Lopoliths are relatively small plutons that typically developed an upper surface that is concave downward. This sagging shape may be attributable to volume reduction when magmas crystallize. The weight of the overlying strata would cause collapse into the volume previously occupied by more voluminous liquid magma.
Lopoliths formed by a similar mechanism to laccoliths, but they are composed of dense, mafic magma that allows depression by the overlying strata on cooling. Many lopoliths contain layered gabbroic rocks. Some lopoliths are very large, with thicknesses of many kilometres. The Bushveldt lopolith in southern Africa is several hundred kilometres across and contains the richest platinum deposits known.
image Bushveldt volcanics