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M any arc volcanoes are characterized by frequent eruptions of small (<0.1 km 3 ) volume. A key question is do such small events represent discrete magma batches or do these magmas share a partial common history and to what depths? The last 1000 years of eruptions at Tongariro volcano, New Zealand, offer a series of time-calibrated 'snap shots' of an arc magmatic system. Complex and abrupt changes in magma chemistry at Tongariro attest to the small size (<1 km 3 ) and short life span (years to decades) of many magma batches and a powerful role for shallow level assimilation, fractional crystallization and mingling in modifying magma compositions on time scales as short as a year.

New K-Ar age determinations indicate that the exposed portion of the Tongariro volcanic complex has grown steadily since at least 275 ka, with intervals of vigorous cone growth at 210-200, 130-70, and 25 ka to the present day.

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 evidenced in the historical record. Each historical eruption typically lasted days to months, alternating with repose periods of years to decades. Major historic eruptions occurred in 1870 1949 1954-1955 and 1973-1975, encom-passing wide variations in eruptive style over short timescales. 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 individual eruptive pulses vary by two orders of magnitude (2.7-280 m 3 s -1 ), reflecting variations in high level magma ascent rates and processes such as degassing, which are, in turn, reflected in contrasting eruptive styles. Lower 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 plumbing system. There are few detailed studies of the growth patterns of young volcanic cones, but such data are essential in understanding the dynamics of andesitic systems. More than 60 lavas and pyroclastic units mapped on different sectors of Ngauruhoe cone have been correlated by flow chronology and their distinctive compositions into five groups. Although the cone has grown rapidly, Ngauruhoe shows little evidence for the existence 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, multidirectional character and erupts small (<0.1 km 3 ) and short-lived (10 0 -10 3 years) magma batches with no simple time-composition relationships between successive batches.