A kinematic model of a volcanic rift zone, based on plate tectonics concepts of crustal accretion, is presented. Quantitative relationships between observable parameters of the model are tested by a comparison with estimates from Iceland of the rate of production of extrusive rocks by eruptions, the width of the volcanic zone, the drift velocity of the lithospheric plates, the regional dips of the flood basalts and the rate of increase of dyke volume fraction with depth in the eastern Iceland lava pile.Approximate calculations of surface heat flow in the volcanic zone of the model and the adjacent lithospheric plates have been made and compared with heat flow observations from various parts of Iceland.The model appears to describe fairly well certain regional structural properties of the Icelandic lava pile, such as regional dips of the flood basalts and the average dyke distribution with depth. It is also compatible with the general pattern of heat flow values in Iceland. It is necessary, however, to assume that crustal accretion has shifted between at least two zones during the time involved in building up the Icelandic lava pile.

Summary Gravity data from Iceland and its surroundings are analysed and modelled with respect to seismic data. A Bouguer gravity map of Iceland is recomputed based on admittance between the topography and the gravity and with corrections for glacial ice sheets. From seismic data and with the help of relations between the residual topography and the depth to seismic boundaries we construct maps of the main seismic boundaries, including the Moho. By inversion calculations we recomputed these maps, assuming different density values for Seismic Layer 4 to fit the observed gravity field. We found that the average density of Layer 4 has to be in the range 3050–3150 kg m−3 in order to fit both seismic and gravity data. Thus we conclude that Layer 4 is a transition zone between the mantle and the oceanic crust in Iceland. Furthermore by assuming that the upper‐mantle density variations necessary to compensate for the gravity effect of crustal layers, are caused by thermal variations in the upper mantle, we calculate the depth to the 1200 °C isotherm to be at 30–50 km depth below Iceland but rising up to less than 20 km below parts of the volcanic zone in Northern Iceland. We conclude that the temperature within the Seismic Layer 4 is close to 600 °C at its top, increasing to approximately 950 °C at its bottom (Moho), which makes a widespread layer of partially molten material within Layer 4 unlikely. By use of cross spectral analysis of the gravity field and the external topographic load at short wavelengths, we conclude that the elastic plate thickness in Iceland can hardly exceed 6 km. In addition we point out that the residual isostatic anomalies have circular forms east of the eastern volcanic zone but are near parallel to the ridge axis on the western side. This form of the anomalies may be caused by pressure from the eastward moving mantle plume below the eastern volcanic zone.

During the early 1960s, when the idea of seafloor spreading was crystallizing out of geophysical data gathered over oceanic areas, similar ideas were beginning to take shape in Icelandic geology, but on the basis of a different kind of data. Geological studies of the structure of the eastern Iceland Tertiary lava pile led Walker (1959, 1960) to suggest that regional dips of the lavas were due to sagging accompanying lava deposition in the active volcanic zone. These ideas were further elaborated by Bodvarsson and Walker (1964), who tried to estimate the amount of crustal drift that might have accompanied the injection of dikes as observed in the eastern Iceland lava pile, taking into account the increase in dike fraction with depth in the crust. Subsequent studies of the Tertiary lava pile in other parts of Iceland have essentially confirmed a structure analogous to that given by Walker for eastern Iceland (Saemundsson, 1978). Observations from two distinct geological regions can be brought to bear on the process of crustal accretion in Iceland. One region is the present active zone of rifting and volcanism, the axial rift zone. The other region is the flanking Tertiary lava pile where the internal structure of the uppermost well-developed crust can be observed to a depth of 1–1.5 km in valleys that have been carved into the crust by glacier action. In the axial rift zones the constructional processes of extrusive volcanism, faulting, and Assuring take place as discrete but closely associated events.

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