The Krumbein phi scale ranks sediment particle sizes (sand, silt, clay) logarithmically, with larger particles having lower numbers. A boulder, with a phi value ≤ -8, is at least 10 inches (256mm) in diameter. This scale helps geologists compare particle sizes across different sediment types.
The image you sent is a screenshot of the Wikipedia page for the Krumbein phi scale. The Krumbein phi scale is a logarithmic scale that geologists use to classify the size of particles in sediment, such as sand, silt, and clay. The scale is based on the diameter of the particles, with larger particles having lower phi values than smaller particles.
The formula for the Krumbein phi scale is:
φ = -log₂(D)
where:
* φ is the phi value of the particle
* D is the diameter of the particle in millimeters
According to the Wikipedia page, a boulder is defined as a rock that has a phi value of -8 or less. This means that the diameter of a boulder is at least 256 millimeters, or about 10 inches.
The Krumbein phi scale is a useful tool for geologists because it allows them to compare the size of particles from different types of sediments. For example, a geologist might use the phi scale to compare the size of sand particles in a beach deposit to the size of sand particles in a river deposit.