The term “boulder” is commonly used to describe any rock too large for a person to lift. However, in the earth sciences, the word has a specific and precise definition based on size. Geologists and civil engineers rely on formal classification systems, known as sedimentology, to categorize sediment particles. This systematic approach ensures universal communication about particle size, which is fundamental for understanding how clastic, or broken, rock fragments were transported and deposited.
Defining the Minimum: Standardized Sediment Classifications
The universally accepted boundary for a particle to be scientifically classified as a boulder is determined by a measurement scale known as the Udden-Wentworth scale. This scale, developed in the early 20th century, classifies sediment based on a doubling series of particle diameters. A particle must have a minimum diameter of 256 millimeters to officially earn the classification of a boulder.
This minimum size is equivalent to approximately 10.1 inches. The 256 mm measurement acts as a strict lower limit, differentiating boulders from the next largest class of sediment. The size of the particle is typically based on the length of its intermediate axis, the middle dimension of the three principal axes.
While the lower limit is precisely 256 mm, the boulder classification itself includes a vast range of sizes. Some sedimentologists further subdivide the boulder class into small, medium, large, and very large categories to better organize extreme examples. For instance, a large boulder may measure over 1,000 millimeters (one meter) in diameter, while classifications have been extended to include particles up to 4,096 millimeters (about 13.4 feet).
The Scale Below: Cobbles, Pebbles, and Gravel
Sediments immediately smaller than a boulder are still part of the Udden-Wentworth scale. All clastic particles larger than two millimeters are grouped under the broad term “gravel,” which is split into several distinct size classes. Just below the boulder class are cobbles, which range from 64 millimeters up to the 256-millimeter threshold.
Cobbles are roughly the size of a loaf of bread. Smaller than cobbles are pebbles, which range from 4 millimeters up to 64 millimeters. Pebbles include fragments from the size of a small pea up to that of a tennis ball.
The smallest particles within the gravel category are granules, defined as having a diameter between 2 and 4 millimeters. These sizes are the coarsest material found in an average bag of gravel aggregate. The transition between these classes reflects how the transport energy of water or ice dictates the resulting sediment size found in a deposit.
Practical Classification and Extreme Examples
Measuring the size of an irregular boulder in the field presents a practical challenge. Since most large rock fragments are not perfect spheres, geologists estimate the particle’s size by measuring its three principal dimensions: the long axis, the intermediate axis, and the short axis. The intermediate axis is the standard measure used to assign the particle to its size class, such as the 256 mm minimum for a boulder.
In engineering and commercial contexts, the size is often determined by the narrowest dimension, as this dictates the size of the opening the boulder can pass through. When an extremely large particle of boulder size is angular instead of rounded, geologists may classify it as a “block” to indicate that it has undergone less transport and abrasion.
While the minimum size is fixed, there is no technical upper size limit for a boulder, and some natural examples can be massive. Glacial erratics, for instance, are boulders transported by ice sheets and deposited far from their source rock, sometimes reaching the size of small houses. The term “megaboulder” is informally used in some geological literature to describe these detached, exceptionally large clasts that still adhere to the classification’s lower limit.