What is an isopach map primarily used to illustrate?

An isopach map is specifically designed to illustrate areas of equal thickness across a surface, typically in relation to geological or engineering contexts. This type of map utilizes contour lines to represent the thickness of a particular layer, such as sediment, rock, or other materials, which can be crucial for resource management, construction projects, and geological studies. By showing variations in thickness, an isopach map allows users to understand spatial distributions and make informed decisions based on this information.

The other choices do not align with the definition of an isopach map. Areas of uniform temperature pertain more to isotherm maps, which focus on temperature distributions. The representation of contours of elevation is characteristic of topographic maps, while variability in soil types would typically be illustrated using soil maps. Thus, B stands out as the correct choice for the function of an isopach map.