Abstract Ngauruhoe cone, in southern Taupo Volcanic Zone, New Zealand, has grown rapidly over the last 2,500 years in an alternation of effusive, strombolian, vulcanian, and sub-plinian eruptions of andesitic magma. At times growth has been ‘staccato’ in fashion as evi- denced in the historical record. Each historical eruption typically lasted days to months, alternating with repose periods of years to decades. Major historic eruptions oc- curred in 1870 1949 1954–1955 and 1973–1975, encom- passing wide variations in eruptive style over short time- scales. The early period of cone building appears to have been dominated by a more continuous form of activity characterised by a series of numerous frequent explosive eruptions, with associated lava flows. The 2.2-km 3 cone has grown in a piecemeal sectorial manner reflecting constant modification to the morphology of the summit, which has funnelled eruption products to specific sectors of the cone. Eruption rates can be calculated on several different timescales. Discharge rates averaged over indi- vidual eruptive pulses vary by two orders of magnitude (2.7–280 m 3 s –1 ), reflecting variations in high level mag- ma ascent rates and processes such as degassing, which are, in turn, reflected in contrasting eruptive styles. Low- er rates (e.g. 0.65 m 3 s –1 ) are obtained by averaging the discharge over an entire eruption lasting several months and may correspond to the ascent rate of magma batch(es) feeding the eruption. The long-term growth rate of Ngauruhoe is 0.9 km 3 ky –1 . This is an average rate reflecting the long-term deep supply rate of magma to crustal reservoirs. By looking at eruption rates on these different timescales we are better able to constrain processes occurring at various depths within the plumb- ing system. There are few detailed studies of the growth patterns of young volcanic cones, but such data are es- sential in understanding the dynamics of andesitic sys- tems. More than 60 lavas and pyroclastic units mapped on different sectors of Ngauruhoe cone have been corre- lated by flow chronology and their distinctive composi- tions into five groups. Although the cone has grown rapidly, Ngauruhoe shows little evidence for the exis- tence of large crustal magma reservoirs and long-lived magma batches. Instead, abrupt and non-systematic changes in magma chemistry and isotopic composition between and within the five groups indicate that the volcano has an open-system, multi-process, multi- directional character and erupts small (<0.1 km 3 ) and short-lived (10 0 –10 3 years) magma batches with no sim- ple time–composition relationships between successive batches. Keywords Andesitic volcanism · Composite cone · Eruption rates · Lava flows · Ngauruhoe · Explosive eruptions Introduction During the past few decades elegant models have been constructed for small scale eruptions at basaltic volca- noes (Head and Wilson 1989; Mangan and Cashman 1996), and large silicic eruptions at rhyolitic calderas (Wilson and Walker 1985) or composite cones (Carey and Sigurdsson 1986; Carey et. al. 1990). Only very re- Editorial responsibility: W. Hildreth B.J. Hobden ( ✉ ) Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand e-mail: b.hobden@waikato.ac.nz Tel.: +64-7-8384024, Fax:. +64-7-8560115 B.F. Houghton Department of Geology and Geophysics, University of Hawaii, 1680 East–West Rd, Honolulu HI 96822, USA I.A. Nairn Institute of Geological and Nuclear Sciences (IGNS), Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand Present addresses: B.J. Hobden, Department of Earth Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand I.A. Nairn, 45 Summit Rd, RD5, Rotorua, New Zealand Bull Volcanol (2002) 64:392–409 DOI 10.1007/s00445-002-0216-3 RESEARCH ARTICLE Barbara J. Hobden · Bruce F. Houghton Ian A. Nairn Growth of a young, frequently active composite cone: Ngauruhoe volcano, New Zealand Received: 25 June 2001 / Accepted: 15 March 2002 / Published online: 9 May 2002 © Springer-Verlag 2002