of the chain lies Mount Steere with a summit elevation of and a rectangular summit caldera. Lie Cliff is a volcanic outcrop on the northeastern flank. Mount Steere is heavily dissected, bears evidence of former glaciation in the form of moraines and cirques have been eroded into its northern and northeastern flanks. As with Mount Rees, volcanic rocks alternate between these that formed subglacially and these formed subaerially. Outcrops feature breccias and lava with numerous intruded dikes. South of Mount Steere is Mount Frakes, with it is the highest peak in the range and the least eroded of the Crary Mountains. It has a wide circular summit caldera and unlike Mount Rees and Mount Steere it bears no evidence of subglacial eruptions, probably due to lack of erosion that could have exposed them. Volcanic rocks crop out both on the southern and western flank, at Morrison Rocks and English Rock respectively. These outcrops are cinder cones that formed on the slopes of Mount Frakes. Volcanic and nonvolcanic boulders on the slopes of Mount Frakes may either be xenoliths or glacial erratics. Boyd Ridge is smaller than the other three volcanoes and located southeast of Mount Frakes and reaches an elevation of . Runyon Rock crops out to its east and is the only area of Boyd Ridge not covered by ice. Cinders and a cliff of hyaloclastite are found there. The volcanoes rise from a platform formed by lava flows and pyroclastic rocks. This platform lies at about elevation and the Boyd Ridge may be a southeastward extension thereof. It appears that the platform — which crops out only on the eastern side of the Crary Mountains — was tilted westward by faulting. Echo and magnetic sounding have imaged the root of the Crary Mountains in the West Antarctic Ice Sheet, finding that the underlying terrain is steep and flanked by narrow troughs. The mountains are associated with a strong magnetic anomaly that may reflect subglacial rocks containing magnetite. Geology Cenozoic volcanism in Marie Byrd Land is related to the West Antarctic Rift and has been explained by the activity of a mantle plume. This plume either underlies Marie Byrd Land and its volcanoes, or it rose to the surface before Antarctica separated from New Zealand during the middle Cretaceous and induced volcanism across the continental borderlands of the Southwest Pacific. In the latter theory, the Marie Byrd Land volcanism