Abstract The aeromagnetic expressions of two diabase dykes in the Abitibi greenstone belt are compared with each other as are those of three granitic plutons in the Eastern Townships in Canada with the benefit of ground magnetic survey data, known magnetic properties and known geology. Similar aeromagnetic expressions for these examples i.e. positive aeromagnetic anomalies for the diabase dykes and a ring-shaped structure associated with two of the granite plutons, are shown to be due to different geological sources in each case. In the case of the two diabase dykes, the concomitant aeromagnetic anomalies are due to a strong downward directed total magnetization in one of the dykes and to strongly magnetized contact zones along the other dyke. In the case of the granitic plutons, the ring-shaped feature of one pluton is due to a more basic, more strongly magnetized ring dyke, whereas for the other pluton it is due to a magnetic halo within the surrounding host rock. Quantitative interpretation of aeromagnetic anomalies without ground follow-up is therefore hazardous. The formation of a magnetic halo in the host rock is an important accompanying feature and is considered in some detail using heat conduction theory.