We report recent mapping of Spiess Ridge with the Hawaii-MR1
sidescan sonar. Spiess Ridge is an unusual elongate 90 by 50
km volcanic feature on the slow spreading Southwest Indian
Ridge near the Bouvet hotspot. The northwest half of the ridge
has a narrow ~7-km neovolcanic zone in MR1 sonar images and
a simple magnetic anomaly sequence including possible
anomalies C1n and C2n. In contrast, the southeast half shows
extrusive volcanism over >40 km with distributed eruption sites,
a broadened central magnetic anomaly with no anomaly C2n,
and a volcanic ridge radiating from the summit of Spiess Ridge,
oblique to the spreading orthogonal trend. The images show no
evidence for large-offset normal faults or an axial rift valley
typical of slow spreading ridges. Overall, Spiess Ridge has an
appearance very unlike that of either fast or slow spreading
ridges and more transitional between a spreading ridge and a
seamount. We compare the morphology of Spiess Ridge to
other large volcanic structures in oceanic rifts. Spiess Ridge and
Bouvet Island represent localized zones of excess melting along
the Southwest Indian Ridge and we discuss their origin in the
light of current ideas on ridge-hotspot interaction.
Mitchell, N. C., and R. A. Livermore, Spiess Ridge: An axial high on the slow-spreading
Southwest Indian Ridge,
J. Geophys. Res., 103, 15457-15471, 1998.