Mid-ocean ridges were categorised primarily by their spreading rate. Each class is characterised by typical morphology, tectonics and volcanism. With decreasing spreading rate, not only the amount of produced melt decreases, but also the spatial and temporal continuity of magmatism. For very small effective rates of less than 12 mm/yr total spreading, melt supply is highly focused. Thus, the formation of basaltic crust occurs only at discrete volcanic centres; in between dominate amagmatic segments, where mantle material is emplaced directly at the sea floor. The approx.1800 km long Gakkel Ridge in the central Arctic Ocean shows not only very low full spreading rates of 6-14.5 mm/yr, but also a remarkable absence of major transform faults and oblique spreading segments. The Gakkel Ridge is therefore an ideal example to study the typical characteristics of these so-called ultra-slow spreading ridges. In this contribution, we present the results of geophysical and petrological studies on the western Gakkel Ridge. Wide-angle seismic measurements in the central valley and a three-dimensional gravity modelling of the central valley, the rift flanks and parts of the adjacent basins provide information about the structure of the oceanic crust and the upper mantle. Regional and local magnetic data as well as the determination of susceptibility and natural remanent magnetisation of selected basalt samples provide insights into the dominant magmatic processes. In the so-called "Western Volcanic Zone (WVZ)" of the ridge, the formation of sea floor began about 35 Ma ago, the robust volcanism found there is more typical for faster spreading ridges. Crustal thickness of up to 6 km and a continuous positive magnetic anomaly over the central valley support models of stable melt supply. In the older part of the ridge east of 3.5° W, where spreading persists for 55 Ma, an alternation of magmatic and amagmatic segments typical for ultra-slow spreading ridges was found. The crust is mostly 1-4 km thin and the magnetic anomalies are generally weak, a positive anomaly over the central valley was found only at the volcanic centres. The border between the two different spreading regimes is very pronounced in the geophysical data sets, the analysis of rock samples indicates a similarly sharp boundary in the upper mantle.