The fact that mines like these exist might surprise some readers; it definitely did surprise me. This is an open surface mine in the state of Wisconsin, but what is so interesting is what they’re actually mining.
Mines are created for lots of reasons; to get minerals, to quarry stone for building, even to gather up gravel that can be used in road building. This mine is none of those. These miners are after one very specific component of rock: sand grains. This is a sand mine.
Sometimes, beaches are hammered by storms, causing large amounts of erosion. One way to restore those beaches is to bring in trucks of sand; sometimes that sand is harvested from other beaches (not always legally!) and other times, the sand can be instead harvested from inland deposits of sandstone, like those here. Other uses include filling in sand traps at golf courses, molds for industrial processes, use in concrete for paving and building, and many others.
The sand in Wisconsin is probably the most widely used in North America. The sands are Cambrian and Ordovician in age and have properties that make them ideal for industrial uses. They are extremely well sorted, giving them a uniform grain size and high silica content, and the grains are also extremely well rounded (makes em comfortable to walk on). The deposits are also close to the surface, so they can be unearthed and mined at low costs.
Sand production in these mines has surged over the past decade due to one additional use: frac sand. The sand grains mined here are broken up, sorted by grain size, and shipped to locations all around the country where they are pumped underground and used to enable natural gas production. This sand is one component injected into the ground as part of fracking fluids - the sand grains get shoved into the rock at depth and hold the rock open, creating permeability that allows oil and natural gas to flow to the well site. At present, over 60% of the sand mined in these sites is used in driving gas and oil production. As a consequence of this high demand, more mines have opened and prices for sand used for beach restoration have skyrocketed. As techniques for fracking have evolved, sand mines have cropped up around the country.
As the mines have expanded, the same types of issues that appear with the expansion of many mines have appeared: additional surface runoff, dust pollution, and potentially spilling of chemicals used in processing the mined materials. In Texas, runoff from some sand mines is thought to have flowed into the San Jacinto river, and may have contributed to flooding on that river during Hurricane Harvey. One of the first government and taxpayer funded projects paid for after that storm was a dredging operation to remove sand from that river, clearing more space for floodwaters during the next storm.
In North Carolina, a mine pulling this same mineral, quartz, out of the ground, is difficult to reach as it is considered a national security site. The high-purity quartz from that site is processed into metallic silicon and is the main ingredient in silicon chips used in microprocessors, likely including the computer or phone you're using to read this post.
Image credit: http://wgnhs.uwex.edu/wp-content/uploads/2014/02/frac-sand-active-mine-pit.jpg
Read more: http://wgnhs.uwex.edu/wisconsin-geology/frac-sand-mining/ http://minerals.usgs.gov/minerals/pubs/commodity/silica/mcs-2014-sandi.pdf https://wgnhs.uwex.edu/wisconsin-geology/frac-sand-mining/frac-sand-mining-process/ https://www.wired.com/story/book-excerpt-science-of-ultra-pure-silicon/
