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A lava fountain is a type of volcanic plume, a jet of centimetre- to metre-sized lava fragments or blobs ejected from a volcanic opening in the ground surface and which then fall back to the ground surface near the volcanic opening. The lava is carried in the plume by volcanic gases. Lava fountains require magma of low viscosity and low gas content. Where these conditions are well met, the lava fragments coalesce on landing to form rootless lava flows.^[1] Where conditions of temperature, viscosity, magma production rate and yield strength (degree of solidification) are not optimal for lava flows, piles of coalesced pyroclasts that retained their heat sufficiently to land still semi-molten, lava spatter piles around the vent, or agglutinate stick together on impact. The height of a lava fountain is proportional to the exsolved gas content of the magma. Lava fountains usually range between 10 and 100 metres in height. The tallest lava fountain observed on the Earth occurred during an eruption of Izu Oshima volcano in Japan, when a lava fountain reached an estimated height of 1,600 metres.

Underground processes

As magma rises to the surface, the associated reduction in pressure causes volcanic gases to exsolve (come out of solution) forming bubbles that provide the energy for the eruption by forming a foam that collapses to produce a single large gas pocket in the conduit. Within a few hundred metres of the surface, the explosive nature of the eruption tears the magma into blobs that are carried in the expanding gas jet (volume of gas and lava mixture is at least 70 per cent gas) and these molten fragments are erupted from the [[vent. Too much gas would cause adiabatic cooling of the molten pyroclasts and the fragments would land as solid scoria rather than molten spatter (agglutinate). Too little gas would not fragment the magma. Lava fountains are common in volcanic eruptions of the hawaiian and strombolian types of volcanic activity in basaltic volcanic areas including Hawaii, Iceland, Reunion and Italy. The most significant gases in lava fountains are water vapour (and carbon dioxide?). Pre-eruption storage of magma in shallow magma chambers, approximately 2 km below the ground surface, erupts lava with low amounts of carbon dioxide, and the erupted gas jets are dominated by water vapour.

Pyroclastic flows

There are several mechanisms that can produce a pyroclastic flow:

• Fountain collapse of an eruption column from a Plinian eruption (e.g., Mount Vesuvius's destruction of Pompeii, see Pliny the Younger). In such an eruption, the material ejected from the vent heats the surrounding air and the turbulent mixture rises, through convection, for many kilometres. If the erupted jet is unable to heat the surrounding air sufficiently, convection currents will not be strong enough to carry the plume upwards and it falls, flowing down the flanks of the volcano.
• Fountain collapse of an eruption column associated with a vulcanian eruption (e.g., Montserrat's Soufrière Hills volcano has generated many of these deadly pyroclastic flows and surges.) The gas and projectiles create a cloud that is denser than the surrounding air and becomes a pyroclastic flow.

References

External links

Wikimedia Commons has media related to Lava fountains.

• Lava fountain in Hawaii – video by USGS