Surtseyan eruptions were observed at Capelinhos in the Azores from 1957 to 1958, but it was the emergence of Surtsey, off southwestern Iceland between 1963 and 1967, that gave them their name. They are most commonly generated by basaltic magmas. Surtseyan and phreatomagmatic eruptions constitute the two main forms of hydrovolcanic activity, and they have often been grouped together. The best displays of Surtseyan activity take place mainly in lakes, shallow seas or, occasionally, above shallow aquifers. The pressure exerted by deep water that inhibits explosions during submarine eruptions is much reduced when the water is shallow. Hydrovolcanic explosions can easily develop where the water is less than 100m deep and, when water invades the vents up which magma is rising, the eruptions are more violent than if they had taken place on land. Surtseyan eruptions are most often generated by basaltic magmas associated on land with mildly explosive activity. The sudden transfer of heat from magma at a temperature of 1200'C to water commonly at less than 20'C transforms the water into steam; its expansion releases energy that quenches and shatters the magma. Large volumes of water can continually gain access to the vent. The explosive expansion of the steam thus takes place near the surface in an open vent where water pressures and temperatures are relatively low, and where the energy generated can be dispersed in eruptions, of varying intensity, repeated at intervals of several minutes, that may last for many months. The upper parts of the conduit are filled with a fluidized slurry of fragments and water close to boiling point, that is repeatedly penetrated by pulsations of magma, each of which provokes an explosion as the water expands into steam. The shallow explosions and limited collapse of the uppermost walls of the vent develop a funnel-shaped conduit, open to the air or water at the top. Whether there is any deep foundering in the conduit is still debated. The shattered fragments of magma accumulate around the vent in mainly fine and thin beds, known as "tuffs", which are characteristically tiny, often 1 n= across, angular and glassy, because of the shattering and quenching processes in their formation. Little of the country rock is exploded, new magma usually comprising over 90 per cent of the fragments ejected.
The explosions commonly take on three main forms, which may occur together or separately. Fragments are expelled upwards in a cloud of ash and steam that can form a billowing column reaching 5 km in height. Many fragments fall back into the vent, and are shattered by more explosions until they are carried away by the wind, or finally accumulate around the orifice. The second form of expulsion occurs when column collapses, or clouds of dry fragments and superheated steam surge from the conduit and radiate from the vent like the billowing collar developed around the bases of nuclear explosions. But the third form is the chief distinguishing mark of Surtseyan eruptions. Thick dark-pointed jets of fragments, often headed by bombs trailing black debris behind them, are repeatedly shot out from the vent. They resemble pointing fingers, the fronds of a cypress tree, or the spreading plumes of a cockerel's tail as they arch out about 1 km from the vent. These jets are usually generated by relatively cool eruptions where the trapped steam condenses around and wets the fragments, which then accumulate in consolidated layers in ruff cones.
Because water is continually available in a lake or the sea, Surtseyan eruptions continue until the magma stops rising, or until an impermeable mass of fragments prevents water from invading the conduit. Without water interference, rising basaltic magma then gives birth to the cinders and lava-flows typical of Strombolian eruptions. Thus, it was only the earlier parts of the eruptions of Capehnhos and Surtsey that were truly Surtseyan; their later stages comprised largely Strombohan eruptions, to which both volcanoes owe their armour-pladng of lava-flows.
Surtsey must have been one of the most closely observed volcanic babies in history, for it could be relied upon to produce spectacular explosions that were far less dangerous than they looked. Surtsey grew from the ocean floor at a depth of 130m and emerged on 14 November 1963. Within four hours the eruption had developed its full fury, and had built up a cone 174rn high when activity abated on 1 February 1964. The baton was taken up by a new vent that constructed a cone as big as the first in five weeks. The tuffs accumulated so quickly that water had increasing difficulty in reaching the vent. The Surtseyan eruptions were doomed and their fate was sealed on 4 April 1964, when lava-flows from Strombolian eruptions were emitted, which have helped protect Surtsey from wave attack.
The shores of many Atlantic islands are scattered with Surtseyan cones and prominent examples include Monte Brasil in Terceira and Monte Guia in Faial in the Azores, and El Golfo in Lanzarote and Taco in Tenerife in the Canary Islands. Surtseyan features also formed in ephemeral lakes during the pluvial periods in the deserts that corresponded to the ice ages in higher latitudes.
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