Fumaroles and solfataras

Fumaroles & Solfataras

Fumaroles are formed when only steam and hot gases are expelled from small vents commonly less than 10 cm across. They are often very numerous, and Yellowstone Park alone has thousands. When assembled in large fuming hollows, they often look more threatening than they really are, and have inspired such exaggerated place names as Colter's Hell at Cody, Wyoming, Devil's Kitchen, or Bumpass Hell on Lassen Peak. Nevertheless, fumaroles are capable of burning and scalding. Those issuing onto the Acqua Calda beach at Vulcano, for instance, are hot enough to scald the sensitive parts of unwary bathers. Most active volcanoes in repose frequently show fumarole activity. Whatever the location of the fumaroles, the water comes mainly from recycled atmospheric precipitation and only rarely from the rising magma. Some fumaroles are as hot as 1000'C and many exceed 100'C. Their gas content often varies according to the temperature. Relatively cool fumaroles between 100'C and 300'C often give off hydrogen sulphide that reacts with the oxygen in the air to produce sulphur deposits. These are known by the Italian name of solfataras.

La Solfatara itself lies in the Phlegraean Fields west of Naples. Its crescent-shaped cone is wrapped around the Piano Sterile, the flat-floored crater, some 750m across, containing about 25 active little vents, from which escape hot gases such as carbon dioxide, sulphur dioxide and hydrogen sulphide, as well as mud, steam and water. This activity has probably continued ever since the crater was formed about 4000 years ago. The most important episode in historic time occurred in 1198 when a big mud geyser spouted 10m into the air for several days after a small earthquake. A similar event took place on 23 July 1930. At present, the Piano Sterile is a wide arena, covered with grey mud, into which a number of bubbling muddy pools are sunk. Many of the solfataras are encrusted with beautiful, if fragile, sulphur crystals. This sulphur, however, reacts with water to produce sulphuric acid, which alters the rocks of the crater floor and forms the white "bianchetto" coating the drier surfaces. The activity of La Solfatara is dominated by the recycling of rainwater, heated to high temperatures, which reach 120'C at a depth of 150m. The predominance of gas and mud is no doubt related to the high subterranean temperatures and the relative scarcity of atmospheric water. Each cluster of vents has its own particular characteristics. The Forum Vulcani is a vigourous steam vent; the Bocca Grande has the hottest emissions at a temperature of 185'C; in the centre of the crater, Fangaia ("mire") has hot mudpots usually about 2m deep. In the active central parts of the crater, hardened mud forms grey mounds between the pots and pools, but the crust is sometimes dangerously thin, resonates underfoot, and masks boiling waters just below the surface. In contrast, the northwestern area of the Piano Sterile is solid enough to support a campsite and bar for those who appreciate sulphurous odours with their leisure.

In several countries, hydrothermal activity has been exploited for heating. Chaudes Aigues ("hot waters") in Auvergne, runs a central-heating system from its hot springs; and those at Larderello in Central Italy, and near Ribeira Grande in Sao Miguel in the Azores, are used for industrial and domestic power. In Iceland, boreholes reaching 2000m in depth have been sunk to tap the hot water to provide cheap, clean, ecological heat for homes, swimming pools, industrial and commercial installations, and greenhouses. All Reykjavik is heated in this way. Electricity is generated from the hydrothermal heat of Krafla volcano. At Hveragerdi, the geothermally heated greenhouses, covering 10ha, have become world-famous because bananas, as well as less exotic tomatoes, have been ripened almost within reach of the Arctic Circle. Iceland, inside and underground, can be quite warm ...

These hydrothermal emissions are often symptoms of waning volcanic activity, or simply mark periods of repose between more violent explosions. However, many volcanic areas are so riddled with hydrothermal fissures and conduits that the whole rock is altered and weakened. Much of El Piton, the summit cone of Teide, for instance, has been transformed into a powdery white mass resembling the "bianchetto" at La Solfatara. But, by way of compensation, hydrothermal alteration of rhyolite in Wyoming formed the "yellow stone" that gave its name to a beautiful volcanic region. Not least of its attractions is the superb canyon cut through this golden stone by the Yellowstone River.

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