Journal of Conference Abstracts

The Geometry of Low-Aspect Ignimbrites Emplaced by Turbulent Parent Flows

W. B. Dade (bdade@esc.cam.ac.uk) & H. E. Huppert (heh1@esc.cam.ac.uk)

Institute of Theoretical Geophysics, Department of Earth Sciences and Department of Applied Mathematics & Theoretical Physics, University of Cambridge, Cambridge CB2 3EQ, England.

The mechanism of emplacement of ignimbrites with a low aspect ratio is currently a vexed issue in volcanology. We have considered (Dade and Huppert, 1995) the end-member scenario of a dilute, radially spreading gravity current with constant total flux and with a driving suspension which can be characterised by a single, average fall speed of individual particles. The thickness of the deposit resulting from such a flow is predicted to diminish exponentially with radial distance from the vent. When a range of grain sizes is considered, moreover, a simple averaging scheme indicates that the probability density function of fall speeds in the initial suspension at the vent can be inferred from the observed deposit geometry. Such an analysis has been applied to fall deposits generated by ash-laden umbrella clouds which spread away from sustained eruption columns at altitude (Koyaguchi, 1994). We apply it successfully to examples of pyroclastic flow deposits laid down by ground-hugging flows associated with some violent eruptions (Dade and Huppert, 1996).

The ability to delineate the initial distribution of particle fall speeds from the geometry of a pyroclastic deposit offers a useful framework for the characterisation of a parent eruption in terms of fragmentation processes in the explosion. Where initial distributions of particle size and fall speed can be independently inferred, on the other hand, the analysis provides a powerful test of whether a pyroclastic deposit is in 'aerodynamic equilibrium' on a regionally-averaged basis.

Dade, W. B., & Huppert, H. E., J. Geophys. Res. (Oceans), 100(C9), 18,597­18,609 (1995).

Dade, W. B., & Huppert, H. E., Nature, 381, 509-512 (1996).

Koyaguchi, T., Bull. Volcanology, 56, 1­9 (1994).

Peralkaline Silicic Lavas from the Tarosero Volcano, N. Tanzania

J. B. Dawson¹ (jbdawson@glg.ed.ac.uk) & J. R. Cooper²

¹ Grant Institute, University of Edinburgh EH9 3JW.

² Department of Geology, University of Manchester M13 9PL.

Tarosero (3^o12'S, 36^o22'E) is a small (25 km diameter) cone rising some 1,000 m. above an alkali-basalt lava plateau. It is a member of the Older Volcanic Series of the N Tanzania Neogene-Quaternary volcanic province (Dawson, 1992), with lavas dated at between 2.4 and 1.9 Ma (MacIntyre et al., 1974).

Principal component analysis of major- and trace-elements in 41 lava samples (Cooper, 1972) has established two main lava groups, which are alkaline to peralkaline, characteristically containing Na-rich amphibole, Na-clinopyroxene and aenigmatite. Group 1 ranges from benmoreite, through quartz trachyte, trachyte and phonolite to pantellerite, linked by pyroxene and feldspar fractionation; the more siliceous are peralkaline, and chevkinite (Ce₄Fe₂Ti₃[Si₂O₇]₂O₈) has been found in one quartz trachyte. Group 2 comprises quartz trachyte, trachyte and phonolite, containing sodalite and eudialyte; all are peralkaline, and relative to Group 1 rocks with the same major element concentrations, are enriched in Cl, Ba, Sr, Rb, REE, Nb, Zr and Pb.

The overall lava sequence, emplaced at this single volcano, covers the same compositional spectrum as the plateau trachytes and phonolites that are extruded on a regional scale on the Kenya Dome to the north. The absence of an overall trend in the extrusion sequence (e.g. the latest extrusions are of quartz trachyte), togther with interbedding of (a) nepheline-bearing and quartz-bearing, and (b) Group 1 and Group 2 lavas, indicates complex evolution and emptying of the sub-volcanic magma chamber.

Cooper, J., Geochemistry of Tarosero volcano, northern Tanzania, Unpubl. M.Sc thesis, University of Manchester (1972).

Dawson, J.B., Tectonophysics, 204, 81-92 (1992).

MacIntyre, R.M., et al, Nature, 247, 354-6 (1974).