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Regional telluric-magnetotelluric measurements performed as a cooperative research project between Brown University and the National Energy Authority of Iceland are interpreted in terms of physical processes in the crust and upper mantle associated with magma genesis and crustal spreading. First, these results confirm, over a larger region than was previously studied, the presence of a relatively conducting crust (25–30 ohm-m) at shallow depth (d ≦ 4.5 km) reflecting electrolytic conduction in hydrothermal pore fluids on a regional scale. Secondly, and perhaps of greatest interest to current work on processes of accretion along plate boundaries, is the presence of an anomalous conducting layer (T ≦ 4 km, ρ ≦ 10 ohm-m) at the base of the crust (8–15km) beneath the neovolcanic zone. This layer we feel represents an accumulation of molten magma. There is some suggestion that it is at shallower depth beneath the most active portion of the neovolcanic zone, and that its depth increases with age of the crust. The presence of this feature suggests that the crust is being underplated by the accretion of molten material to its base which, as it solidifies, contributes a significant component of crustal thickening. We feel that the molten phase residing in this basal zone represents an available source of magma for high level intrusions. Finally, our results confirm that, beneath the zone of magma accumulation, the resistivity changes little with depth in the mantle. Although there may be an indication that the upper few tens of kilometres of the mantle is somewhat more conducting, reflecting the presence of a minor amount of melt, this is not a firmly resolved feature of our analysis. Nevertheless, the mantle has almost a constant resistivity to a depth exceeding 100 km. Assuming a homogenous composition for the mantle beneath Iceland, the vertical gradient of temperature is of the order of only a few °Ckm‐ or less.

Ground-based observations of magnetic disturbance fields at auroral latitudes, as well as at lower latitudes, have, for a number of years, led workers to infer the rapid spatial movement of electric current systems in the ionosphere. However, relatively little attention has been directed toward understanding the associated electromagnetic induction problem, namely the effects on electric- and magnetic-field components observed at the Earth's surface, caused by time-varying current systems moving above the earth of finite conductivity. The following discussion considers this problem and presents a method for determining the electromagnetic field components at the surface and within the interior of an n-layered earth for a current system flowing east—west and moving with a uniform velocity to the north or south. The results for a line current source, which are given in detail, are used to derive expressions for the fields of a Gaussian electrojet, a ribbon electrojet and several asymmetric electrojets. This development differs substantially from previous work in that one can allow independent time variations of the source strength itself, which are essentially decoupled from the effects of its motion. Based on our own routine inspection of high-latitude magnetograms, we have found that moving sources are the rule and stationary sources are the exception. Theoretical calculations imply significant spectral broadening of temporal variations from moving sources relative to their stationary counterpart. Synthetic magnetograms visually demonstrate temporal contraction of ground-based observations relative to the true temporal morphology of the source itself. Moreover, there are significant phase lags and leads introduced between local field maxima observed on the ground and the time of the actual maximum strength of the source itself. Finally, magnetotelluric measurements are affected in a positive way by moving sources, in that the source appears to have a larger ‘effective width’ through averaging induction effects over a larger volume of the Earth during its lateral movement. For a harmonically time-varying source having a width of 500 km and a lateral movement of 500 m/s, the magnetotelluric relation appears valid to periods as long as 5 hr.