Nyiragongo is an active strato volcano located 20 km north of Lake Kivu and forming part of the Virunga volcanic field (No. 6). It forms a 3470 m high cone, with the lesser cones of Baruta and Shaheru on the northern and southern flanks, as well as close to 100 smaller satellite cones. Flows that originated from Nyiragongo and its satellite centres occupy a rectangular area of about 26x15 km which extends southwards to the shores of Lake Kivu. Some flows were able to advance 6-7 km eastwards before being diverted to the south by older flows from Karisimbi. In the west and north flows pass beneath younger Nyamuragira lavas. The cone of Shaheru is covered by dense forest and thick deposits of deeply weathered ash with very little exposure. A steep-sided crater 700 m across and 80 m deep has been partly invaded by a flow from the main Nyiragongo volcano. The walls of the crater display layers of coarse ash and cinders. The base of Shaheru is ringed by satellite cones and disappears beneath flows emanating from them. Baruta is a complex cone the main crater of which is 1100 m across and 300 m deep. The inner sub-vertical walls expose massive lavas which are partly vegetation covered, as is the floor of the crater. The northwest wall of the crater is fissured and cut by dykes. Nyiragongo itself is terminated by a crater of 1140x960 m and over 1000 m deep, although the overall depth has varied considerably in the last several decades according to the level of the lava lake. The crater walls in the southwest and south expose agglomerates and coarse tuffs with only about 10% of interstratified lavas. The other walls, however, reveal up to 80% flows with only thin tuff beds. Within the crater is a horizontal annular upper platform, or terrace, separated from a lower platform by a circular fault, although the lower platform has sometimes been inundated by a lava lake which, when at lower levels, occupies a pit within the lower platform. Since the lava lake was observed in 1948 it has varied widely in level, and details of variations in lake level, and other activity within the crater, will be found in Tazieff (1975, Appendix 3) and Richard and Padang (1957), while activity in 1982, when the lava lake rose to cover the lower platform, is described by Tazieff (1984). The structures apparent within the main cone pit crater are described in some detail by Demant et al. (1994), who also give a resume of the regional flows that emanated from the lower flanks of the volcano. Numerous photographs, many in colour, taken during expeditions into the crater are given in a book by Tazieff (1975), and Sahama and Meyer (1958) and Sahama (1978) also present useful general accounts of the crater geology. A good idea of the scale of the inner walls of the crater is given in a photograph in Demant et al. (1994). All the lavas of Nyiragongo are feldspathoidal, with no modal feldspar detected. Many petrographic terms have been used for the various rock types but Sahama (1978) adopted a simplified system, along the lines of the recommendations of the IUGS Subcommission on the Systematics of Igneous Rocks, the principal types of which are nephelinite, leucitite and melilitite with broad gradations between them. This system has been further refined by the detailed work of Demant et al. (1994) who were able to define Lower, Intermediate and Upper units within the walls of the pit crater. The Lower unit comprises melilitites in the lower part above which are heterogeneous melilitites, the former characterised by phenocrysts of melilite, nepheline and kalsilite with scarce pyroxene and olivine and the latter by phenocrysts of pyroxene and melilite. The matrix of the melilitites includes nepheline, pyroxene, olivine, titanomagnetite and leucite and aggregates of kalsilite, melilite and titanomagnetite, whereas the heterogeneous melilitite matrix contains neither leucite nor olivine but abundant calcite. The Intermediate unit comprises nephelinites with phenocrysts of nepheline, leucite and pyroxene in a matrix of the same minerals plus magnetite. The Upper unit is of melilite nephelinites with phenocrysts of nepheline, melilite and in some rocks leucite; the matrix contains, in addition, pyroxene and titanomagnetite. A distinctive feature of the pit crater wall units are aggregates of various minerals such that this type of nephelinite was called 'nepheline-aggregate-lava' by Sahama (1978). The mineralogy of the aggregates in the various units is detailed by Demant et al. (1994) and involves various combinations of nepheline, pyroxene, titanomagnetite, apatite, leucite, melilite, olivine and kalsilite. The lavas occurring on the flanks of the volcano are petrographically similar to those of the pit crater, but those of adventive cones display differences. One aligned series of cones on the lower southwestern flanks, known as the Rushayo chain, is the type locality for rushayite, an olivine-rich melilitite. In general lavas erupted from parasitic vents are olivine- and/or clinopyroxene-phyric whereas those on the main cone generally contain aggregates of nepheline and/or leucite which, Demant et al. (1994) suggest, is indicative of fractionation. A number of dykes and irregular intrusions cut the inner walls of the crater and include bergalite, leucite nephelinite and nephelinite. Modal data for all the principal rock types will be found in Demant et al. (1994) and the names, distribution and petrography of many individual flows is given by Sahama and Meyer (1958). Xenoliths, found either as ejected blocks or inclusions in flows, representing fragments of the underlying basement are principally of fused granite (Sahama, 1978). A coloured geology map on a scale of 1:50,000 accompanies Thonnard and Denaeyer (1965). Analyses of rocks from Nyiragongo are given by Denaeyer et al. (1965), Denaeyer (1972), Sahama and Meyer (1958), Demant et al. (1994) and Sahama (1978), who also gives chemical data on nepheline, leucite, clinopyroxene, melilite, olivine, magnetite and perovskite with details of other minerals including gotzenite, combeite, delhayelite and andremeyerite. Gee and Sack (1988) discuss the Denaeyer et al. (1965) whole rock chemical data in terms of their experimental work on melilite nephelinites. Numerous strontium isotope data are given by Bell and Powell (1969). A detailed discussion of kalsilite is that of Sahama (1960) who also considers the petrogenesis of the lavas (Sahama, 1973). Vollmer and Norry (1983) present and discuss the significance of Nd, Sr and Pb isotopic ratios, while geophysical aspects of the volcano are covered in papers edited by Hamaguchi (1983).