All rise for the case of the missing magma

Abstract

A detailed geological analysis of a ridge in the Indian Ocean suggests that compositional variations in Earth's mantle have a surprisingly crucial role in the uplift of a bathymetric bulge along the ridge. See Article p.195 The Marion Rise, a 3,100-kilimetre-long stretch of the Southwest Indian Ridge beneath the Southern Indian Ocean, is as large as the more extensively studied Icelandic Rise, but has a deep rift valley along most of its length rather than an axial 'high'. Oceanic rises are elevated regions that are normally associated with mantle hotspots and a thickened crust, indicating that they are the result of thermal anomalies in the mantle. Huaiyang Zhou and Henry Dick now show that, surprisingly, the crust is generally thin and often missing on the Marion Rise. They conclude that the rise must be largely an isostatic response to ancient melting events that created low-density depleted mantle beneath the ridge.