Hydrovolcanic eruptions are generated by the intereaction of magma with either groundwater or surface water. Explosive hydrovolcanic eruptions of basaltic lava are sometimes called Surtseyan, after the eruption off Iceland in 1963. Surtseyan eruptions are considered to be the "wet" equivalents of Strombolian-type eruptions, although they are much more explosive. This high explosivity is a hallmark of hydrovolcanic activity. As the water is heated, it flashes to steam and expands explosively, thus fragmenting the magma into exceptionally fine-grained ash. When the volcanic island of Surtsey was born in the Atlantic, the initial hydrovolcanic eruptions were spectacularly explosive. As the volcano grew, however, the rising lava in the central vent interacted with water to a lesser degree, so that the waning stages of the eruption became more Strombolian in character.

Hydrovolcanic eruptions are not restricted to the underwater development of oceanic islands. Many explosive Surtseyan events are generated on land by rising conduits (diapirs) of basaltic magma that interact with water-bearing strata (e.g., aquifers) at shallow levels beneath the surface.

A radial cloud emanating from the base of the eruptive column is a phenomenon known as a base surge. These surges are analogous to the ring-shaped, ground-hugging clouds that travel radially away from the vertical columns generated by nuclear explosions. Base surges are derived from the gravitational collapse of the "wet" eruptive column, which is denser than those associated with "dry" eruptions. Base surge deposits are wedge-shaped, with their thickest end near the vent. Dune-shaped deposits are common near the vent, indicated lateral transport analogous to the lateral movement and deposition of sand grains along the face of a moving sand dune. The bedding is often disrupted by "bomb sags" containing large ejected blocks which followed ballistic trajectories unrelated to the lateral surge. Spherical accumulations of wet, accreted ash, called accretionary lapilli, are also common to many base surge deposits.

Maars and Tuff Rings

Hydrovolcanic explosions generate maars and tuff rings. These vent types are large circular depressions with low rims of ejected debris. Maars are excavated into the substrate, thus exposing older rocks along their inner walls. Tuff rings, however, are built above the substrate. Maars contain a greater proportion of fragmented basement rocks in the ejecta blanket. This suggests that maars are derived from steam blasts (phreatic eruptions) generated well above the diapiric intrusion. Tuff rings, on the other hand, contain a greater proportion of magmatic (juvenile) fragments (see for example, palagonite tuff). Such deposits are consistent with explosions derived from a combination of heated groundwater and vesiculating magma (phreatomagmatic eruptions) from relatively shallower intrusions.

Ciruclar-shaped maars and tuff rings appear to be generated above rising columns of magma (diapirs). However, if the magma rises along a linear fracture zone, the interaction with groundwater may result in the reaming out of the fissure. Such eruptions can be even more destructive than those associated with maars and tuff rings, as exemplified by the highly explosive Tarawera eruption which buried three villages and killed 150 people on New Zealand's North Island in 1886.

Littoral Cones

Although littoral cones have a hydrovolcanic origin, they are not true hydrovolcanic vents. They are composed of basalt tephra that accumulate as ejected debris from the explosive interaction of moving lava and seawater. Although pahoehoe flows generally enter the water in a relatively passive manner, a'a flows often enter the sea explosively to produce cone-shaped piles of ejected debris. The irregular cracks bounding cindery blocks of a'a allow water to penetrate into the interior of the hot flow where it then flashes to steam explosively. Basalt exiting from lava tubes can also generate littoral cones from episodic explosions due to disruption of the lava stream by incoming waves or swells. The littoral cones that develop from these explosive ejections are typically open on the seaward side.

© Dr. Vic Camp, How Volcanoes Work, http://www.geology.sdsu.edu/how_volcanoes_work/ 2004