=.31:.::6------NEWSANDVIEWS------'N.:::A:.:..ru~RE~v~o:!::L.~306~24~N~o~VE~M~B~E~R.!.:t98:!::3

thorough documentation of volcanic erup­ ing can be significant and sometimes sur­ be superheated or undercooled, which can tions, and their products. These studies are prising. Repeated episodes of mixing can cause distinctive textures, as well as unex­ the more persuasive since much other generate incompatable trace element com­ pected orders of mineral crystallization. geophysical and observational data can be positions which depart significantly from Mixing is a process that is no longer safe to used to enable convincing interpretations those expected by crystallization or ignore. 0 to be made. Fitton eta/. 's documentation melting4. The topology of phase diagrams, R.S.J. Sparks is in the Department of Earth of the 1982 Mount Cameron eruption is where liquid paths are characteristically Sciences, University of Cambridge, Downing one example of such a study. Perhaps the non-linear, require that mixing can pro­ Street, Cambridge CB2 3EQ. best recent documentation of the relation­ duce compositions which are impossible to l. Filion, J.G., Kilburn, C.R.J., Thirlwall, M.F. & Hughes, D.J. Nature306, 327 (1983). ship between volcanism and magma mixing produce by fractionational crystallization 2. Bell, B.R. Nature 306,323 (1983). is provided by Soviet scientists who studied of any particular parental melt. Further­ 3. The Great Tolbachick Fissure Eruption (eds Fedotor, S.A. & Markhinin, Ye) (Cambridge University Press, 1983). the 1975-1976 fissure eruption of more such mixed magmas must generally 4. O'Hara, M.J. Nature :166, 503 (1977). Tolbachik Volcano in Kamchakta3 • A surprisingly simple picture has emerg­ Science in Africa ed from these studies which, in hindsight, appears all too obvious. Most volcanoes are underlain by complex plumbing Historical science in Sokoto, systems which allow magma from the man­ tle to ascend to the surface. Magma often gets caught in traps (magma chambers) on the way to the surface where cooling and from Beverly Halstead crystallization cause differentiation. At in­ tervals, varying from a few years to tens of THE northwestern corner of Nigeria, beginning of June 1983 and stressed the thousands of years, the system is recharged , holds a special place in the importance of bone-bearing horizons in by new magma rising from depth. As , as it was from here the uppermost Cretaceous and the basal recharge occurs the fluid pressure in the that the great Jihad or Holy War took place Palaeocene. These have enormous plumbing system and magma chambers in­ during the early part of the last century potential in helping resolve the nature of creases until the volcanic edifice fails and under the leadership of Usman Dan Fodio. the Cretaceous - Tertiary transition in an eruption begins. In many cases recharge It was in this same region that early in the tropical latitudes. The paucity of Palaeo­ also causes mixing between magmas of dif­ present century fossil reptiles and fish from cene vertebrates throughout the continent ferent composition in the system by both the uppermost Cretaceous and basal of Africa also makes Sokoto a region of natural and forced convection. It is thus Tertiary were discovered by officers of the especial interest. The abundance and hardly surprising that so many studies Geological Survey of Nigeria and sub­ importance of fossils was recognized and as show that magma mixing occurs shortly sequently described in the 1930s by staff of a consequence of the seminar it was before or during eruptions. Most eruptions the British Museum (Natural History); decided that there should be a place for occur because the plumbing system is ac­ W.E. Swinton1 andE.I. White2 • such palaeontological studies as an integral tivated by ascent of new magma. It was not until half a century later that part of the bureau's future researches and The Tolbachik and Mount Cameron expeditions were mounted specifically to that part of the planned museum should be eruptions provide excellent examples of the recover further material, those in the 1970s devoted to the palaeontological and geo­ general model outlined above. During the producing the skulls of the mosasaur logical history of the State. Tolbachik eruption, seismic data show Goronyosaurus, the dyrosaur crocodile The establishment of the Sokoto State how a high temperature magnesian basalt Sokotosuchus and the pelomedusid turtle History Bureau and its current policies ascended from a reservoir at the base of the Sokotochelys. The 1977-1978 Inter­ mark a major step forwards in intellectual crust, 30 km below. A fissure eruption of national Palaeontological Expedition to development in Nigeria and in a sense this basalt began but after a few weeks the Sokoto State ran into severe difficulties as points the way for the rest of West Africa: a new magma entered a high-level reservoir the party was apprehended by the Nigerian worthwhile way forward for local scholar­ containing already differentiated basalt. A security authorities. Happily the ship, wherever leading politicians have mixture of the two magmas erupted for a misunderstandings were finally resolved both an awareness of their country's heri­ while and gradually became richer in the amicably, as a consequence of the personal tage and an appreciation that scholarship is differentiated basalt end member. The intervention of His Excellency the Military international, and that knowledge should high-level chamber eventually drained and Governor, the late Brigadier Umaru Alhaji have no barriers of nationality or race. caused a 1. 7km diameter caldera to form at Mohammed, who provided all the neces­ It is hoped that the academic endeavours the summit of Tolbachik volcano. A sary logistic support. It was the intention to of Sokoto State will receive an appropriate broadly similar series of events was seen in continue such joint collaboration directly welcome and practical support and the Mount Cameron eruption documented with the Sokoto State Government and encouragement by the academic by Fitton et al. except that the mixed there was a plan to construct a museum to community both in Nigeria and elsewhere. basanite lava became richer in the more house and display the collections3 • There have been local difficulties in primitive magma with time. Since that time, new civilian govern­ exploiting the fossil riches of Sokoto State It is likely that most magmatic systems ments have been elected throughout but it is hoped that with the enthusiastic are open. The main cause of volcanic erup­ Nigeria and are now due for re-election. It nucleus now in Sokoto, these important tions is the increase in fluid pressure caused was not until 1982 that the Sokoto State deposits can be fully studied and the results by recharge of the plumbing system. Com­ Government convened an intenational made available to the international com­ positional diversity of the magmas is still seminar on the archaeology and early munity. In the years to come Sokoto could likely to be mainly brought about by partial history of Sokoto State under the auspices become an important centre in West Africa melting and fractional crystallization dur­ of the Sokoto State History Bureau - for the study of palaeontology. 0 ing the long periods of quiescence in the 'history' in this context being taken to magma reservoirs beneath the volcano. include geological and palaeontological Beverley Halstead is in the Department of Magma mixing will, however, be an in­ history. Sokoto state is unique among the Geology, University of Reading, Reading RG6 evitable consequence of the recharge and nineteen States of the Federation of 2AB. eruption process, and make interpreting Nigeria in having set up a state academic l. Swinton, W.E. Bull. geol. Surv. Nigeria 13, l (1930). the origin of igneous rocks that much more body concerned with the region's history. 2. White, E.l. Bull. geol. Surv. Nigeria 14, I (1934). difficult. The consequences of magma mix- The first seminar was hosted by them at the 3. Niger. Fld Monogr. 1, I (1976).

0028-08361831470316-01$01.00 <0 1983 Macmillan Journals Ltd