Gravel is a widely used material, often seen in driveways, roads, and construction projects. It consists of loose fragments of rock, typically found in various sizes and shapes. This versatile material forms naturally through geological processes or can be produced artificially through mechanical means.
Gravel as a Size Classification
Gravel is not a specific type of rock, but rather a classification based on the size of individual rock fragments. Geologists and engineers define gravel by its particle diameter, distinguishing it from finer materials like sand or coarser ones like cobbles and boulders. Generally, gravel particles range in size from about 2 millimeters (0.08 inches) to 64 millimeters (2.5 inches) in diameter. This size classification means gravel can include a variety of rock types within its composition, depending on the local geological environment.
Common Rock Components of Gravel
The actual rock types that make up gravel depend heavily on the geology of the area where it originated. Igneous rocks, formed from cooling magma or lava, frequently contribute to gravel. Granite, with its interlocking mineral grains, and basalt, a dense volcanic rock, are common examples due to their hardness and resistance to weathering.
Sedimentary rocks, formed from the accumulation and compaction of sediments, also often become gravel. Limestone, composed primarily of calcium carbonate, and sandstone, made from cemented sand grains, are frequently found in gravel deposits. These rocks can be quite durable depending on their composition and cementation. Metamorphic rocks, which have been transformed by heat and pressure, are another significant component. Quartzite, a very hard and durable rock formed from sandstone, and gneiss, a banded metamorphic rock, contribute significantly to gravel due to their robust nature.
The Formation of Gravel
Gravel forms through both natural geological processes and human-engineered methods. Naturally, larger rocks are broken down into smaller fragments through weathering, which involves physical and chemical processes like freezing and thawing, or chemical reactions with water and air. Once broken, these rock fragments are then transported by erosional forces such as flowing water in rivers, moving ice in glaciers, or strong winds. During this transport, the sharp edges of the rock fragments are gradually rounded as they collide with other particles and the surrounding environment.
Rivers are particularly effective in creating rounded gravel as they tumble and abrade rock pieces downstream. Glaciers also grind and transport vast quantities of rock, depositing them as glacial till, which often contains a wide range of gravel sizes. Gravel is also produced artificially by crushing larger rocks obtained from quarries. Mechanical crushers break down bedrock into specific sizes, creating angular gravel that is often used in construction for its interlocking properties.