barium, relative to high-field-strength elements, such as zirconium, niobium, hafnium, rare-earth elements (REE), thorium, uranium, or tantalum. Andesite is particularly characteristic of volcanic arcs, though it sometimes also occurs in regions of crustal extension. In the rock record, volcanic arcs can be recognized from their thick sequences of volcaniclastic rock (formed by explosive volcanism) interbedded with greywackes and mudstones and by their calc-alkaline composition. In more ancient rocks that have experienced metamorphism and alteration of their composition (metasomatism), calc-alkaline rocks can be distingiushed by their content of trace elements that are little affected by alteration, such as chromium or titanium, whose content is low in volcanic arc rocks. Because volcanic rock is easily weathered and eroded, older volcanic arcs are seen as plutonic rocks, the rocks that formed underneath the arc (e.g. the Sierra Nevada batholith), or in the sedimentary record as lithic sandstones. Paired metamorphic belts, in which a belt of high-temperature, low-pressure metamorphism is located parallel to a belt of low-temperature, high-pressure metamorphism, preserve an ancient arc-trench complex in which the high-temperature, low-pressure belt corresponds to the volcanic arc. Petrology In a subduction zone, loss of water from the subducted slab induces partial melting of the overriding mantle and generates low-density, calc-alkaline magma that buoyantly rises to intrude and be extruded through the lithosphere of the overriding plate. Most of the water carried downwards by the slab is contained in hydrous (water-bearing) minerals, such as mica, amphibole, or serpentinite minerals. Water is lost from the subducted plate when the temperature and pressure become sufficient to break down these minerals and release their water content. The water rises into the wedge of mantle overlying the slab and lowers the melting point of mantle rock to the point where magma is generated. While there is wide agreement on the general mechanism, research continues on the explanation for focused volcanism along a narrow arc some distance from the trench. The distance from the trench to the volcanic arc is greater for slabs subducting at a shallower angle, and this suggests that magma generation takes place when the slab reached a critical depth for the breakdown of an abundant hydrous mineral. This would produce an ascending "hydrous curtain" that accounts for focused volcanism along the volcanic arc. However, some models suggest that water is continuously released from the slab from shallow depths down to , and much of the water released at