Volcanic dam

This classification generally excludes other, often larger and longer lived dam-type geologic features, separately termed crater lakes, although these volcanic centers may be associated with the source of material for volcanic dams, and the lowest portion of its confining rim may be considered as such a dam, especially if the lake level within the crater is relatively high.

The emplacement, internal structure, distribution and longevity of such dams can be related variously to the amount, rapidity and duration of (primary) geothermal energy released, and the rock material made available; other considerations include the rock types produced, their physical and toughness characteristics, and their various modes of deposition.

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.

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.

Upstream of the dam this material would rapidly accumulate to fill the lake, and downstream it would tend to erode its slopes and base.

While evidence of pyroclastic dams occur within the geologic record,[5] such as Lake Reporoa in New Zealand,[6] they are best known and studied in relation to recent and current volcanic eruptions.

[8] The caldera lake associated with Taal Volcano, which was previously open to the South China Sea, was permanently closed by a pyroclastic dam during the 1749 eruption, and remains in equilibrium at a higher level to this day,[9] while the pyroclastic dam comprising the low rim of crater Lake Nyos in Cameroon is considered less stable.