Here is an explanation of some of the terms used on the site; the list will be added to as the site develops.
Assimilation: some of the rock ascending magma passes though (country rock) is sloughed off and mixed with the magma, changing the magma’s composition.
Basic rocks: rocks or lava which contain a low level of silicon dioxide (SiO2), such as basalt.
Caldera: a large elliptical depression caused by dropping of the magma reservoir roof when magma leaves the reservoir, usually during an eruption. The magma may erupt via a summit or flank vents.
Constructive plate margin: the boundary between two separating tectonic plates where new rock is added by magma ascent.
Cryptodome: a bulge caused by ascending unerupted magma.
Decompression melting: a reduction in pressure reduces the melting point of rock and causes partial melting.
Degassing: the release of volatiles (gases) from magma or solution when the constraining pressure reduces.
Destructive plate margin: the boundary between two colliding tectonic plates where the denser plate sinks below the other and is ultimately destroyed in the process.
Fire fountain: vertical jet of gas and incandescent lava released from a fissure or vent.
Fractional crystallisation: as magma cools, minerals crystallise out depending on their melting point. This changes the composition of the remaining magma.
Holocene: the current period of the Quaternary period, which started around 10,000 BC, with the Holocene Glacial Retreat.
Hydration melting: the addition of water reduces the melting point of rock to cause partial melting.
Ignimbrite: rock formed by the deposit of hot ash and pumice.
Intermediate rocks: rocks or lava which lie between basic and silicic rocks, such as andesites.
Jökulhlaup: a glacial outburst flood. Accumulated melt water caused by heat from magma close to the surface breaks through the glacier in a flood. The term is used in Iceland. Lahars are a similar phenomenon.
Large Igneous Province: a large area of lava s and intrusions which are thought to originate from mantle processes, e.g. a mantle plume; they may be basaltic or silicic.
Magma mixing: one or more batches of magma combine in the magma chamber / reservoir, changing the overall composition of the magma.
Obduction: one plate is thrust over the other when two tectonic plates collide.
Ophiolite: a remnant of old oceanic crust and mantle that has brought to the surface by obduction.
Phreatic eruptions: Steam driven eruptions, caused by heat from magma flashing ground water to steam. No new lava is emitted.
Phreato-magmatic eruptions: As for phreatic eruptions with the emission of new lava.
Pyroclastic flows: also known as pyroclastic density currents, turbulent ground hugging flows or currents of ash, rock and gases from an eruption. They may be hot or cold.
Silicic rocks: rocks or lava which contain a higher proportion of silicon dioxide (SiO2), such as rhyolite.
Subduction: the descent of one tectonic plate / lithospheric slab beneath another at a destructive plate margin.
Tephra: fall out deposits from explosive eruptions.
Volcano Explosivity Index (VEI): a measure of the explosivity of volcanic eruptions, calculated based on plume height.
Wadati-Benioff zone: the zone where earthquake foci delineate the descending plate in a subduction zone.