Gravel constitutes a loose aggregation of rock fragments, occurring naturally on Earth due to sedimentary and erosive geological processes; additionally, commercial production yields considerable quantities as crushed stone. This basic raw material finds diverse applications.
Gravel Definition and Physical Properties
The technical definition of gravel varies by region and application, though geologists define gravel simply as loose, rounded rock particles exceeding 2 mm (0.079 in) in diameter. The Udden-Wentworth scale classifies gravel into granular (2–4 mm) and pebble gravel (4–64 mm). The American construction industry distinguishes natural gravel from crushed stone, produced mechanically.
- Particle Size and Classification: ISO 14688 grades gravels as fine (2-6.3 mm), medium, and coarse (20-63 mm).
- Bulk Density: The bulk density of gravel ranges from 1,460 to 1,920 kg/m3.
- Hydraulic Conductivity: Natural gravel exhibits high hydraulic conductivity, sometimes surpassing 1 cm/s, which makes them excellent aquifers.
Gravel Origin and Formation
Most gravel originates from bedrock disintegration through weathering processes. Quartz commonly constitutes the prevalent mineral in gravel, exhibiting hardness, chemical inertness, and absence of cleavage planes. Rock fragments round rapidly during river transport, often within tens of kilometers from source outcrops.
- Deposition Environments: Gravel deposition occurs in stream channels as gravel blankets, alluvial fans, near-shore marine settings, and deltas of swift-flowing streams.
- Aeolian Processes: Wind-formed (aeolian) gravel “megaripples” in Argentina may parallel features on Mars.
Gravel Production and Commercial Use
Gravel serves as a major raw material in construction, yet official statistics frequently combine sand and gravel, distinguishing crushed stone as a separate category. In 2020, sand and gravel accounted for 23% of U.S. industrial mineral production, with approximately 960 million tons of construction sand and gravel produced, exceeding industrial varieties. Almost half finds use as aggregate for concrete.
- Applications: Road construction utilizes gravel extensively, either as road base or blacktop. Construction fill and other minor applications also consume significant quantities.
- Availability and Economic Factors: Gravel’s wide distribution, predominantly in river deposits, flood plains, and glacial deposits, means environmental considerations and quality determine economic viability against alternatives like crushed stone. Recycled gravel gains importance, with crushed stone already displacing natural gravel in the eastern United States.
Gravel Types and Composition
Different varieties distinguish by their composition, origin, and use cases.
- Bank Gravel: Naturally deposited gravel intermixed with sand or clay, found in and next to rivers.
- Bench Gravel: Gravel beds located on valley sides above current stream bottoms, marking former stream bed locations.
- Crushed Stone: Rock crushed and graded, used for road surfacing.
- Fine Gravel: Gravel consisting of particles with diameters between 2 and 6.3 mm.
- Lag Gravel: Surface accumulations of coarse gravel due to finer particle removal.
- Pay Gravel: Nickname for gravel rich in gold and other precious metals.
- Pea Gravel: Clean gravel similar in size to garden peas, for concrete surfaces and walkways.
- Piedmont Gravel: Coarse gravel carried down from high places by mountain streams.
- Plateau Gravel: Gravel layer on a plateau, above stream-terrace gravel heights.
- Shingle: Coarse, loose, well-rounded sediment composed of smooth pebbles and cobbles.
Gravel and Plant Life
In areas with predominant gravelly soil, plant life tends to be sparse because gravels retain less moisture and have fewer mineral nutrients. This leads to challenging conditions for vegetation.
Gravel in the Geologic Record
Sediments containing over 30% gravel, lithified into solid rock, classify as conglomerate. Conglomerates are widely distributed in sedimentary rocks but are usually a minor component. Alluvial fans likely contain the largest gravel accumulations.
Citations:
Gravel (/ˈɡrævəl/) is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentary and erosive geological processes; it is also produced in large quantities commercially as crushed stone.
Gravel is classified by particle size range and includes size classes from granule- to boulder-sized fragments. In the Udden-Wentworth scale gravel is categorized into granular gravel (2–4 mm or 0.079–0.157 in) and pebble gravel (4–64 mm or 0.2–2.5 in). ISO 14688 grades gravels as fine, medium, and coarse, with ranges 2–6.3 mm (0.079–0.248 in) for fine and 20–63 mm (0.79–2.48 in) for coarse. One cubic metre of gravel typically weighs about 1,800 kg (4,000 lb), or one cubic yard weighs about 3,000 lb (1,400 kg).
Gravel is an important commercial product, with a number of applications. Almost half of all gravel production is used as aggregate for concrete. Much of the rest is used for road construction, either in the road base or as the road surface (with or without asphalt or other binders.) Naturally occurring porous gravel deposits have a high hydraulic conductivity, making them important aquifers.
English
Etymology
From Middle English gravel, grauel, from Old French gravele, diminutive
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