Sensational Silt

Whether or not it is known by the proper name, we all know what silt can do.

Silt, deposited by the yearly flooding along the Nile River, created the rich and fertile soil of the Nile Delta that help the ancient Egyptians survive.

Silt, deposited by the flooding of the Mississippi River, helped make the farmlands along the river and at the mouth of the river so productive. A system of levees along the Mississippi has reduced the flooding. Those levees have also reduced the amount of silt deposited at the mouth of the Mississippi, which has reduced the amount of protective wetlands and barrier islands surrounding New Orleans.

Silt has also forced federal, state and local governments to spend billions of dollars to clear the material out of shipping and boating lanes in coastal waters.

So while we all know what silt can do, just what is it? It turns out there’s quite a bit to learn about silt science.

Silt is a granular material between the size of clay and sand. Specifically it is between 0.0039mm and 0.0625mm in diameter. That puts it somewhere between sand and clay. The mineral origin of silt is quartz and feldspar.

Silt is created by a variety of physical processes that are capable of splitting the sand-sized pieces of rock. Those processes involve chemical weathering, in which water and other chemicals physically start to break down the sand-sized quarts. There is also a significant physical weathering process. There’s erosion, which is the chemical weathering of our quartz crystals, as well as the physical weathering caused by frost shattering.

But researchers say the main process is abrasion through transport. That is, as the tiny rocks are blown by the wind or washed through a stream, the pieces of rock bang into each other and get smaller. The tiny particles pile up and silt is born!

— Frank Graff

Frank Graff is a producer/reporter with UNC-TV, focusing on North Carolina Science Now, a weekly science series that airs Wednesdays, beginning in August 2013, as part of North Carolina Now on UNC-TV. In addition to producing these special segments, Frank will provide additional information related to his stories through this North Carolina Science Now Reporter's Blog!

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