Into the Badlands
[Note: This entry was posted on Aug 2, 2019. It is backdated to keep it in chronological order.]
by George Taniwaki
On our trip to Rapid City (see Real Numeracy, Oct 2018), Sue and I saw notable geology and topography. Assuming you believe in evolution, the earth’s history can be seen written in land formations such as mountains, caves, and canyons. On the other hand, if you don’t believe in evolution, then there is no coherent explanation for what you see, it just is.
During our field trip around Rapid City we saw rocks with ages that span 2 billion years. There are not too many places on earth where you can see such a wide range of rock ages and types within a few miles of driving. (Denver, where I grew up is another such place. I didn’t realize how special it was until I moved away.)
Geologic map showing Devils Tower, Jewel Cave, Wind Cave, and Badlands. Image from Mira Costa College
Late Precambrian—Igneous inclusions at Black Hills
The earth consists of a molten ball of iron and other heavy metals called the core. A layer of molten silica and other lighter materials called the mantle floats on top of it. The very top layer is a thin shell, less than 50 km (30 miles) thick that has cooled off and is solid. This layer, called the crust, is all we can normally see.
Geologic time starts about 4.8 billion years ago, when the earth was still a molten mass and the crust had not yet formed. The timeline is divided into 5 eons. The eons are subdivided into 11 eras, then periods, then epochs, then ages.
The story of the Northern Plains starts at the Orosirian period, between 2,050 to 1,800 million years ago. During this period, hot magma rose to the surface and solidified into two igneous rocks, granite and pegmatite. Under heat and pressure, some of these rocks metamorphosed into gneiss (pronounced nice). These form the bedrock making up the Black Hills.
Remember though, we don’t know what the surface of this area looked like back then. It may have been underwater. It probably was not mountainous like it is today, otherwise the inclusions would have eroded away by now.
Granite in the Black Hills. Photo from Deadwood Connections
Paleozoic-Missing layers
There are no rocks aged between 1,800 million years to 225 million years old visible in the area around the Black Hills. They all became buried or eroded away.
Mesozoic—Inland sea and limestone
During the late Triassic through the Cretaceous period, 225 to 65 million years ago, the earth was warmer than now. Dinosaurs ruled the land. The continents began to drift apart and the middle part of what is now the North American continent was below sea level, part of an inland seaway. Salt water bacteria and invertebrate marine animals converted carbon dioxide into calcium carbonate. As they became buried under the sediment the shells became limestone, rich in fossils.
At the same time thin beds of mud, sand, and silt also built up creating layers of black shale, red sandstone, and red siltstone. These striped sedimentary rocks are visible at the base of the Black Hills.
Cenozoic—Inclusions at Devils Tower
During the Paleogene period, 65 to 23 million years ago, the earth began to cool. The dinosaurs became extinct and mammals of all sorts became the predominant land animals.
From the action of plate tectonics, the land to the west rose and the inland sea receded. Magma rising from the mantle circulated to the Earth’s crust forming inclusions and volcanoes. Nearby Devils Tower is a prominent example. The igneous rock is harder than the surrounding sedimentary formations that have eroded away, leaving a butte.
Devils Tower, perhaps once a volcano or an alien beacon. Photo from Pattys-photos
Cenozoic—Volcanic ash at Badlands
Also during the Paleogene period, after the extinction of dinosaurs, a wide variety of mammals lived in the northern plains. Many of the species are now extinct, including saber-toothed cats, rhinos, tapirs, and three-toed horses. Periodically, a volcano would erupt killing them suddenly. Their remains can be found in fossil beds that can be dated by measuring the ratio of isotopes in the layer of ash they are found in.
The ash and lava are harder than the underlying sandstone or limestone. Places where the lava eroded away, the water and wind carved out steep canyons, called badlands.
I’ve been to Badlands National Park as a child. Today, it looks pretty much the same, rugged but beautiful.
The Badlands in 1967 (top) and today (bottom)
Cenozoic—Supervolcanoes at Yellowstone caldera
During the Neogene period, 23 to 2.5 million years ago, continued tectonic action caused the land to rise just west of Rapid City, ultimately to 2,000m (6,500 ft) above sea level, creating the Black Hills.
This was accompanied by significant volcanic activity in the Rocky Mountains. We didn’t visit Yellowstone National Park on this trip as it is 600 km (360 miles) away, but it is worth mentioning. As you travel west through Wyoming, the mountains get taller and the volcanoes larger. The largest volcano is Yellowstone, the caldera of a supervolcano that had its last supereruption very recently on the geologic timescale, about 630,000 years ago. In that event, it ejected 1000 km3 of rock, dust, and ash, or about 250 times as much debris as the 1980 Mt. St. Helens eruption. An earlier supereruption 2.1 million years ago is believed to have been 10 times larger than that.
Today, the Yellowstone volcano is quiet, but the area is teeming with geysers, hot springs, and mud pots.
Notable geysers with live-stream webcams. Photo from National Park Service
Cenozoic—Limestone caverns at Jewel and Wind Cave
As the glaciers advanced and receded over the past 2.5 million years, the current period called Quaternary, groundwater percolated through the limestone. The water reacted with the calcium carbonate, dissolving it and carrying it away, leaving hollow areas called karsts. Further erosion caused the karsts to grow larger and more numerous. Eventually they formed sinkholes on the surface and caves underground.
Mineral saturated water dripped from the ceilings of caves. Some of the carbonates precipitated out, leaving stalactites and stalagmites. (Simple mnemonic to remember which is which, stalactites hang tight to the “c”eiling, stalagmites are like mites on the “g”round.)
There are two well-known cave systems in the Black Hills, Jewel Cave National Monument and Wind Cave National Park. If you only have time or inclination to visit one, here’s a guide to help you choose. (TL;DR, it’s a tie.)
Jewel Cave on a bright day. Photo from National Park Service
For more geology field trips, see Science field trip–North Cascades (Aug 2017) and Science field trip—Bay of Fundy (Jun 2012).
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Dinosaur Park
Back in the city, we drove along Skyline Drive, a winding road that leads out of the city. At the top of the hill is a large parking lot and gift shop. Across the road is a staircase that leads to Dinosaur Park. There are six life-size, but not realistically shaped, concrete dinosaur sculptures painted in garish green and white. The whole site was built in the 1930s as a WPA project. There are spectacular views of the city, and it’s free, so it is worth spending an hour to visit.
A brontosaurus or something like it
SDSMT campus
South Dakota School of Mines and Technology has a Geology Museum that has an excellent collection of fossils, many found at the Badlands. You can take a virtual tour here.
SDSMT also has a well equipped Industrial Engineering lab. Stuart Kellogg, the department chair, was nice enough to give me a tour. The lab includes 3D printers, laser engravers, and CNC machines.
A nearly complete brontothere; IE department video on YouTube
Finally, no trip to an engineering college campus is complete until you find the bent monument that honors the local Tau Beta Pi chapter.
Get bent. Photo by Susan Wolcott
[Update: Rearranged the sections to be in geologic chronological order. Moved some of the text to related Aug 2017 blog post.]