Silt

Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. It can be found as soil mixed with sand or clay, or as sediment suspended in water. When dry, silt has a floury feel and lacks plasticity when wet.

Silt makes up 45% of average modern mud. It is common in river deltas and as wind-deposited accumulations in places like central Asia, north China, and North America. It forms through various processes, including dust storms and glacial grinding.

Loess, which is soil rich in silt, creates some of the most fertile agricultural land on Earth. However, silt is easily eroded and poses challenges for construction. Environmental pollution from windblown and waterborne silt can be worsened by poor farming practices.

Description

Silt is made from tiny pieces of weathered and eroded rock. It is smaller than sand but bigger than clay minerals. Most silt particles are broken pieces of quartz. When dry, silt feels floury, and when wet, it does not stick together like clay does.

Scientists measure silt by its size, usually between very small pieces you can barely see. Different groups, like the USDA and the ISSS, have slightly different size ranges for what they call silt. But all agree that silt is gritty and not smooth like clay.

Occurrence

Silt is a very common material, with an estimated abundance of a billion trillion trillion (10³³) grains worldwide. It is often found in places where rivers spread out, such as the Nile and Niger River deltas, and in areas like Bangladesh, northern China, central Asia, and North America. Silt is less common in tropical regions.

In rivers, silt can be found floating in the water and makes up over 0.2% of river sand. It is also found in types of rock called mudrock, either in thin layers, small groups, or spread throughout the rock. When silt makes up most of a sedimentary rock, it is called a siltstone.