Return to Scale View
About

How big is a Silt Particle?

50 micrometers!

View History Report
Silt Particle

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.

Sources

Scale of the Universe > Silt Particle
Skin Cell
The outermost layer of skin, which is also the only visible layer of skin, is made up of dead cells. That's because dead cells are the strongest, and stronger skin provides more protection! These cells continuously fall off. In fact, dust is 95% dead skin cells!
Thou
Although the word "thou" is most commonly known as being an old form of the word "you", a thou is also a unit of measurement. It is equal to one thousandth of an inch; in fact, that's where it gets its name. THOUsandth. Pretty cool.
Twip
A twip is a twentieth of a typographical point. That means that 12-point font is approximately 240 twips high. Also, TUIP can stand for "The Universe in Perspective”, an old name for the Scale of the Universe. TUIP and twip are pronounced the same.
Width of Human Hair
Human hair is amazing. Straight hair is almost perfectly cylindrical. On the other hand, curly hair is flatter, which allows it to curl, like a ribbon. Did you know that you probably have 50,000 to 200,000 strands of hair on your head? You can count!
Thickness of Paper
Paper is very thin. It's so thin it can give you a paper cut. You can do a lot of things with paper, including writing on it, drawing on it, and even folding it into shapes like squares and triangles and dodecahedrons and others.
Smallest Object Visible to the Naked Eye
Under optimal conditions, the naked eye can see objects as small as 0.1 millimeters. That means that anything smaller than this circle is impossible for you to see without a microscope. You're missing out quite a lot, you know!

Discussion