A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breakerexpedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridgesystem, predictions were that magmatism should progressively diminish as the spreading rate decreases along the ridge, and thathydrothermal activity should be rare. Instead, it was found that magmatic variations are irregular, and that hydrothermal activity isabundant. A 300-kilometre-long central amagmatic zone, where mantle peridotites are emplaced directly in the ridge axis, liesbetween abundant, continuous volcanism in the west, and large, widely spaced volcanic centres in the east. These observationsdemonstrate that the extent of mantle melting is not a simple function of spreading rate: mantle temperatures at depth or mantlechemistry (or both) must vary significantly along-axis. Highly punctuated volcanism in the absence of ridge offsets suggests thatfirst-order ridge segmentation is controlled by mantle processes of melting and melt segregation. The strong focusing of magmaticactivity coupled with faulting may account for the unexpectedly high levels of hydrothermal activity observed.