Gravel particles are considerably larger than sand particles. This size difference is based on a standardized scientific framework used across geology and engineering to classify earth materials. Understanding this size distinction requires examining the precise boundaries of particle diameter established by sedimentologists, where the exact measurement determines the classification within a consistent hierarchy of increasing diameters.
Sediment Classification and Particle Size Hierarchy
Scientists use a systematic approach, most commonly the Udden-Wentworth scale, to categorize sediments based on their grain size. This scale establishes a universal language for geologists, engineers, and soil scientists to accurately communicate about earth materials. The framework organizes clastic sediments into classes that range from the microscopic to the massive, where each class boundary is defined by a specific diameter in millimeters.
The size classes follow a sequential order, beginning with the smallest particles like clay and silt, moving up to sand, and then progressing to gravel. This classification is often geometric, meaning each grade is roughly twice as large as the preceding one. This hierarchy ensures that every piece of natural sediment fits into a designated size category. The largest particles, such as cobbles and boulders, occupy the highest levels of this size hierarchy.
Defining the Size Range of Sand
Sand is formally defined as any sediment particle with a diameter falling between 0.0625 millimeters and 2.0 millimeters. This size range is significant because the lower boundary separates sand from the finer material known as silt, which is too small to be seen easily without magnification. Particles at the upper end of the sand scale are just at the threshold of becoming gravel.
Within this defined range, sand is further subdivided into five grades, such as very fine, medium, and very coarse sand. For instance, very fine sand is categorized as particles between 0.0625 mm and 0.125 mm. Very coarse sand ranges from 1.0 mm up to the 2.0 mm limit.
Defining the Size Range of Gravel
Gravel begins precisely where the size range for sand ends, starting at a minimum diameter of 2.0 millimeters. The upper boundary for gravel is extensive, reaching up to 4096 millimeters, though it is commonly subdivided at 64 millimeters to separate it from even larger stones. This broad size class encompasses a wide variety of rock fragments and is often broken down into several sub-classifications.
The smallest gravel particles, known as granules, range from 2.0 mm up to 4.0 mm in diameter. Moving up the scale, the next major sub-class is pebbles, which are defined as fragments between 4.0 mm and 64 mm. Beyond the pebble size, the sediment transitions into the larger categories of cobbles, which extend from 64 mm to 256 mm, and then boulders, which are the largest classified sediment particles.