The geology of Longonot volcano, Central Kenya: a question of volumes

Abstract
The Quaternary central volcano Longonot is situated on the floor of the Eastern Rift Valley of Kenya, at 36°26'E, 0°55'S, within the Nakuru-Naivasha peralkaline volcanic province. The volcano consists of peralkaline trachyte lavas and pyroclastics, accompanied by minor volumes of mixed, hawaiite-peralkaline trachyte lavas. Longonot has been mapped in detail as a prerequisite to the collection of a sample suite representative of the volumes of liquid erupted during specific periods of activity. Particular emphasis has been given to establishment of the time relations and original volumes of its volcanic products. Seven episodes of activity are recognized: (i) growth of a primitive, peralkaline trachyte composite cone (280 km 3 ) ; (ii) caldera collapse at the summit of the composite cone, accompanied by late-stage pyroclastic activity (> 50 km 3 ) ; (iii) eruption of mixed, hawaiite-peralkaline trachyte lavas on the northern Longonot plains (0.5 km 3 ) ; (iv) growth of a peralkaline trachyte lava-pile on the partly infilled, eastern caldera floor (16 km 3 ) ; (v) lava-pile grow th term inated by a phase of ash eruptions ( > 2 km 3 ) ; asymmetric collapse o f the lava-pile and ash-cover summit followed, forming a pit crater; (vi) eruption of very recent, mixed hawaiite—peralkaline trachyte lava onto the pit-crater floor ( > 0.34 km 3 ) ; (vii) flank eruptions of two peralkaline trachyte flows (0.045 km 3 ). Liquid equivalent volumes of volcanic products erupted during each episode of activity are calculated ; the results indicate that the products formed from the greater part of the total liquid volume erupted are not exposed. This presents a serious problem for the collection of a sample suite representative of the volumes of liquid erupted throughout the entire history of the volcano. A sample suite truly representative of liquid volum es erupted can only be collected from those products erupted during and after the late-stage caldera pyroclastic activity. These products, how ever, account for only 14.5 % of the total liquid volume. This unavoidable undersampling must be taken into account when the petrogenesis of the entire Longonot volcanic succession is considered.

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