VOLCANOES 565 VENUS See SOLAR SYSTEM: Venus VOLCANOES G J H McCall, Cirencester, Gloucester, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Volcanoes are a major component of the Earth’s present surface geology, both active and extinct; volcanic processes can be recognized throughout geological history and are important in generating certain types of mineral deposits (e.g., metallic sulphides, sulphur) They are studied in their relationship to both mantle (see Earth: Mantle) and crustal (see Earth: Crust) processes of igneous rock generation and plate tectonics Volcanic eruptions comprise one of the most important of natural hazards (see Engineering Geology: Natural and Anthropogenic Geohazards) threatening populations living close to them Volcanoes have been recognized on other bodies of the solar system – Venus (see Solar System: Venus), Mars (see Solar System: Mars) and Jupiter’s satellite Io (see Solar System: Jupiter, Saturn and Their Moons) – the latter is the most volcanically active body in the solar system Mars has the largest single volcano, Olympus Mons (440 km diameter; 24 km altitude) Venus, which we can only study from radar images on account of its dense volcanogenic CO2-rich atmosphere, appears like Mars, to have no active volcanoes now, though there are many inactive ones Volcanoes and the Mantle Liquid rock, poured out from volcanoes as lava (see Lava), makes up only a small portion of the planet, though a large part of the core is molten metal The outer layer of the Earth, the lithosphere, is relatively cool, but the mantle below is so hot that rocks lose their cohesion Indirect evidence obtained from seismology suggests that they move very slowly; and the theory of plate tectonics requires such movements to take the form of convection currents and localized upwellings (hot spots, mantle plumes) (see Mantle Plumes and Hot Spots) However, there is no agreement among geoscientists as to the depth of the lower boundary of these circulations The hot, soft, solid material of the mantle, of peridotite and related compositions, only partially liquifies when the temperature exceeds the melting point of minerals in the rock Because it is lighter than the solid rock above it, it will rise towards the surface, entering the lithosphere It may pass through the crust directly and swiftly appear at the surface as a volcano, or it may be halted and form a large concentration of molten rock down in the crust, a magma chamber: from this it may later burst out to the surface as a residue, changed after some crystallization in the magma chamber In narrow conduits to the surface, it may cool and crystallize, forming wall-like intrusions (dykes) or sheet-like intrusions (sills) Intrusive rocks formed by crystallization of the magma in the magma chamber, dykes, and sills, form the underworks of volcanoes, and may be all that is left after erosion: they frequently comprise ring complexes as on the Isle of Rum in Scotland A quirk of erosion has left these underworks exposed, surrounded by Precambrian granite, in a hollow central to the remains of the lava and tuff pile in the 100-km wide Miocene Kisingiri volcano in western Kenya – Howel Williams called this the best preservation yet seen of a volcano complete with its upper- and under-works Distribution of Volcanoes Volcanoes not occur everywhere on the Earth’s surface The global distribution shown in Figure reflects the present distribution of more than a dozen tectonic plates, rigid plates of the lithosphere, which move laterally across the Earth’s surface at minute velocities in the order of centimetres per year Of course, they cannot this indefinitely without colliding with each other and this pattern is controlled by upwelling of magma on the lines of separation (the mid-ocean ridges) and either the diving down of the spreading oceanic plate under the one it spreads against (subduction) or, where continental parts of plates meet, collisions such as formed the Himalayas In subduction there is both