Longonot

Longonot volcano has a well-developed cone with a 2 km diameter summit crater defined by very steep to vertical walls between 75 and 350 m high. It rises about 880 m above Lake Naivasha, to which it extends in the north, and occupies an area of approximately 350 km2, but airfall ashes may occur over an area as great as 3000 km2 (Scott, 1980). The volcanic rocks of its eastern flank extend onto, and are intimately involved with, the eastern margin of the rift. Arcuate lava flow fronts up to 40 m high form distinct topographic features and arcuate ridges on the western and southern sides of the cone mark the buried rim of an earlier caldera within which the cone was built. The volcano comprises peralkaline trachyte lavas and pyroclastic rocks with minor mixed lavas of hawaiite and peralkaline trachyte. Scott (1980) distinguished seven episodes of activity, which he describes in considerable detail, and these are presented in simpler form by Clarke et al. (1990), on which the following brief account is based. (1) Building of an early shield of welded pyroclastic rocks and lavas, which are encountered in boreholes. (2) Caldera collapse with concomitant eruption of trachytic ignimbrites and associated fall deposits. (3) Building of a cone of pyroclastic rocks with later stages predominantly of lavas. The earlier pyroclastic rocks include surge deposits the later predominantly ashfalls. The lavas comprise both trachytes and mixed types (alkali basalt - peralkaline trachyte) (Scott and Bailey, 1984). (4) Formation of a summit crater preceded or accompanied by ash eruption. (5) Eruption very recently of mixed hawaiite-trachyte lava onto the crater floor and of trachyte flows onto the flanks of the volcano. The ignimbritic trachytes of stage (2) contain crystals of alkali feldspar, amphibole, a green clinopyroxene and probable aenigmatite. Pumice beds of stage (3) include syenitic lithic fragments containing phenocrysts of anorthoclase, augite, sodalite, magnetite and rare fayalite with sodic plagioclase and aegirine-augite in the groundmass. Deuteric alteration of these rocks led to the development of interstitial to poikilitic fayalite, aegirine, arfvedsonite and aenigmatite (Clarke et al., 1990). Trachyte lavas include phenocrysts of anorthoclase, fayalite, Fe-rich pyroxene and titanomagnetite with a groundmass of alkali feldspar and pyroxene, aenigmatite and sodic amphibole (Scott, 1982). The mixed lavas of stage (3) are located on the lower north-northeastern flanks of the volcano and represent co-eruption of alkali basalt and pantelleritic trachyte lavas; they are described in detail by Scott and Bailey (1984), who present mineral and rock analyses. In section the rocks are seen to comprise either two contrasting groundmass components each supporting a unique phenocryst assemblage, or a uniform groundmass with variable proportions of all the phenocryst types. The heterogeneous lava groundmass comprises a basalt component with zoned plagioclase phenocrysts with bytownite cores, forsteritic olivine, salitic pyroxene and an opaque phase, and a trachytic component of alkali feldspar, sodic pyroxene, fayalitic olivine, aenigmatite and an opaque phase. The synchronous caldera-forming events at Longonot have been investigated in detail by Scott and Skilling (1999) using tephrachronology.