The style of oceanic crustal formation in the North Atlantic is controlled by interaction between the Iceland mantle plume and the lithospheric spreading. There are three main tectonic regimes comprising: (a) oceanic crust formed without fracture zones, with spreading directions varying from orthogonal up to 30° oblique to the ridge axis; (b) oceanic crust with a normal slow-spreading pattern of orthogonal spreading segments separated by fracture zones; and (c) 20–35 km thick crust generated directly above the centre of the mantle plume along the Greenland-Iceland- Fseroe Ridge. I show that the main control on the tectonic style is the temperature of the mantle beneath the spreading axis. A mantle temperature increase of as little as 50 °C causes an increase of about 30% in the crustal thickness, and thereby allows the mantle beneath the crust at the ridge axis to remain sufficiently hot that it responds to axial extension in a ductile rather than a brittle fashion. This generates crust without fracture zones and with an axial high rather than a median valley at the spreading centre. Using gravity, magnetic, bathymetric and seismic refraction data I discuss the mantle plume temperatures and flow patterns beneath the North Atlantic since the time of continental breakup, and the response of the crustal generation processes to these mantle temperature variations.