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Geology Word Bank

Felsic Rocks: Igneous rocks that contain >65% silica (SiO2), common minerals include quartz and alkali-feldspar. Rich in Si, Al, K and Na and light in colour they are less dense and more viscous than their mafic equivalents.

Mafic Rocks: Igneous rocks containing ~59% silica, common minerals include olivine, pyroxene and plagioclase feldspar. Rich in Fe, Mg and Ca they are dark in colour and denser and less viscous than their felsic equivalents.

Mantle Plume: An upwelling of abnormally hot molten rock from within the mantle of the Earth. Plumes are often used to explain volcanic activity away from a tectonic plate boundary e.g. Hawaii.

Fractional Crystallisation: The process by which different minerals crystallise out of a body of molten rock (magma) as it cools. Fe and Mg-rich minerals crystallise first (high temperature) leaving progressively more Si-rich melt behind.

Pyroclastic Density Current: A flow of hot ash, rock and gas that forms when the ash column produced by an explosive volcanic eruption

Pumice: A felsic volcanic rock which is light in colour and has an extremely low density due a high abundance of small bubbles in the rock structure. Bubbles represent gasses trapped in the magma during eruption due to the high viscosity of felsic melts.

Scoria: A mafic volcanic rock which is dark in colour and contains a high proportion of bubbles. Formed during explosive eruptions of mafic magma scoria is more dense than pumice due to its Fe and Mg-rich compostion.

Vesicles/Vesicularity: Vesicles are voids in a volcanic rock that resulting from gas bubbles forming within a magma while it is molten. When the magma is erupted it cools rapidly and the edges of the bubbles are frozen in place. Vesicularity of a rock is what % of a rocks volume is taken up by vesicles.

Xenolith: A chunk of rock found within an igneous body (lava flow, pyroclastic density current, or plutonic body) that has a different origin to the surrounding igneous rock. Usually representing the rocks through which the molten material travelled to reach the surface.

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