Fig. 5. Once a shallow magma chamber has formed (evolved from a sill or sills, for example), it injects many thin dykes and inclined sheets. Here, a fossil magma chamber (A) and an associated dense swarm (B) of inclined sheets and dykes (about 80% of the rock is composed of the sheet-like intrusions). The top of the fossil magma chamber, a gabbro body, is located at about 2 km depth in the palaeorift zone in Southeast Iceland.