BILL CUNNINGHAM / US GEOLOGICAL SURVEY / SCIENCE PHOTO LIBRARY BILL CUNNINGHAM / US GEOLOGICAL SURVEY / SCIENCE PHOTO LIBRARY
Fracking sand sample. Fracking or hydraulic fracturing is a drilling technique used to extract gas from shale deposits. A mixture of sand, chemicals and water is pumped under high pressure from the surface to underground horizontal wells. The pressure forms fissures (fractures) in the shale. The sand keeps the fissures open, allowing gas to flow into the well and rise to the surface. Fine-grained silica sand is used. The ability of the sand to prop open fractures sand is known as a 'proppant', and is essential to allow the oil and gas to flow into the well. Photographed in Arkansas, USA.
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