Coarse-grained material porosity is typically:

• A. Low porosity
• B. High porosity
• C. Moderate porosity
• D. Variable porosity

Answer: (A)

Porosity is the fraction of a rock or sediment’s volume that is void space. With coarse-grained material such as sand and gravel, the grains are larger and tend to pack more efficiently, leaving fewer total void spaces per unit volume. Although the gaps between large grains can be sizable, there aren’t as many of them in the overall volume as there are micro-voids among fine particles. Because of this, the total pore space is typically smaller overall than in fine-grained materials, making porosity lower on average for coarse-grained sediments. (Note that permeability can be high even with lower porosity because the larger pore channels transmit fluid more readily.)