Aeromagnetic anomaly maps show variations in the Earth’s magnetic field that are caused mainly by magnetic minerals in crustal rocks. These anomalies vary with amount and type of magnetic material and the geometry and depth of the magnetic body. Igneous and metamorphic rocks frequently are magnetic enough to cause anomalies, whereas sedimentary rocks commonly are nonmagnetic. Aeromagnetic anomaly maps are important tools for mapping surficial and buried rocks, for determining geologic structure, and for discovering some types of mineral deposits. Regional geologic features may become evident after individual aeromagnetic surveys are compiled and plotted at the same scale in a consistent way.