The Lajes-Angra Ignimbrite (LAI) was produced during the last major explosive episode of Pico Alto volcano, on Terceira Island (Azores), about 19,000-23,000 14C years BP.
The LAI, which consists of >~ 0.3 km3 DRE of comenditic trachyte magma, comprises widespread deposits that cover most of the island, except the western part that is occupied by Santa Barbara volcano. The pyroclastic flows that deposited this ignimbrite reached the sea at the northern and southern coasts of the island, where the LAI is well represented. However, a large volume of the ignimbrite was probably deposited offshore or inside Pico Alto caldera that is now covered by recent lava domes and coulees.
This low-aspect-ratio ignimbrite varies in thickness from more than 25 m to less than 1 m, and for all of its extent is strongly controlled by the pre-eruptive topography. There are normally two main depositional layers in the ignimbrite, separated by a high concentration of coarser porphyritic pumice at the top of each layer. The juvenile clasts of the ignimbrite can range from light-grey highly vesicular to black dense vitrophyric clasts. The LAI matrix varies from non-welded and light-grey to densely welded and dark-grey, almost forming a black vitrophyre. The lower layer is generally poorer in coarse juvenile clasts than the upper layer. The latter is usually welded at the bottom but becomes less welded towards the top, with the degree of welding depending on its thickness. The presence of these two distinctive layers suggests that the deposition of the ignimbrite occurred in two different eruptive pulses. Where the deposits are less than 1 m thick, the LAI consist of an altered crystal-rich ignimbritic veneer deposit that covers the landscape up to 300m above sea level. Thus the presence of these various deposit facies and its rapid variation along flow paths represent the effects of different eruptive and depositional conditions interacting with a highly irregular topography.
The pyroclastic flows from which the LAI was deposited were mainly channelised within depressions, spreading out from Pico Alto caldera and traveling mostly towards the north and south coasts. They were also able to overrun ridges, leaving behind a thin ignimbritic veneer. However, the transition from the valley-ponded to the thin veneer facies can also occur even over subtle changes of the local topography.