A cut block of trachyandesite lava from a volcano in Auvergne, France, used as building stone, forming part of the walls of Clermont-Ferrand Cathedral, France
Trachyandesite is field S3 in the TAS diagram

Trachyandesite is an extrusive igneous rock with a composition between trachyte and andesite. It has little or no free quartz, but is dominated by sodic plagioclase and alkali feldspar. It is formed from the cooling of lava enriched in alkali metals and with an intermediate content of silica.[1][2]

The term trachyandesite had begun to fall into disfavor by 1985[1] but was revived to describe extrusive igneous rocks falling into the S3 field of the TAS classification. These are divided into sodium-rich benmoreite and potassium-rich latite.[3]

Trachyandesitic magma can produce explosive Plinian eruptions, such as happened at Tambora in 1815.[4] The Eyjafjallajökull 2010 eruption (VEI-4[5]), which disrupted European and transatlantic air travel from 14–18 October 2010,[6] for some time was dominated by trachyandesite.[7]


Trachyandesite is characterized by a silica content near 58% and a total alkali oxide content near 9%. This places trachyandesite in the S3 field of the TAS diagram.[8][3] When it is possible to identify the minerals present, trachyandesite is characterized by a high content of sodic plagioclase, typically andesine, and contains at least 10% alkali feldspar.[1] Common mafic accessory minerals are amphibole, biotite or pyroxene. Small amounts of nepheline may be present and apatite is a common accessory mineral.[2] Trachyandesite is not a recognized rock type in the QAPF classification, which is based on the actual mineral content. However, latite is recognized in this classification, while benmoreite would likely fall into either the latite or the andesite fields.[8]

Trachyandesite magmas can have a relatively high sulfur content, and their eruption can inject great quantities of sulfur into the stratosphere.[9] The sulfur may take the form of anhydrite phenocrysts in the magma.[10] The 1982 El Chichón eruption produced trachyandesite pumice rich in anhydrite, and released 2.2 × 107 metric tons of sulfur.[11]


Sodium-rich trachyandesite (with %Na2O > %K2O + 2) is called benmoreite, while the more potassic form is called latite. Feldspathoid-bearing latite is sometimes referred to as tristanite.[12] Basaltic trachyandesite is transitional to basalt and likewise comes in two varieties, mugearite (sodium-rich) and shoshonite (potassium-rich).[8]


Trachyandesite is a member of the alkaline magma series, in which alkaline basaltic magma experiences fractional crystallization while still underground. This process removes calcium, magnesium, and iron from the magma.[13][14][15] As a result, trachyandesite is common wherever alkali magma is erupted, including late eruptions of oceanic islands[16][14] and in continental rift valleys and mantle plumes.[17]

Trachyandesite is found in the Yellowstone area as part of the Absaroka Volcanic Supergroup,[18] and has been erupted in arc volcanism in Mesoamerica[11] and at Mount Tambora.[19]


  1. ^ a b c McBirney, Alexander R. (1984). Igneous petrology. San Francisco, Calif.: Freeman, Cooper. p. 503. ISBN 0198578105.
  2. ^ a b Neuendorf, Klaus K.E.; Mehl, James P. Jr.; Jackson, Julia A. (2011). Glossary of geology (Fifth revised ed.). American Geological Institute. ISBN 9781680151787.
  3. ^ a b Le Bas, M. J.; Streckeisen, A. L. (September 1991). "The IUGS systematics of igneous rocks". Journal of the Geological Society. 148 (5): 825–833. Bibcode:1991JGSoc.148..825L. doi:10.1144/gsjgs.148.5.0825. S2CID 28548230.
  4. ^ Self, S.; Gertisser, R.; Thordarson, T.; Rampino, M. R.; Wolff, J. A. (2004). "Magma volume, volatile emissions, and stratospheric aerosols from the 1815 eruption of Tambora" (PDF). Geophysical Research Letters. 31 (20): L20608. Bibcode:2004GeoRL..3120608S. doi:10.1029/2004GL020925. hdl:20.500.11820/6925218f-d09e-4f9d-9f2e-3ab8419b223f. S2CID 56290102.
  5. ^ Eyjafjallajokull. Eruptive history. Global Volcanism Program. Retrieved 26 August 2020.
  6. ^ Woodhouse, M. J.; Hogg, A. J.; Phillips, J. C.; Sparks, R. S. J. (January 2013). "Interaction between volcanic plumes and wind during the 2010 Eyjafjallajökull eruption, Iceland: VOLCANIC PLUMES AND WIND" (PDF). Journal of Geophysical Research: Solid Earth. 118 (1): 92–109. doi:10.1029/2012JB009592.
  7. ^ Donovan, Amy R; Oppenheimer, Clive (March 2011). "The 2010 Eyjafjallajökull eruption and the reconstruction of geography: Commentary". The Geographical Journal. 177 (1): 4–11. doi:10.1111/j.1475-4959.2010.00379.x.
  8. ^ a b c Philpotts and Ague 2009
  9. ^ Schmincke, Hans-Ulrich (2004). Volcanism. Berlin, Heidelberg: Springer Berlin Heidelberg. p. 262. ISBN 9783642189524.
  10. ^ Carroll, M. R.; Rutherford, Malcolm. J. (1 October 1987). "The Stability of Igneous Anhydrite: Experimental Results and Implications for Sulfur Behavior in the 1982 El Chichon Trachyandesite and Other Evolved Magmas". Journal of Petrology. 28 (5): 781–801. doi:10.1093/petrology/28.5.781.
  11. ^ a b Luhr, James F.; Logan, M.Amelia V. (September 2002). "Sulfur isotope systematics of the 1982 El Chichón trachyandesite: an ion microprobe study". Geochimica et Cosmochimica Acta. 66 (18): 3303–3316. Bibcode:2002GeCoA..66.3303L. doi:10.1016/S0016-7037(02)00931-6.
  12. ^ Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge University Press. pp. 140–141. ISBN 9780521880060.
  13. ^ Macdonald, Gordon A. (1983). Volcanoes in the sea : the geology of Hawaii (2nd ed.). Honolulu: University of Hawaii Press. pp. 51–52. ISBN 0824808320.
  14. ^ a b Philpotts and Ague 2009, pp. 369-370
  15. ^ Rondet, Morgane; Martel, Caroline; Bourdier, Jean-Louis (December 2019). "The intermediate step in fractionation trends of mildly alkaline volcanic suites: An experimental insight from the Pavin trachyandesite (Massif Central, France)". Comptes Rendus Geoscience. 351 (8): 525–539. Bibcode:2019CRGeo.351..525R. doi:10.1016/j.crte.2019.07.003.
  16. ^ MacDonald 1983, pp. 51-52
  17. ^ Philpotts and Ague 2009, pp. 390–394
  18. ^ Nelson, Willis H.; Pierce, William Gamewell (1968). "Wapiti formation and Trout Peak Trachyandesite, northwestern Wyoming". U.S. Geological Survey Bulletin. 1254-H. doi:10.3133/b1254H.
  19. ^ Self, S.; Rampino, M. R.; Newton, M. S.; Wolff, J. A. (1 November 1984). "Volcanological study of the great Tambora eruption of 1815". Geology. 12 (11): 659–663. Bibcode:1984Geo....12..659S. doi:10.1130/0091-7613(1984)12<659:VSOTGT>2.0.CO;2.