Bysmaliths are considered to be conical or cylindrical laccoliths. They develop when highly viscous magma is injected into strata. Because lateral spreading along the bedding is limited by viscosity, the magma moves upward to form the cylindrical shape. Overlying rock layers are fractured. The walls of bysmaliths slope away from each other with depth, which makes their diameter increasingly large at greater depths.
[image Black Mesa bysmalith (Henry Mountains, Colorado Plateau, Utah), close-up of Wolf porphyry, Mixes Baldy-Anderson Peak Bysmalith close to the Blankenship Divide; ref, ref 2, gallery, Henry Mountains Wilderness]