An area's potential for geologic hazards may also be assessed from a geologic map. Likewise, earthquakes are most likely to occur in areas where faults displace recent rocks or deposits, such as river gravels. High concentrations of radon gas are commonly associated with rocks, such as particular types of granite, that have an anomalously high uranium content. Determining the suitability of a site for a large construction project, such as a bridge, requires an adequate knowledge of the rock types that compose the foundation. A geologic map shows the surface rocks types and suggests which rock types lie at depth.
Geologic maps are an effective way to convey large amounts of geologic information about an area.
The geologic history of an area is recorded by the types of rocks deposited there and by the geologic relationships between those rocks. A geologic map might show, for example, that a limestone was formed under the sea, but was later uplifted and covered by lava flows. This is exactly what the Geologic Map of Arizona implies by the proximity of the pink and light-blue areas in northern Arizona. On the Geologic Map of Arizona, the Colorado Plateau includes the large region shown in light blue and various shades of green. The southern boundary of the Colorado Plateau is the Mogollon Rim, which is represented on the map by the southern limit of the light-blue color.