Volcanoes

A volcano is constructed from rock materials produced by the eruption and solidification of molten rock (magma) at the surface of the Earth (or other planetary body). Earth’s 1545 active volcanoes are not distributed randomly. They are only found above zones of partial melting within the upper mantle caused by plate tectonics or mantle convection. While partial melting generates magma of basaltic composition, subsequent igneous processes beneath volcanoes lead to a wide range of compositions being erupted. Volcanic eruptions range from relatively benign effusive eruptions of lava, to discrete explosions, and to continuous explosive discharge of magma. The style of eruption is determined by the magma’s chemical composition, viscosity, volatile content and volatile solubility, the magma’s ascent rate, the degree to which gas stays trapped within the rising magma, and environmental controls on the cooling and confinement of the magma once it has left the vent. These variables, and hence eruptive style, may change during an eruption. Eruption size, measured on a magnitude scale based on the mass of erupted magma, covers about 10 orders of magnitude (105 to 1015 kg). The largest magnitude historic eruption (of Tambora volcano in 1815) produced about 7 × 1013 kg (33 km3) of magma; it caused 61,000 deaths and had global environmental impacts. Volcanic eruptions pose hazards to life, societies and environmental systems. Hazards derive from the covering of land by lava flows; the effects of explosions and pyroclastic density currents; and the injection of volcanic gas and particles into the troposphere and stratosphere. Volcanoes are monitored using field geology and measurements of seismicity, ground deformation, thermal emissions and gas emissions. These techniques are the basis of increased understanding of volcanoes and the development of methods to better forecast future eruptions