Scientists divide the Earth’s naturally occurring materials into three main groups: igneous, metamorphic, and sedimentary rocks. Understanding where a common substance like gravel fits requires applying the specific language of geology. Geological classification is based not on appearance, but on the method of formation. This explanation clarifies the difference between loose sediment and solid rock and defines gravel within this system.
What is Gravel?
Gravel is defined strictly by its physical properties, specifically its size, and is a collection of loose, unconsolidated sediment. It is not a rock in the geological sense because it is an aggregate of separate fragments lacking a permanent, cohesive bond. The fragments that make up gravel are known as clasts, which are pieces of rock broken down from larger sources.
Geologists use the Udden-Wentworth grain size scale to classify sediments, designating gravel as any clast larger than 2 millimeters in diameter. This category includes smaller granules, pebbles (which range up to 64 millimeters), and even larger cobbles and boulders. The composition of these clasts can be highly variable, often reflecting the types of bedrock found in the source area. Gravel may be composed of durable minerals like quartz, or fragments of parent rock such as granite, basalt, or limestone.
The Formation and Definition of Sedimentary Rock
Sedimentary rock is defined by the process of its creation, which transforms loose material into a solid mass. This process begins with the breakdown of pre-existing rocks through weathering, followed by erosion and transport of the resulting fragments. These sediments are eventually deposited in layers, often in basins, riverbeds, or ocean floors.
As more material accumulates on top, the pressure from the overburden squeezes the lower layers, a process called compaction. This action reduces the space between the grains and expels trapped water. The most important step in the formation of sedimentary rock is lithification, which means “turning into stone.”
Lithification is completed by cementation, where minerals dissolved in groundwater precipitate into the remaining pore spaces between the sediment grains. These mineral crystals, such as calcite, silica, or iron oxides, act as a natural glue, binding the loose clasts together into a cohesive, structurally sound rock mass. This final, permanent binding distinguishes a sedimentary rock, like shale or sandstone, from a pile of loose sediment.
Gravel: A Classification of Size, Not Rock Type
The distinction between gravel and sedimentary rock lies in whether the material is classified by its dimension or by its formation. The term “gravel” is a size-based classification for sediment particles larger than 2 millimeters, regardless of what they are made of or where they are located. Conversely, the term “sedimentary rock” describes a material that has undergone the entire chemical and physical process of lithification.
Gravel is considered a type of sediment, which is the raw material used to make a sedimentary rock. Loose gravel lacks the cementation required to be classified as a rock, possessing neither the structural integrity nor the permanence of a lithified formation. Without the mineral cement to bind the clasts, the material will easily separate when handled or subjected to natural forces.
Therefore, gravel cannot be a sedimentary rock because it is defined by its state as an unconsolidated, fragmented material. The name describes the size of the component pieces, while the name of the rock describes the process that permanently fused those pieces together. Loose gravel is simply the precursor to a rock, representing a transitional stage in the greater rock cycle.
When Gravel Becomes Rock
When gravel-sized particles are subjected to deep burial, compaction, and cementation described by the process of lithification, they form specific types of sedimentary rock. The resulting rock is generally classified based on the shape of the original gravel clasts. The most common rock formed from lithified gravel is conglomerate.
Conglomerate is characterized by rounded clasts, indicating that the gravel traveled a significant distance from its source, allowing water or wind to smooth its edges. If the original gravel clasts are angular and sharp, the resulting rock is called breccia. This suggests the material was deposited and lithified quickly, close to where it was first broken down. The binding agents responsible for turning the loose gravel into these solid rocks are typically mineral compounds like silica, calcium carbonate, or iron oxide cements.