About 380 volcanoes were active during the last century, and around 50 of them were active per year (Andres and Kasgnoc 1998). The distribution of active volcanoes is linked to the active zones of plate tectonics, and more than two-thirds of the world's volcanoes are in tropical regions of the Northern Hemisphere. Emissions of gases depend on thermodynamic factors such as temperature and pressure, and on the magma type (basaltic, felsic or andesitic), which in turn depends on the tectonic environment. In general, most basaltic volcanoes (with magmas rich in Mg and Fe and poor in Si) occur along mid-ocean ridges; sub-aerial eruptions (into the atmosphere) may occur in areas such as the Azores and Iceland or in intra-plate volcanoes such as the island chain of Hawaii. These volcanoes mostly show effusive eruptions with a low gas content (mostly CO2 and sulphur), and only in rare cases do volcanic aerosols reach the stratosphere. Felsic (rich in silicate and alkali) and andesitic (intermediate silicate content) magmas are typical of volcanoes along converging oceanic plates (e.g. Indonesia) or where a continental plate overrides an oceanic plate (e.g. in the Andes). These volcanoes erupt less frequently than basaltic volcanoes, but they play a major role in global climate change and in the composition of the atmosphere in the Southern Hemisphere; because their eruptions are often more explosive, they can inject large amounts of ashes and gases directly into the stratosphere. These volcanoes emit gases during non-explosive phases, contributing a large amount of sulphur to global volcanic emissions. Although the Northern Hemisphere is usually a negligible source of aerosol to high latitudes in the Southern Hemisphere, large explosive volcanic eruptions may affect the composition of the Antarctic atmosphere.
The main compounds (by volume) of volcanic gases at the vent are 50-90 % water vapour (H2O), 1-40 % CO2,2-35 % sulphur gases (SO2,H2S and SO42-) and 1-10 % HCl (Symonds et al. 1994). The amount of sulphur gases is important to the global sulphate aerosol burden and represents by far the most relevant species as far as the climate impact of active volcanoes is concerned. Although the emission of sulphur from a single volcano may vary, according to its state of activity, the total amount released into the atmosphere from quiescent degassing and eruptions has been estimated to be in the range of 7.2-14.0 tg S year-1 (Spiro et al. 1992; Andres and Kasgnoc 1998). The concentration of anthropogenic sources of SO2 is higher than that of volcanic sulphate but the latter, with a contribution of about 35 % to the tropospheric sulphur burden, has only a slightly smaller radiative effect (Graf et al. 1997). Indeed, volcanic sulphate aerosols in the upper troposphere contribute to the formation of ice particles and consequently, to an indirect radiative effect. There is evidence (Sassen 1992) that volcanic aerosols are involved in cirrus cloud formation, and years with high-level clouds are usually associated with intense explosive volcanic activity.
Cataclysmic volcanic eruptions which inject ash and gas into the stratosphere are sporadic and unpredictable, usually occurring a few times per century. However, there is evidence (Graf et al. 1998) that, besides explosive eruptions, many felsic and andesitic volcanoes emit gases during non-explosive phases and may contribute a large amount of sulphur to total global volcanic emissions. Sulphate has a residence time of a few years in the stratosphere, and it can affect global climate in this period through a transient cooling of surface temperatures (for a review, see Robock 2000). The stratospheric sulphate aerosol mass and optical thickness reached peak values about 3 months after the Mt. Pinatubo eruption; materials stored in the stratosphere then gradually returned to the troposphere and it took about four years for radiative forcing (about -4 W m-2) to decay exponentially to background values (McCormick et al. 1995).
Since the discovery (Hammer 1977) that major volcanic events are recorded in polar ice sheets as sulphuric acid layers, ice cores from Antarctica have been widely used to study the link between climate and volcanism (e.g. Langway et al. 1988; Moore et al. 1991; Stenni et al. 1999; Zhang et al. 2002). Delmas et al. (1992) detected 23 major volcanic eruptions by applying physi cal and chemical analytical techniques to a 1,000-year ice core drilled at the South Pole. The 19th century was the period most affected by global explosive volcanic activity, and several eruptions were tentatively identified by comparing similar Antarctic and Greenland records. However, ice core records are noisy, and small volcanic events of regional significance may produce the same signal as distant large eruptions. In continental Antarctica, besides an active volcano (Mt. Erebus on Ross Island, southern Victoria Land), there are sites with fumaroles in Marie Byrd Land (LeMasurier and Rex 1982) and northern Victoria Land (Mt. Melbourne and Mt. Rittmann; Bargagli et al. 1996a) which indicate quite recent volcanic eruptions. Moreover, the amount of volcanic sulphate (non-sea salt sulphate; nssSO42) deposited on Antarctic snow is a small fraction of that arising from sea-salt aerosols. Robock and Free (1996) used data from both hemispheres to produce a new Ice core Volcanic Index (IVI); Robertson et al. (2001) introduced the volcanic aerosol index (VAI) by combining historical observations, ice core data from both hemispheres and satellite data to estimate the stratospheric optical depth for the past 500 years. Figure 31 shows the explosive volcanic eruptions south of 20° N, most commonly detected in Antarctic ice cores.
Major volcanic eruptions also play a very important role in the global atmospheric cycle of Hg and of many other trace metals. Based on the emission of metals from Kilauea, Hinkley et al. (1999) suggested a revision of the
estimated worldwide output by quiescent (non-explosive) volcanoes. More recently, Matsumoto and Hinkley (2001) found that the mass and proportion of Cd, Pb and other metals in pre-industrial Antarctic ice match the output rate to the atmosphere by quiescent degassing of volcanoes, and that the iso-topic composition of Pb in ice is similar to that in emissions of a suite of ocean island volcanoes, mostly located in the Southern Hemisphere.
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