Studies of relatively young volcanic centers were conducted in a variety of areas during this quadrennium: on the island of Hawaii [ Hildenbrand et al., 1993]; at Long Valley caldera, California [ Miyazaki, 1991a, 1991b]; at Portland, Oregon [ Blakely et al., 1995]; in New Zealand [ Rout et al., 1993; Woodward and Mumme, 1993]; and in West Antarctica [ Blankenship et al., 1993; Behrendt et al., 1991, 1994].
The ability of the magnetic method to define concealed structures was perhaps best demonstrated by the aerogeophysical study of the West Antarctic rift system conducted by Blankenship et al. [1993]. Their study was part of the CASERTZ program (Corridor Aerogeophysics of the Southeastern Ross Transect Zone). They discovered a variety of spatially coincident features: a depression in the surface of the West Antarctic ice sheet , a peak in subglacial topography, a 600-nT positive aeromagnetic anomaly, and a broad positive gravity anomaly. Blankenship et al. [1993] concluded that these diverse data reflect an active volcano, part of a 23-km-wide caldera situated at the base of the ice. The depression in the surface of the ice sheet apparently is caused by recent volcanism; i.e., increased heating by the volcano at the base of the ice is matched by an increased flux of ice toward the volcano from above. Such geothermal features potentially could modify the dynamics of the West Antarctic ice sheet by, for example, modifying the water supply available for saturating the underlying sediments responsible for ice streaming. Monitoring the geothermal flux of the West Antarctic rift system and mapping its regional tectono-magmatic framework are important factors in understanding the position and character of ice streams and their control on ice sheet stability [ Blankenship et al., 1993].
Hildenbrand et al. [1993] produced a comprehensive analysis of aeromagnetic data over the Island of Hawaii. Aeromagnetic data are useful here for delineating the lateral extent of shield structures, such as rifts, summit calderas, pit craters, and vent fissures. The active East Rift of Kilauea volcano is reflected in the aeromagnetic data but becomes especially clear when the regional anomaly caused by the topographic expression of the island is removed. Magnetic lows that parallel the rift may reflect chemical alteration of magnetic minerals by hydrothermal fluids. Using spectral analysis, Hildenbrand et al. [1993] determined that the magnetic lows originate at depths of about 1 km beneath the rift, just above a concentration of earthquake hypocenters at depths of 2 to 4 km thought to be caused by a magma conduit. Other linear magnetic lows may indicate the location of ancestral rifts on Hawaii.