Basaltic intrusions more than 20 m thick are relatively rare in Iceland and are mainly associated with central volcanic complexes. A literature survey shows that the majority of large (⩾1 km2 in area) basaltic intrusions are intruded into soft and structureless host rocks such as tuffaceous hyaloclastites, sediments, vent and caldera agglomerates, hydrothermally propylitized lavas, and hot and still partly liquid silicic intrusive material. It appears that, upon entering host rock that breaks irregularly, the upwelling magma may expand within the host rock rather than penetrate upward along a narrow fracture to form a dike. This suggests that magma may tend to spread out laterally within the highly altered base of seismic layer 2 (lava layer) rather than penetrate the progressively harder (less altered) lava pile. The boundary of layers 2 and 3 (intrusive layer) could therefore be controlled by a metamorphic front at which the degree of alteration makes the lava pile lose its coherence and accommodate large intrusions. This model is compatible with the correspondence previously observed in Iceland between depths to seismic layer 3 (VP= 6.5 km/s) and geothermal gradients as measured in bore holes.