It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals,[1] and is formed by the rapid cooling of lava (or shallow intrusions) enriched with silica and alkali metals.

[5] Trachyte is common wherever alkali magma is erupted, including in late stages of ocean island volcanism[6][7] and in continental rift valleys,[8] above mantle plumes,[9] and in areas of back-arc extension.

These chemical differences are consistent with the position of trachyte in the TAS classification, and they account for the feldspar-rich mineralogy of the rock type.

The ferromagnesian minerals rarely occur in large crystals, and are usually not conspicuous in hand-sized specimens of these rocks.

Two types of groundmass are generally recognized: the trachytic, composed mainly of long, narrow, subparallel rods of sanidine, and the orthophyric, consisting of small squarish or rectangular prisms of the same mineral.

Sometimes granular augite or spongy riebeckite occurs in the groundmass, but as a rule this part of the rock is highly feldspathic.

It was first given to rocks of this class from Auvergne, and was long used in a much wider sense than that defined above, so that it included quartz-trachytes (now known as liparites and rhyolites) and oligoclase-trachytes, which are now classified as andesites.

[14] It is rarely in crystals large enough to be visible without the aid of the microscope, but in thin sections it may appear as small hexagonal plates, which overlap and form dense aggregates, like a mosaic or like the tiles on a roof.

They often cover the surfaces of the larger feldspars or line the vesicles of the rock, where they may be mingled with amorphous opal or fibrous chalcedony.

Glassy forms of trachyte (obsidian) occur, as in Iceland, and pumiceous varieties are known (in Tenerife and elsewhere), but these rocks as contrasted with the rhyolites have a remarkably strong tendency to crystallize, and are rarely to any considerable extent vitreous.

[7] Trachyte also occurs in areas of back-arc extension, such as the northern Aegean Sea[9] and the Aeolian arc of Italy.

In Britain they occur in Skye as lava flows and as dikes or intrusions,[23] but they are much more common on the continent of Europe, as in the Rhine district and the Eifel, also in Auvergne, Bohemia and the Euganean Hills.

The latter differ in no essential respect from their modern representatives in Italy and the Rhine valley, but their augite and biotite are often replaced by chlorite and other secondary products.

Alkaline rocks with an age close to 570 million years are common around the perimeters of many continental shields and are evidence of worldwide rifting at that time.