Unlike lava dams, which are formed by coherent, molten liquid gravity surface flow, filling the valley bottom directly and solidifying rapidly from the outside inward, pyroclastic dams are produced by less coherent airborne gravity currents or falls of tephra particles from the atmosphere, which solidify on the surface more slowly from the inner portion outward; pyroclastics are also deposited both in the valley bottom and widely distributed on the adjacent slopes. Their airborne nature is less restricted to the immediate drainage and they may roil over drainage boundaries; their particulate components allow for continued redistribution after initial placement by gravity and water. The explosiveness of pyroclastic eruptions, both laterally and vertically, range from fiery surges, to hot flows, to warm falls of tephra; the former may tend to emplace a dam directly while the latter tends to enhance placement or provide additional material.