By Richard I. Gibson
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After the Trans-Hudson and Penokean mountain building episodes, which we talked about a few days ago, weathering attacked the mountains and eroded them. After about 150,000,000 years, much of what is now the southern edge of the Superior Craton was a low-lying, subsiding area crossed by braided streams. These rivers flowed between about 1.76 billion and 1.63 billion years ago
The streams carried quartz grains, sand, that created a deposit as much as 3,000 meters thick in what is now Minnesota, South Dakota, Iowa, and Nebraska. That’s nearly two miles of Proterozoic sandstone, so intensely cemented and lithified that it’s called quartzite. The package is called the Sioux Quartzite, and Sioux Falls drops over a resistant escarpment in this rock.
Similar rocks are found on the surface around Baraboo, Wisconsin, as well as in Arizona and New Mexico. It’s likely that much of North America was a low shield being eroded by shallow streams around 1.7 billion years ago. The countryside around those rivers would have been bleak by modern standards – no life at all, no trees, no grass, no plants, no animals. Not even any soil as we would recognize it, since modern soil contains a lot of organic matter. Just loose rocks and grains of resistant quartz.
The Sioux Quartzite is so thick and resistant that the area where it crops out has been a persistent relatively high area for much of earth history. Any sediments that were laid down across it during high stands of seas have been stripped off so that today Sioux Falls and the surrounding area are a window into the past.
The Sioux Quartzite is a pretty, pinkish rock that has been used in many historical buildings in the city of Sioux Falls and surrounding areas.
Today, January 23, is also the anniversary of the most deadly earthquake in human history. In 1556, in Shaanxi, north central China, a quake estimated at a 7.9 magnitude killed at least 830,000 people. 7.9 isn’t that intense, as quakes go, but at the time many of the people there were living in caves dug into soft earth, and many died in the collapse of those caves.
The quake was ultimately a result of the interaction of India and Eurasia. The geologic map shows the area around Xian to be near the intersections of several major faults, which were strained and continue to be strained by the northward push of India even though its collision was hundreds of kilometers to the southwest.
Quartzite photo by Andrew Wickert, via Wikipedia under Creative Commons license.
Here’s the podcast:
Transcript:
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| Sioux quartzite |
The streams carried quartz grains, sand, that created a deposit as much as 3,000 meters thick in what is now Minnesota, South Dakota, Iowa, and Nebraska. That’s nearly two miles of Proterozoic sandstone, so intensely cemented and lithified that it’s called quartzite. The package is called the Sioux Quartzite, and Sioux Falls drops over a resistant escarpment in this rock.
Similar rocks are found on the surface around Baraboo, Wisconsin, as well as in Arizona and New Mexico. It’s likely that much of North America was a low shield being eroded by shallow streams around 1.7 billion years ago. The countryside around those rivers would have been bleak by modern standards – no life at all, no trees, no grass, no plants, no animals. Not even any soil as we would recognize it, since modern soil contains a lot of organic matter. Just loose rocks and grains of resistant quartz.
The Sioux Quartzite is so thick and resistant that the area where it crops out has been a persistent relatively high area for much of earth history. Any sediments that were laid down across it during high stands of seas have been stripped off so that today Sioux Falls and the surrounding area are a window into the past.
The Sioux Quartzite is a pretty, pinkish rock that has been used in many historical buildings in the city of Sioux Falls and surrounding areas.
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| Geologic map of China showing location of 1556 quake. |
The quake was ultimately a result of the interaction of India and Eurasia. The geologic map shows the area around Xian to be near the intersections of several major faults, which were strained and continue to be strained by the northward push of India even though its collision was hundreds of kilometers to the southwest.
Quartzite photo by Andrew Wickert, via Wikipedia under Creative Commons license.
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