In its large caldera, Newberry volcano (Oregon, USA) has two small volcanic lakes, one fed by volcanic geothermal fluids (Paulina Lake) and one by gases (East Lake). These popular fishing grounds are small windows into a large underlying reservoir of hydrothermal fluids, releasing carbon dioxide (CO2) and hydrogen sulfide (H2S) with minor mercury (Hg) and methane into East Lake.
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Source: ScienceDaily