gravel

gravel

[grav-uhl]
gravel, particles of rock, i.e., stones and pebbles, usually round in form and intermediate in size between sand grains and boulders. Gravel is composed of various kinds of rock, the most common constituent being the mineral quartz. Deposits of gravel are formed as a result of the weathering of rocks and the erosive and concentrating action of rivers and waves. Sometimes gravel becomes consolidated into the sedimentary rock called conglomerate. Gravel is used extensively in building roads and in making concrete. For road building it is crushed into angular particles of uniform size. One or more layers of gravel underlie the road surface. A small percentage of clay must be present to act as a binder when gravel is used in macadam for road surfaces. When used as a coarse aggregate for concrete, gravel must be clean and free from clay and organic matter. Commercially, it is classified according to the size of the particles. In areas where natural deposits are inadequate, gravel is produced by quarrying and crushing durable rocks, such as sandstone, limestone, or basalt.

Aggregate of more or less rounded rock fragments coarser than sand (i.e., more than 0.08 in., or 2 mm, in diameter). Gravel beds in some places contain heavy metallic ore minerals, such as cassiterite (a major source of tin), or native metals, such as gold, in nuggets or flakes. Deposits accumulate in parts of stream channels or on beaches where the water moves too rapidly to permit sand to remain. Because of changing conditions, gravel formations generally are more limited and more variable in coarseness, thickness, and configuration than sand or clay deposits. In many regions gravel terraces (or raised beaches) extend great distances inland, indicating that the sea at one time stood higher than it does today. Gravels are widely used building materials.

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Gravel is rock that is of a specific particle size range. In geology, gravel is any loose rock that is larger than two millimeters (2mm) in its largest dimension (about 1/12 of an inch) and no more than 63 millimeters (about 2.5 inches). Sometimes gravel is restricted to rock in the 2-4 millimeter range, with pebble being reserved for rock >4-63 millimeters. The next smaller size class in geology is sand, which is >0.063 mm to 2 mm in size. The next larger size is cobble, which is >63 millimeters to 256 millimeters (about 2.5 to ten inches). One cubic foot (28.32 dm3) of gravel typically weighs about 100 pounds (45 kg).

Gravel is an important commercial product, used in many applications. Many roadways are surfaced with gravel, especially in rural areas where there is little traffic. Globally, far more roads are surfaced with gravel than with concrete or tarmac; Russia alone has over 400,000 km of gravel-surfaced roads.

Geological Formation

Large gravel deposits are a common geological feature, being formed as a result of the weathering and erosion of rocks. The action of rivers and waves tends to pile up gravel in large accumulations. This can sometimes result in gravel becoming compacted and concreted into the sedimentary rock called conglomerate. Where natural gravel deposits are insufficient for human purposes, gravel is often produced by quarrying and crushing hard-wearing rocks, such as sandstone, limestone, or basalt. Quarries where gravel is extracted are known as gravel pits. Southern England possesses particularly large concentrations of them due to the widespread deposition of gravel in the region during the Ice Ages.

Modern Production

China is currently the world's leading producer of gravel, displacing the USA as the world's chief supplier.

Etymology

The word comes from the French gravelle, meaning 'coarse sand'.

Types of gravel

Multiple types of gravel have been recognized, including:

  • Bank gravel: gravel intermixed with sand or clay.
  • Bench gravel: a bed of gravel located on the side of a valley above the present stream bottom, indicating the former location of the stream bed when it was at a higher level.
  • Creek rock: This is generally rounded, semi-polished stones, potentially of a wide range of types, that are dredged or scooped from river beds and creek beds. It is also often used as concrete aggregate and less often as a paving surface.
  • Crushed Rock: Rock that is mechanically broken into small pieces then sorted by filtering through different size mesh.
  • Crushed stone: This is generally limestone or dolomite that has been crushed and graded by screens to certain size classes. It is widely used in concrete and as a surfacing for roads and driveways, sometimes with tar applied over it. Crushed stone may also be made from granite and other rocks. A special type of limestone crushed stone is dense grade aggregate, or DGA, also known as crusher run. This is a mixed grade of mostly small crushed stone in a matrix of crushed limestone powder.
  • Fine gravel: gravel consisting of particles with a diameter of 1 to 2 mm.
  • Lag gravel: a surface accumulation of coarse gravel produced by the removal of finer particles.
  • Pay gravel: also known as "pay dirt"; a nickname for gravel with a high concentration of gold and other precious metals. The metals are recovered through gold panning.
  • Pea Gravel:gravel that consists of small, rounded stones used in concrete surfaces. Also used for walkways or driveways.
  • Piedmont gravel: a coarse gravel carried down from high places by mountain streams and deposited on relatively flat ground, where the water runs more slowly.
  • Plateau gravel: a layer of gravel on a plateau or other region above the height at which stream-terrace gravel is usually found.
  • River run gravel: naturally deposited gravel found in and next to rivers and streams.

References

See also

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