Terceira shows a great diversity of lavas and pyroclastics for an oceanic island and is noteworthy for voluminous production of peralkaline salic magma. Four volcanoes form the island; three are composed of both basic and salic rocks and one has only salic rocks exposed. Since the emergence of the island a compositionally bimodal population of rocks has been represented.
The products of over 100 eruptions in the upper Terceira Group have been recognised. These include ignimbrites, pumice fall deposits, salic lava extrusions, strombolian scoria deposits, basaltic lava flows and littoral (surtseyan) basaltic tuffs. Basaltic activity is concentrated along a fissure zone which bisects the island diagonally from NW to SE and may be the subaerial expression of the Terceira Rift. Volumetric studies show the rate of accumulation of new crust along this small spreading centre; 5.46 km3 of new material has been erupted on the island in the past 23 000 years, of which over 4 km3 is of comendite–pantellerite composition.
The pumice fall deposits are of sub-plinian type characterised by smaller dispersal areas than plinian deposits, inverse grading of pumice and internal stratification. The pumice deposits were produced by eruptions of lower explosivity than those which occur on other Atlantic islands, which are interpreted as resulting from lower magmatic gas contents.