An aeromagnetic study of the Island of Hawaii provides new insight on magnetic properties of subsurface rock and geologic structure. On a regional scale, spectral-depth analysis delineates two shallow magnetic zones, each roughly 1.5 km thick, lying at a depth of 1 km. One zone (of unknown origin) lies in the center of the island and correlates with a regional magnetic high. The other zone coincides with pronounced magnetic lows paralleling Kilauea's active east rift zone. These magnetic lows probably depict rocks chemically altered by hydrothermal fluids, in which titanomagnetite has been destroyed. Analysis of magnetic terrain effects indicates that magnetization also decreases with depth within Mauna Kea and Mauna Loa. We estimate that magnetization is reduced by about half at a depth of 1 km. The magnetic method is particularly useful in delineating the lateral extent of local shield structures, such as rifts, summit calderas, pit craters, and vent fissures. Rifts possess characteristic magnetic patterns, primarily long-wavelength linear magnetic low zones. We propose that alteration processes reduce magnetizations along the flanks of rifts. On the other hand, along young rifts (e.g., Kilauea's east rift zone), short-wavelength magnetic anomalies probably reflect slowly cooled, unaltered instrusions. Altered rock may also produce magnetic lows that help define buried summit calderas.