The size of a rock fragment determines its name, with geologists using precise measurements to categorize these pieces of earth. The term used to describe a small rock is not arbitrary but is part of a standardized geological classification system. This scale, known as the Udden-Wentworth scale, defines rock fragments based solely on their diameter. The continuum of rock fragment sizes provides a uniform language for scientists and engineers.
Classification of Sediment Particles
The smallest materials form the baseline of the size classification system. Clay is the smallest particle size, defined as anything less than 0.004 millimeters in diameter. These particles are too small to be seen individually without specialized equipment, and their size contributes to their ability to remain suspended in water for long periods.
Silt particles are slightly larger, ranging from 0.004 millimeters up to 0.0625 millimeters. Silt feels smooth and powdery when dry but lacks the plasticity of clay when wet. The next step up is sand, which spans from 0.0625 millimeters up to 2 millimeters, marking the approximate limit of what is easily visible to the human eye. These three smallest categories—clay, silt, and sand—are collectively known as fine-grained sediments.
The Standard Small Rock: Gravel and Pebbles
The size range for what is broadly considered small rocks begins with the category of gravel, encompassing all fragments larger than 2 millimeters in diameter. Gravel includes several subdivisions, with the smallest being granules, which measure from 2 millimeters to 4 millimeters. These are often the size of coarse sugar or small seeds.
Pebbles represent the most common type of small, handheld rock, defined by the Udden-Wentworth scale as having a diameter between 4 millimeters and 64 millimeters. A pebble is a piece of rock that fits easily in the palm of a hand and is frequently rounded and smoothed by natural processes like water transport. The term “pebble” is the most accurate answer for a loose, typical small rock found on a beach or riverbed.
Larger Fragments: Cobbles and Boulders
Moving up the size scale, the next classification is the cobble. Cobbles are defined as rock pieces that measure between 64 millimeters and 256 millimeters in diameter. This size marks the transition to fragments that are still manageable but too large to be comfortably held in one hand, roughly comparable to a tennis ball or a small loaf of bread.
Fragments larger than 256 millimeters are classified as boulders. Because a boulder’s diameter exceeds ten inches, it is generally too large for a single person to move manually.