Glossary (Reference)

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.

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