Sand and gravel are small, weathered pieces of rock. Unlike rock itself, which is defined by its composition, sand and gravel are defined by their particle size, ranging from 0.002 to 4 inches in diameter. This means that, depending on where the material comes from, sand and gravel can be made of many different types of rock.
Sand consists of tiny particles of rock between 0.002 and 0.08 inch in diameter, which is about the size of a grain of salt. In the natural world, sand results from larger particles of rock that has been weathered down to this size. Most commonly, sand contains igneous rocks, such as quartz, feldspar and mica, which come from a granite source. Sand also can be made mechanically by crushing larger pieces of rock.
Gravel particles are larger than sand, ranging from 0.08 to 4 inches in diameter. It can be further classified by diameter size as granules (0.08 to 0.16 inches), pebbles (0.16 to 2.5 inches) and cobbles (2.5 to 10 inches). Rock particles larger than 10 inches in diameter are considered boulders.
Gravel occurs naturally the same way sand does, only it has not been around long enough to be weathered as small. However, gravel is considered to be any assortment of rocks of this size, which includes everything from crushed limestone to debris picked up and later deposited by a glacier.
While most sand and gravel commonly comes from granite sources, such as mountains and exposed rocks, any material of the appropriate particle size can be considered sand or gravel. For example, many beaches contain sand that is made from coral or sea shells. The black sand beaches of Hawaii result from volcanic rock that has been weathered down from lava. On the other side of the color spectrum, the white sand dunes in New Mexico are made of gypsum.
Uses of Sand and Gravel
Sand and gravel are part of the aggregate commodities, which are often used for construction. Major uses of sand and gravel include highways and homes. The materials are mined from open pits and quarries, where it is sorted by size and shipped out.
Aggregate manufacturers also make their own gravel and sand by crushing larger rocks into the appropriate particle size. This results in less smooth pieces, but it allows full control over the composition of the final product.
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