Growth of a young, frequently active composite cone: Ngauruhoe volcano, New Zealand

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 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.

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“…-1 for the past 2.5 k.y. at Ngauruhoe (Hobden et al, 2002). The highest quantifi able short-term Mazama rate is the ~8 km 3 k.y.…”

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“…This gives an approximate construction rate of 0.22 km 3 ka (1 . Since Ngauruhoe eruptions commenced about 2500 years ago, it has grown to a size of 2.2 km 3 i.e., at a rate of 0.9 km 3 ka (1 without even considering the other parts of the Tongariro Volcanic Centre (Hobden et al 2002). There have been no eruptions of Ngauruhoe since 1975, so the rate is zero for the last 34 years.…”