Volcanistic Terminology

SOME VOLCANICLASTIC TERMINOLOGY

PYROCLAST – Juvenile crystals, crystal fragments, glass or rock fragments produced during a volcanic eruption.

BOMB – A pyroclast with a mean diameter exceeding 64mm. It may be rounded to angular.

BLOCK – A pyroclast with a mean diameter exceeding 64mm. Its angular to subangular shape indicates that it was ejected as a solid.

LAPILLI – Pyroclasts of any shape with mean diameters of 2 to 64 mm.

ASH GRAINS – Pyroclasts with mean diameters of 1/16 – 2 mm.

DUST PARTICLES – Pyroclasts with mean diameters smaller than 1/16th mm.

PYROCLASTIC DEPOSITS – These are consolidated and unconsolidated layers containing more than 75% by volume pyroclasts.

TEPHRA – A collective term for unconsolidated pyroclastic deposits.

PYROCLASTIC BRECCIA – A pyroclastic rock with angular clasts whose average size exceeds 64 mm.

AGGLOMERATE – A pyroclastic rock in which rounded clasts predominate and the average pyroclast size exceeds 64 mm.

AGGLUTINATED SPATTER – A pyroclastic rock in which the clasts are welded together.

LAPILLI TUFF – A pyroclastic rock whose average pyroclastic size is between 2 and 64 mm.

TUFF (ASH TUFF) – A pyroclastic rock whose average clast size is less than 2mm.

DUST TUFF – A pyroclastic rock whose average grain size is less than 1/16th mm.

MONOMICT (MONOLITHOLOGIC) – Composed of more than 75% clasts of one rock type.

POLYMICT (MULTILITHOLOGIC) – Composed of more than 75% clasts of different types.

VITRIC TUFF – A tuff composed dominantly of glass fragments.

CRYSTAL TUFF – A tuff composed dominantly of crystals and crystal fragments.

LITHIC TUFF – A tuff composed dominantly of lithic fragments.

JUVENILE MATERIAL – Composed of the lava erupted.

LITHIC MATERIAL – Composed of older pre-existing rock fragments usually blasted from the bottom of the crater.

EPICLASTS – Crystals, crystal fragments, glass and rock fragments weathered or eroded from pre-existing rocks and transported by gravity, air, water and ice.

EPICLASTIC DEPOSIT – A consolidated or unconsolidated aggregate of epiclasts.

REFERENCES:

LeBas, M.J. and Sabine, P.A., 1980, Progress on the Nomenclature of Pyroclastic Materials: Geological Magazine, vol. 117, No. 4, p. 389-391.

Schmidt, R., 1981, Descriptive nomenclature and classification of pyroclastic deposits and fragmentals: Recommendations of the IUGS Subcommission on the systematics of igneous rocks: Geology, vol. 9, p. 41-43.

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