Oriented core samples were collected from 18 lava-dike contacts in Iceland, in locations of low to moderate (<100¿C) regional hydrothermal alteration. Alternating field demagnetization of these samples shows that they generally carry a strong and stable remanence. Magnetic directions in the lavas follow a simple pattern, demonstrating that the remanence of the contact zone has only been reset within very short distances from the contact surface. On average, this distance may be only one tenth of the thickness of a typical dike (4 m). Other magnetic properties (remanence intensity, susceptibility, Curie point) of the contact zone are affected to a lesser or a less regular degree than the remanence direction. It is concluded that for the crustal depth of this study, thermal conduction models do not apply to temperature anomalies set up by dike intrusions or fissure eruptions in Iceland, presumably because of rapid advection of groundwater into the active fissures. These results also indicate that the magnetic anomalies observed over Iceland are only to a small extent caused by dikes.