How big is Silt Particle? | Scale of the Universe

Silt is sediment made of grains finer than sand but coarser than clay, and it is defined by the size of its grains. The 50 micrometers shown here is a coarse silt grain, at the upper limit of silt in the system of the U.S. Department of Agriculture (USDA). A micrometer is a millionth of a meter, so 50 micrometers is 0.05 millimeters.

Size

The size is the nominal diameter of one grain, the measure used to sort grains into size classes. Several systems of size classes exist, and they set slightly different limits.

The USDA's manual for soil surveys counts grains from 2 to 50 micrometers across as silt. Smaller grains are clay, and larger ones, up to 2 millimeters, are sand. Grains of 20–50 micrometers are coarse silt.

Geologists use the Wentworth scale, published in 1922, in which silt runs from about 4 to 62.5 micrometers. A 50-micrometer grain counts as silt in both systems.

Size comparisons

A human hair, 80–100 micrometers wide, is 1.6–2 times as wide as a 50-micrometer silt grain.
A 50-micrometer silt grain is about 1.4 times as wide as a flat skin cell from the surface of the body, about 35 micrometers across.
The coarsest silt grains are 25 times as wide as the finest, 2 micrometers across in the USDA system, where silt meets clay.
Twenty 50-micrometer silt grains in a row would span 1 millimeter.
If a 50-micrometer silt grain were enlarged to the size of a basketball, the finest silt would be about 1 centimeter (0.4 inches) across. The largest sand grains would be almost 10 meters (31 feet) across.

Where silt comes from

Glaciers make silt. They grind the bedrock beneath them into a fine powder called rock flour, made of silt-sized particles of rock. Meltwater streams carry away most of what a glacier wears off, and the rock flour in them gives glacial rivers their gray color.

Wind picks up silt as well. Windblown silt that settles in deposits thick enough to recognize and map is called loess. Loess can be many tens of meters thick, and much of it began as rock flour. Most of the loess built up during glacial periods, the cold stretches when ice sheets spread, when there was more loose sediment and the land was drier, windier and less covered by plants.

Silt in soil

In the field, soil scientists judge how much sand, silt and clay a soil holds mainly by feel, rubbing it between their fingers. Sand feels gritty, and its grains can be seen one by one. Silt feels smooth, whether dry or wet, and a single silt grain cannot be seen without magnification. Soils rich in silt get their own names, such as silt loam.