Water and Volcanism
Volcanoes and their eruptions were familiar phenomena to homo sapiens in the species' African development, but among the first recorded speculations, Empedocles identified volcanic activity as a manifestation of fire, one of the four roots responsible for all structures in the world. Impressed by the maritime proximity of many European volcanoes, a belief developed that interaction between the sea and magma is critical in triggering explosive volcanism - nowadays this is termed phreatomagmatic activity. Following the eighteenth-century triumph of the plutonists over the neptunists with respect to the origin of igneous rocks, several nineteenth-century geologists advocated exsolution of H2O, previously dissolved at depth in the magma rather than via shallow interactions with seawater, as the main driving force for explosions. The enormous expansion accompanying transformation of liquid to gaseous H2O provides the energy for this explosive activity. We know considerable amounts of H2O (and other volatile gases such as helium) were accreted during formation of the Earth. Magmas transport these gases towards the surface, and lowered pressure results in exsolution of volatiles.
Igneous petrology and volcanology made little contribution generally to the development of plate tectonic theory, but pioneers such as Arthur Holmes, Harry Hess and Robert Coats were early advocates of crustal recycling, the importance of H2O in serpentinising the oceanic crust and global recycling of H2O via subduction zones respectively. Release of H2O from a subducted plate triggers melting and magma formation in the overlying mantle. Magmas subsequently erupted in arcs developed over subduction zones are much richer in H2O than at ridge or hot-spot settings. It appears the most important continental crustbuilding blocks are the basalt-andesite-dacite-rhyolite suites formed in arcs. The bulk composition of the continental crust is known to be equivalent to andesite, and the distinctive trace element geochemical characteristics of the continental crust are only matched by arc magma types. The aphorism 'no water, no granites - no oceans, no continents' summarises the importance of H2O in global recycling and the genesis of continent-forming arc magmas. Earth seems to be the only planet with plate tectonics and a fundamental question is whether a liquid hydrosphere (unique to Earth among the terrestrial planets) is a necessary condition for this style of tectonism.
In addition to H2O, other potentially volatile and fluid-mobilised elements and compounds (including ore-forming metals) are transported in supercritical fluids exsolved from arc magmas. Beneath the sea floor, these fluids become mixed with recycled seawater to emerge in hydrothermal vent ('blacksmoker') systems. On large scales, precipitation of metallic sulfides from hydrous fluids in the crust leads to large, mineable Au-Cu-rich ore deposits known as 'porphyry coppers.' In the past century, the possible scale of explosive eruptions driven primarily by exsolution of H2O, has become recognised. We can extrapolate from observed small events such as Mt St Helens of the Oregon Cascades, through Taupo in New Zealand (50 km3, 1.8 Ka BP; 500 km3, 26.5 Ka BP), to the Oligocene (30 Ma) 5000 km3 of the Fish Canyon Tuff in Colorado. These large eruptions can have significant environmental effects.
- Insights Vol 3 Article 15 (last modified: 4 October 2010)