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The chemism of the original magma: magmas that are rich in silicates, so-called acid magmas, contain more highly volatile gases than basaltic magmas like sulphur dioxide, hydrogen sulphide, carbon dioxide, nitrous oxide, and water vapour. This chemism depends on the plate tectonic situation. Acid magmas are formed over the subduction zones of ocean plates, e.g. over the circum-Pacific "ring of fire" with the Andes and the North American Cordilleras in the east and the island arcs of Japan, the Philippines, and Indonesia in the west. The high fluid content of these magmas makes them highly explosive and their eruption columns extend much further into the atmosphere than in the case of basaltic volcanoes like Etna or Kilauea. The very rare flat basaltic eruptions are an exception. Compared with the sulphur fraction, other gases generally have much less impact on the climate.
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