GeologyRocks

Magma

A melt, generally containing suspended crystals and other volatiles, formed by total or partial melting of solid mantle or crustal material.

Mare

Area of dark basaltic lava on the Moon (pl. Maria).

Nephelinite

A silica-undersaturated volcanic igneous rock containing nepheline and pyroxene.

Definition provided by: Tim Ivanic (University of Edinburgh)

Nuees Ardentes

See Pyroclastic Flows.

Obsidian

A super-chilled igneous rock, black in colour and glassy in appearance. Fractures conchoidally. Often formed due to contact with cold seawater as the silicate minerals cool almost immediately, restricting mineral growth. Can also be called volcanic glass. Due to the rapid rate of cooling no crystals form. Obsidian forms from felsic, silica-rich, magma. Over time, crystals do form in a snowflake-like pattern, so-called “snowflake obsidian.

Pahoehoe

A solidified lava flow with a smooth, ropey surface, formed from less viscous lava than aa. The name is of Hawaiian origin.

Phenocrysts

Unusually large, often euhedral, crystals in an otherwise fine-grained igneous rock. The resultant texture is a porphyritic texture. Phenocrysts are the result of slower crystallisation of the magma than the fin-grained matrix. See Porphyritic texture.

Porphyritic

A type of texture exhibited by igneous rocks where some minerals occur as significantly larger crystals in an otherwise fine matrix. See Phenocryst.

Pyroclastic Flow

Extremely hot (700°C) clouds of gas and ash particles occurring during some explosive volcanic eruptions. They can travel at speeds comparable to a hurricane (100-150 mph). Also known as Nuees Ardentes.

Rare Earth Elements

Elements with atomic numbers 57 (Lanthanum) to 71 (Lutetium) inclusive plus scandium and yttrium.