Eruptive hummocks: Building blocks of the upper ocean crust

Abstract

The spreading axis at many slow-spreading mid-ocean ridges is marked by an axial volcanic
\nridge. In this study, we use a combination of high-resolution remote sensing methods to elucidate
\nthe detailed nature of volcanoes in such a ridge. We fi nd that the “hummocks” described in
\nprevious sidescan sonar studies are dome- or cone-shaped edifi ces, 5–150 m high with diameters
\nof 30–330 m. We estimate they form quickly, in single eruptions, each of which may produce
\nseveral hummocks. Hummock collapse is common and hummocks of all heights are prone to
\nfailure. Collapses generally occur down the regional seafl oor slope, suggesting control by local
\ntopography. Approximately 33% of hummocks lose ~40% of their volume by collapse, so ~12%
\nof all material erupted on the axial volcanic ridge is rapidly converted to talus. The higher porosity
\nof these deposits may increase average upper crustal porosity by several percent, contributing
\n>0.5 km s–1 to seismic velocity decrease in the upper oceanic crust, and may be one of the
\ndominant mechanisms for increasing porosity in upper slow-spreading oceanic crust.