As the name suggests, hydrovolcanic activity is activity (eruptions) of volcanoes associated with water (hydro). The two main-groups are constituted by Surtseyan and phreatomagmatic eruptions.
Surtseyan Eruptions 
(Fig. 2.20) Surtseyan Eruption. Surtseyan eruptions take place mainly in shallow seas and lakes. Because the water is shallow, as opposed to submarine eruptions, the pressure exerted by water is relatively low. Hydrovolcanic explosions can easily develop when the water is less than 100m deep. The sudden transfer of heat from magma at a temperature of 1200C to water at less than 20C causes the water to transform into steam. The expansion of water releases energy which shatters the magma. The explosive expansion of the steam thus takes place near the surface in an open vent. The generated energy can be dispersed into eruptions which may last until the magma stops rising or until a mass of fragments prevent water to invade the conduit. Shattered fragments of magma accumulate around the vent in fine and thin beds known as tuffs. During the eruption ash and steam clouds may form reaching to 5km in height. However, particular to Surtseyan eruptions are the thick,dark-pointed jets of fragments, often headed by bombs, shot out of the vent. Surtseyan eruptions are named after activity of Surtsey, a volcano situated in Iceland which grew from the ocean floor at a depth of 130m to the surface.
Phreatomagmatic Eruptions
Phreatomagmatic eruptions are characterized by violent steam explosions. Surface water may flow through fissures in the Earth's crust and interact with the rising magma causing the explosions. Country rocks, steam and shattered pieces magma are ejected. Such eruptions are often brief because the water is limited compared with the amounts available during Surtseyan activity. Nevertheless, phreatomagmatic eruptions are violent and they may form maars on the surface. The phreatomagmatic eruption ends when the water supply is exhausted, and not, as in most other eruptions when the magma stops rising.
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