Three Quaternary volcanic centres (Sverrefjell, Sigurdfjell and Halvdanpiggen) in northwestern Spitsbergen formed by off-ridge alkali basaltic volcanism associated with nearby ocean-floor spreading and subsequent tectonism. The Sverrefjell volcano is a unique stratovolcano constructed of primitive alkali basaltic pyroclastics with subordinate lavas. About 15-20 vol.% of this volcanic (and the other centres) comprises a wide variety of upper mantle and lower crustal xenoliths. A new estimate of the age of the Sverrefjell volcano places the time of formation at a minimum of 100,000-250,000 years ago, earlier than previous estimates. The occurrence of interbedded pillow lavas and pahoehoe flows at all levels of Sverrefjell suggests that the volcano grew concurrently with the rise of the surrounding glaciers. Basaltic sequences occurring as nearby mountain cappings represent earlier volcanism (2.5-11.5 Ma), also probably a result of thermal disturbance related to nearby ocean-floor spreading. Geothermobarometry calculations on xenoliths from the three Quaternary localities define a very high geotherm consistent with their location near the Yermak hot spot, and provide a good datum point for the geothermal gradient adjacent to such a thermal anomaly.