Rock Properties: Porosity and Permeability Explained

What determines the ability of a rock to hold water in its cracks and spaces?

A. Density and hardness

B. Color and texture

C. Porosity and permeability

D. Shape and size

Answer:

C. Porosity and permeability

The rock's ability to hold water depends on its porosity and permeability, the volume and connectivity of spaces within the rock, as well as the cohesive and adhesive properties of water that enable capillary action.

Explanation:

The ability of a rock to hold water in its cracks and spaces is a function of the rock's porosity and permeability. Porosity refers to the volume of spaces (pores and cracks) within the rock where water can be stored and depends on the size and shape of the rock's grains and how they are arranged.

On the other hand, permeability describes how easily a fluid can move through those spaces. Highly porous and permeable rocks like sandstones and certain types of limestone can hold and transmit substantial amounts of water. This makes them ideal for forming groundwater aquifers. Water's cohesive and adhesive properties play a role in this capacity, as these forces help water to 'cling' to the rock and move through its pores. Cohesion refers to the attraction between water molecules due to hydrogen bonding, while adhesion is the attraction between water molecules and other substances, including the minerals that make up a rock. These properties allow water to overcome gravitational pull and rise upward in narrow rock fissures or pores, a phenomenon known as capillary action.