Volcanic geology of Mount Suswa, Kenya

Abstract

This thesis describes the geological history of Mount Suswa, a Quaternary, central-type volcano in the Rift Valley of Kenya.Four major eruptive episodes are distinguished, the earliest of which gave rise to quartz-bearing, flood, trachyte lavas; these constitute a plateau foundation onto which the later, felspathoid-bearing, phonolite lavas of Mount Suswa were erupted.The first eruptive episode of the volcano produced a primitive shieldshaped cone consisting mainly of lava flows. The significance of an unusual fragmental rock occurring at the surfaces of these flows is discussed with reference to the recent controversy on froth flows. The second episode accompanied the formation of a caldera in the summit of the early cone; it gave rise to thick pumice deposits and to lava flows which originated at a ring-fracture zone lying concentrically outside the caldera escarpment. The significance of the depositional relationships of these rocks is discussed with reference to the origin of calderas. The third, post-caldera episode produced phonolite lavas with a different flow morphology, petrography and chemistry to those of the earlier periods. Two post-caldera groups are distinguished; these are thought to have been derived from different phonolite magmas which entered the magma chamber on successive occasions. The origin of a later ring graben which lies within the older caldera is also discussed.-3 -Contents page (Fig. 1). The position of the cone relative to other volcanoes of central-type within the East African Tertiary to Recent alkaline volcanic field is shown in Fig. 2.Willis (1936) diVides the volcanic field into two major areas, both of which consist of contrasting volcanic sequences resting on "Basement System" rocks of probable Pre-Cambrian age (Baker 1965). Of these two areas, a southern sector in Tanzania includes the "Crater Highlands" and Kilimanjaro, and a northern sector in Kenya straddles a "super-elevated dome, (Willis op. cit). This dome is described as "...an oval area of high-land centred on Nakuru an area of upwarping which was uplifted repeatedly in Mount Suswa is one of a series of Quaternary central-type cones confined to the Rift Valley floor (Fig. 2), which includes Longonot, Menengai, Kilombe and Lopokino. In all these volcanoes cauldron collapse structures occur in the summits of primitive cones that are composed of phonolitesand trachytes. TOPOGRAPHY AND GEOGRAPHYMount Suswa is the largest and most complex member of the group, with products extending across almost the whole width of the Rift Valley floor and probably covering a total area of the order of 1000 sq. km. It forms an asymmetric, low-lying, shield-shaped cone, and rises from the floor of the Rift Valley (between 1525 m and 1615 m above sea level) to a maximum altitude of 2357 mat 01 Doinyo Nyukie trignometric point (Fig. 1).The asymmftry of the volcano is due mainly to the southerly dip of the Rift Valley floor on which the cone has grown. This is best exemplified by the lavas erupted down the eastern flank-of the vol...

Geological evolution of the trachytic caldera volcano Menengai, Kenya Rift Valley

“…Each possesses a caldera between 7 and 12 km across. The embayed margins of the Suswa (Johnson 1969) andLongonot (Scott 1980) calderas suggest that they, like Menengai, are Krakatoan. Syn-caldera ash-flow tuffs have been described from Menengai and at least one ash-flow tuff is present at Longonot which covers a comparable area, and is probably as voluminous as those of Menengai.…”

Open System evolution of peralkaline trachyte and phonolite from the Suswa volcano, Kenya rift

“…2; Johnson, 1969;Nash et al, 1969;Omenda, 1997; Macdonald and Scaillet, 2006). The subject of this study, Suswa, is the southernmost volcano in the CKPP.…”

“…RTG (S8) phonolites are strongly porphyritic, similar to the ODNG (S7) rocks (Omenda, 1997). Table 1 Suswa volcanic stages (following Johnson, 1969, andSkilling, 1993 Skilling (1988Skilling ( , 1993.…”

“…Salic volcanism in the CKPP has been focused on five centers. These volcanic centers include: (a) two dome fields: the Eburru Volcanic Complex composed of trachyte and pantellerite (Omenda, 1997;Ren et al, 2006;Velador et al, 2003), and the Greater Olkaria Volcanic Complex (GOVC), dominated by comendites with minor outcrops of trachyte (Clarke et al, 1990;Macdonald et al, , 2008aMarshall et al, 2009;Omenda, 1997); and (b) three trachytic caldera volcanoes: Menengai with a comenditic to pantelleritic trachyte composition (Leat et al, 1984;Macdonald et al, 2011a), Longonot composed of pantelleritic trachyte and mixed (trachyte/ hawaiite) lavas (Clarke et al, 1990;Heumann and Davies, 2002;Rogers et al, 2004;Scott and Bailey, 1984), and Suswa, with trachyte and phonolite lavas and tuffs (Johnson, 1969;Nash et al, 1969;Scott and Skilling, 1999;Skilling, 1988Skilling, , 1993. The mafic lava fields Elmenteita, Ndabibi, and Tandamara, lie in the rift floor adjacent to Eburru, Olkaria and Suswa, respectively, and are composed of alkali and transitional basalt, basaltic trachyandesite and trachyandesite.…”

“…Mount Suswa is a large (>700 km 2 ) trachytic shield volcano with two nested summit calderas that erupted peralkaline trachyte and phonolite lavas and tuffs from approximately 240 to b100 ka (Baker et al, 1988;Johnson, 1969;Nash et al, 1969;Skilling, 1988Skilling, , 1993. Suswa is the southernmost volcano within the Central Kenya Peralkaline Province (CKPP), an area within the Kenyan branch of the east African rift system (EARS) between approximately 0°and 1.5°S latitude ( Fig.…”