Phreatic and phreatomagmatic eruptions
Phreatic explosions expel steam and shattered country rocks with no new magma; phreatomagmatic eruptions eject greater proportions of shattered new magma in addition to steam and country rocks, which still, however, often comprise some 90 per cent of the fragments. Both are characterized by violent steam explosions generated when water interacts with rising magma below ground. Many eruptions occurring on land have some phreatic or phreatomagmatic component because fissures in the Earth's crust allow surface water to seep down to meet the magma. Such eruptions are usually brief because the water is limited in volume and restricted in supply, compared with the amounts available in Surtseyan activity. They are, however, often very violent because they take place in a confined space under ground. These eruptions form diatremes below ground and explosion craters, or maars on the surface, surrounded by a rim of shattered fragments consisting of varying proportions of new magma and country rocks.
The explosive vigour of virtually all types of magma is greatly increased by their interaction with water, which is suddenly changed to steam. But, proportionately, as in Surtseyan activity, the greatest increases in violence are added to basaltic magmas, which would usually otherwise only be mildly explosive.
Events begin with the encounter of two fluids at different temperatures at depths commonly exceeding 250m. Fuel-coolant interactions take place. The coolant, the water, is at temperatures ranging, say, between 20'C and 100'C. The magma is quenched; heat is suddenly transferred from magma to water, which expands explosively into steam. The thermal energy of the magma is transformed into the considerable mechanical energy of the steam in a fraction of a second. The magma is shattered, increasing the area in contact with the water, thus accelerating steam formation that, in turn, causes more shattering. The rocks are very quickly ruptured, a trumpet-shaped conduit is formed and a circular explosion crater is blown out at the surface. Fine, angular fragments of country rock with some new magma re shot out of the vent. They form columns of ash and steam rising 5 km or more into the air, which sometimes collapse and radiate from the vent like the collars spreading from the base of a nuclear explosion. The explosions begin violently and often unpredictably, and reach their maximum power in seconds, but then wane rapidly so that each pulsation rarely lasts more than half an hour. A succession of blasts often occurs, separated by increasing intervals of repose, but none is as violent as the first.
Repeated explosions weaken the walls of the conduit, which collapse and slump downwards along circular faults that develop. Thus, the conduit is widened, but it is also often choked temporarily with rock debris. The blockages are, however, then removed by further explosions at greater and greater depths as more water continues to infiltrate. The whole episode only lasts a few hours, or a few days at the most - primarily because the restricted water supply is soon eliminated by the explosions. Thus, the eruption ends when the water supply is exhausted, and not, as in most other eruptions, when the magma stops rising. A collapsed and shattered mixture of magma and country rock fragments then chokes the lower parts of the conduit, often to depths exceeding 1000m, which thus forms a diatreme piercing the crust. Coarse angular fragments of country rock and a small proportion of shattered new magma come to rest in a thin rim or crescent accumulated on a sector around the deep, circular explosion crater, which is commonly 1 km across. Rainwater quickly forms lakes in these craters, which are usually known as maars ("meers") from their name in the Eifel region in western Germany where they were first described. In that area, 72 maars were formed where streams filtered down to meet rising basalts, but, where water did not infiltrate, the same magma formed 160 Strombohan cones. The maars are often 500m to l km across and up to 200m deep: Pulvennaar, for instance, is 900m in diameter and Gemundenermaar is 204m deep. Most were formed between about 10000 and 12500years ago, during the decline of the last ice age when abundant water became available when the permanently frozen ground began to melt. They are also common on similar volcanic plateaux in Auvergne, where over 20 maars have been identified in, for example, the Chain of Puys. Farther south, in Velay, lie the Lac du Boucher, 800m across, 30m deep and enclosed in a wooded rim of fragments, and the Lac d'Issarles which fills a fine crater 110m deep and 1 km in diameter. Lake Nyos, one of 29 maars in the Grassfields region of western Cameroon, has become one of the best known maars in recent years - not, however, for the phreatomagmatic events which created it some 500 years ago, but for its notorious emission of carbon dioxide in 1986. Lake Nyos is 208m deep, and is larger than most maars, with a diameter of 1. 8 km from north to south and 1 km from east to west. Its rim is marked by a crescent where basaltic fragments form thin layers between large shattered blocks of the granitic country rock.
Phreatic eruptions are frequent precursors of magmatic eruptions on many strato-volcanoes; and one of the most sustained periods of phreatic activity this century occurred on the dacitic dome of Lassen Peak in the Cascade Range. The first 170 explosions between 29 May 1914 and 14 May 1915 were phreatic, expelling steam and fragments shattered from the dome that sometimes rose 3km above the summit. The main explosions occurred at quite regular intervals: 14 June, 18 July, 19 August, 21 September, 22 October ... perhaps representing the time required for snowmelt and rainwater to infiltrate the dome and meet the magma. At first, the fragments ejected were not even warm enough to melt the summit snows; and the first hot ash did not emerge until 22 October 1914, showing that magma was approaching the surface. On 14 May 1915, rapid snowmelt caused the last phreatic throat-clearing explosion that opened the vent for the first emission of magma and a change of eruptive style.
Phreatic explosions were expressed differently in September 1979 on Etna, whose summit was then marked by two deep craters, the Voragine and the Bocca Nueva. On 2 September 1979 part of the damp walls of the Bocca Nuova collapsed, thereby blocking the emission of steam and gases from that vent. With the temperatures of 1000'C prevailing at depth, the collapsed material was soon heated to incandescence and its humidity was converted to steam that joined the gases unable to escape. Great pressure consequently built up until it was released by a sudden explosion which occurred without warning at 17.15 on 12 September. The blocking rocks were shattered and thrown from the vent, killing nine tourists and injuring many others as they fled. More than a hundred comparable eruptions, fortunately without fatalities, have apparently occurred on Etna during the past century.
A less violent form of phreatomagmatic activity occurs when molten lava-flows invade marshy ground, or a shallow lake where the lavas are thicker than the depth of the water. The waters are trapped beneath the lavas and suddenly heated to steam, which then explodes through the flow. As a result, some of the lavas shatter into fine tuffs, forming gently sloping low cones between 10m to 50m high, although a few rare examples reach 100m. They are called rootless cones because their vents do not extend below the base of the flow. South of the Chain of Puys, the lava-flow that erupted about 13 700 years ago from Puy de Tartaret invaded the valley of the River Couze Chambon and formed about 30 rootless cones. There are perhaps 10000 rootless cones in Iceland where abundant surface water and frequent lava eruptions provide necessary ingredients. One of the finest series was formed in Lake Myvatn when the Younger Laxa lava-flow erupted from the Threngslaborgir fissure and invaded its shallow waters about 2000 years ago.
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