Salt and gypsum underlie about ____% of the contiguous United States?

• A. 20%
• B. 60%
• C. 40%
• D. 80%

Answer: (C)

Salt and gypsum form as evaporites when seawater or lake water in closed basins concentrates and evaporates, leaving minerals like halite and gypsum behind. Over geologic time, large arid basins across what is now the contiguous United States accumulated these thick evaporite sequences, especially in the western regions, so they blanket a substantial portion of the subsurface. The estimate that best matches geologic mapping is that evaporite rocks underlie roughly forty percent of the contiguous U.S. This widespread occurrence has practical implications for groundwater flow and engineering, since salt- and gypsum-rich layers can influence stability and permeability in the subsurface.