The dense root of the Iceland crust

Bathymetry and topography in the North Atlantic Ocean around Iceland are compared to estimates of crustal thickness in the area. Iceland lies much lower than expected based on crustal thickness. This suggests an anomalously low density contrast between crust and mantle beneath Iceland [W. Menke, Geophys. Res. Lett. 26 (1999) 1215–1218]. The relationship between bathymetry and depth to Moho along ridges adjacent to Iceland suggests a normal density contrast there. Continuity of this relationship leads to the conclusion that most of the change occurs in the crust, i.e. the abnormally low density contrast between crust and mantle within Iceland is primarily due to a heavy crust, not light mantle. Gravity modeling also requires the average density of the crust to be unusually high. I argue that this is in fact not abnormal. The lower crust in Iceland is denser because of a higher degree and/or depth of melting beneath Iceland than at adjacent ridges, because of phase transformations occurring within the thick crust and possibly due to fractionation processes in the crust.