Trachyandesite

The term trachyandesite had begun to fall into disfavor by 1985 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.

Trachyandesitic magma can produce explosive Plinian eruptions, such as happened at Tambora in 1815. The Eyjafjallajökull 2010 eruption (VEI-4), which disrupted European and transatlantic air travel from 15-20 April 2010, for some time was dominated by trachyandesite.

Petrology

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. 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. Common mafic accessory minerals are amphibole, biotite or pyroxene. Small amounts of nepheline may be present and apatite is a common accessory mineral. 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.

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