Volcanic Dam - Pyroclastic Dam

Pyroclastic Dam

Pyroclastic dams are created in an existing drainage either by their direct emplacement or by the accumulation of widely variable pyroclastic particles, broadly termed tephra. 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 air-born 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 air-born 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. Unless violently expelled and generally speaking, larger sized tephra falls closest to the crater and smaller tephra landing farther away, with its distribution more highly influenced by prevailing wind velocity and direction.

Once initially established, a pyroclastic dam's continued longevity remains a balance between its slowly consolidating hardness and toughness, and the amount and velocity of flowing water's erosive capacity to remove it from its outset. Unconsolidated tephra is quickly moved by precipitation and flowing water in drainages, at times creating a lahar. Upstream of the dam this material would rapidly accumulate to fill the lake, and downstream it would tend to erode its slopes and base. The often rapid accumulation of unconsolidated pyroclastic material on steep sideslopes tends to be inherently unstable over time; pyroclastic dams may be emplaced by the landsliding of such material into rivers and streams. Pyroclastic material, given sufficient time to consolidate or 'weld' into hard rock, produce assemblages variously classified as ignimbrites, variously brecciated or agglomerated, along with various types of tuffs and volcanic ash, and are mostly of felsic composition.

While evidence of pyroclastic dams occur within the geologic record, they are best known and studied in relation to recent and current volcanic eruptions. Examples world-wide include associations with El Chichon in Mexico, and the Karymsky Volcano in Russia. The caldera lake associated with Taal Volcano, which was previously open to the East China Sea was permanently closed by a pyroclastic dam during the 1749 eruption, and remains in equilibrium at a higher level to this day, while the pyroclastic dam comprising the low rim of crater Lake Nyos in Cameroon is considered less stable.