Potassic trachybasalt from the July–August 2001 eruption of Mount Etna, Italy
Satellite image of Bayuda volcanic field in Sudan where nepheline-rich trachybasalt lavas have been erupted during the Holocene epoch[1]

Trachybasalt is a volcanic rock with a composition between trachyte and basalt. It resembles basalt but has a high content of alkali metal oxides. Minerals in trachybasalt include alkali feldspar, calcic plagioclase, olivine, clinopyroxene and likely very small amounts of leucite or analcime.[2]


TAS diagram highlighting the trachybasalt field

An aphanitic (fine-grained) igneous rock is classified as trachybasalt when it has a silica content of about 49% and a total alkali metal oxide content of about 6%. This places trachybasalt in the S1 field of the TAS diagram. Trachybasalt is further divided into sodium-rich hawaiite and potassium-rich potassic trachybasalt, with wt% Na2O > K2O + 2 for hawaiite.[3][4][5] The intrusive equivalent of trachybasalt is monzonite.[6]

Trachybasalt is not defined on the QAPF diagram, which classifies crystalline igneous rock by its relative content of feldspars and quartz.[3][4][5] However, the U.S. Geological Survey defines trachybasalt as a mafic volcanic rock (composed of over 35% mafic minerals) in which the quartz-feldspar-feldspathoid fraction of the rock is less than 20% quartz and less than 10% feldspathoid, and in which plagioclase is between 65% and 90% of the total feldspar content.[7]


Trachybasalt is common in continental volcanism and is also found on some ocean islands.[8] It is abundant at Mount Etna[9] and at Mount Taylor (New Mexico).[10] It has also been found on Gale crater on the planet Mars.[11]


  1. ^ "Bayuda Volcanic Field". Global Volcanism Program – Volcanoes of the World database. Smithsonian Institution. 5 June 2020. Retrieved 6 August 2020.
  2. ^ Trachybasalt
  3. ^ a b Le Bas, M. J.; Streckeisen, A. L. (1991). "The IUGS systematics of igneous rocks". Journal of the Geological Society. 148 (5): 825–833. Bibcode:1991JGSoc.148..825L. CiteSeerX doi:10.1144/gsjgs.148.5.0825. S2CID 28548230.
  4. ^ a b "Rock Classification Scheme - Vol 1 - Igneous" (PDF). British Geological Survey: Rock Classification Scheme. 1: 1–52. 1999.
  5. ^ a b Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 139–143. ISBN 9780521880060.
  6. ^ Forsythe, Nathan; Spry, Paul; Thompson, Michael (15 January 2019). "Petrological and Mineralogical Aspects of Epithermal Low-Sulfidation Au- and Porphyry Cu-Style Mineralization, Navilawa Caldera, Fiji". Geosciences. 9 (1): 42. Bibcode:2019Geosc...9...42F. doi:10.3390/geosciences9010042.
  7. ^ "Geologic units containing Trachybasalt". U.S. Geological Survey. Retrieved 3 June 2022.
  8. ^ Allaby, Michael (2013). "trachybasalt". A dictionary of geology and earth sciences (Fourth ed.). Oxford: Oxford University Press. ISBN 9780199653065.
  9. ^ Orlando, Andrea D'Orazio; Armienti, Pietro; Borrini, Daniele (29 August 2008). "Experimental determination of plagioclase and clinopyroxene crystal growth rates in an anhydrous trachybasalt from Mt Etna (Italy)". European Journal of Mineralogy. 20 (4): 653–664. Bibcode:2008EJMin..20..653O. doi:10.1127/0935-1221/2008/0020-1841.
  10. ^ Goff, Fraser; Kelley, Shari A.; Goff, Cathy J.; McCraw, David J.; Osburn, G. Robert; Lawrence, John R.; Drakos, Paul G.; Skotnicki, Steven J. (2019). "Geologic map of the Mount Taylor volcano area, New Mexico". New Mexico Bureau of Geology and Mineral Resources Geologic Map. 80.
  11. ^ Edwards, Peter H.; Bridges, John C.; Wiens, Roger; Anderson, Ryan; Dyar, Darby; Fisk, Martin; Thompson, Lucy; Gasda, Patrick; Filiberto, Justin; Schwenzer, Susanne P.; Blaney, Diana; Hutchinson, Ian (14 September 2017). "Basalt-trachybasalt samples in Gale Crater, Mars". Meteoritics & Planetary Science. 52 (11): 2931–5310. Bibcode:2017M&PS...52.2931E. doi:10.1111/maps.12953. hdl:2381/40838.