the Lakota referred to the mountain as Thuŋkášila Šákpe, Lakota for The Six Grandfathers. The mountain named by Lakota medicine man Nicolas Black Elk after seeing a vision “... of the six sacred directions: west, east, north, south, above, and below. The directions were said to represent kindness and love, full of years and wisdom, like human grandfathers.”
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As you drive up to the main entrance looking northeast, you first pass the profile view of Washington. From this direction you get a good view of the mountain sans most of the demolition work that was done with the carvings. You can see what would be the normally weathered surface of the granite before the carving brings out the fresh surface. This surface is also more reminiscent of the uncarved mountain seen in the image above. About 1.6 billion years ago, during the Proterozoic, magma began to work its way up through the rocks in this area. While still well below the surface of the earth, that magma cooled slowly to form the granite that makes up the carving stone for Mount Rushmore. This rock unit is known as the Harney Peak Granite.
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While the magma was cooling, it cooled unevenly. This caused some portions of the rock to cool quickly, producing smaller, fine-grained, crystals, while other parts of the granite cooled more slowly with very large grained crystals. These large grained crystal granites are known as pegmatites. The upper portion of Mount Rushmore is comprised mostly of the fine grained crystal variety, which is an easier rock to carve from. This large body of magma is known as a batholith.
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Between the formation of the granite 1.6 billion years ago and 500 million years ago, new rocks were deposited and eroded on top of the Harney Peak Granite batholith. However, due to the extreme hardness of the granite, the Harney Peak Granite remained behind while these other rocks had been lost to erosion and time. After this period of time, between 500 and 100 million years ago, there were some rocks deposited from which we do have remains of. Immediately on top of the granite is the green rock seen in the geological map above. This green rock, titled the "limestone plateau" on the map, can be seen surrounding the central granite bullseye. The "limestone plateau" is made up of several different rock layers and will be discussed in more detail in the Wind Cave National Park post (since that is where Wind Cave is located). After deposition of these rocks, the whole region started to be uplifted around 70 million years ago. This uplift is related to the uplifts seen across the Rocky Mountains at the same time.
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Google Earth image of the Black Hill dome.
The uplift formed the dome that we had discussed above. This dome is easily noticeable in the aerial image of the region as well, as seen in the Google Earth Image above. This dome structure stretches across South Dakota and Wyoming, even up to the area in which Devils Tower is located.
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Cross section of the Black Hills. Image courtesy of A Textbook of Geology.
In geological terms, a dome is an anticlinal structure where the rocks dip gently away from the center in all directions. After folding, fracturing, and faulting, this causes the overlying rocks to break apart in the middle, allowing for easier erosion of the them. Once these younger rocks have eroded away, the older rocks are exposed with the oldest rocks exposed in the center. As before, due to the extreme hardness of the Harney Peak Granite, they withstood erosion and remained around much longer. Their hardness is also why the Black Hills have these granitic mountain peaks that have not eroded away.
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As you walk up to the main entrance to the main viewing platform, you come across a rather more grand entrance sign than the wooden one at the park entrance. Another interesting decision that was made for the refurbishment of the memorial in the 1980's and 1990's was the inclusion of several areas encased with granitic blocks. These granitic blocks are found within the Visitor’s & Interpretive Center, the Avenue of Flags, and Grand View Terrace, which, while they are granite, are not the Harney Peak Granite of the mountain. These granitic blocks were trucked in from elsewhere.
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Here you can see the granitic blocks in the framing of the memorial at the distal end of the Avenue of Flags. In construction terms, these granitic rocks are known as "Rockville Beige granite" and are from the quarry company Coldspring. Quarried from Rockville, MN, this granite is more commonly known in geology as the Rockville Granite.
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Closeup of the Rockville Granite at Mount Rushmore
Known for its very large mineral crystals and minimal amount of metamorphism, the Rockville Granite presents as a very nice, consistent granite comprised mainly of quartz, feldspar, biotite, and hornblende with other accessory and background minerals. Minnesota has several granitic bodies that all date to around the same age. The Rockville Granite in particular is dated to being 1.812 billion years old (Ga), which is considered the Late Penokean of the Paleoproterozoic Era. When compared to the varying cooling rates of the Harney Peak Granite producing varying textures with metamorphic inclusions throughout, it is no wonder whey they decided to use a more picturesque granitic rock for the entrance and viewing terrace.
References
https://www.nps.gov/moru/learn/nature/geologicactivity.htm
https://www.govinfo.gov/content/pkg/GOVPUB-I29-PURL-gpo79412/pdf/GOVPUB-I29-PURL-gpo79412.pdf
https://blog.nativehope.org/six-grandfathers-before-it-was-known-as-mount-rushmore
https://etc.usf.edu/clipart/63000/63044/63044_black_hills.htm
https://coldspringusa.com/products/rockville-beige/
https://coldspringusa.com/case_study/mount-rushmore/#!
https://msaweb.org/AmMin/AM24/AM24_303.pdf
https://conservancy.umn.edu/server/api/core/bitstreams/cc610018-1b5b-431a-97ff-fbb4a9b67777/content