Onetapu Formation: The last 2000 yr of laharic activity at Ruapehu volcano, New Zealand

<jats:title>Abstract</jats:title> <jats:p> The Onetapu Formation represents a sequence of at least 17 volcanic diamictons that were emplaced over the last c. 2000 <jats:sup>14</jats:sup> C yr by lahars in the Whangaehu River catchment, covering part of the eastern flank of Ruapehu volcano and its adjacent ring plain. Interbedded andesitic tephras from the Tufa Trig Formation help in reconstructing the chronology of volcanogenic events that led to the formation of the Crater Lake basin and its subsequent evolution. Onetapu Formation contains deposits laid down by lahars spanning several orders of magnitude. Channel deposits left by the largest lahars are bouldery to distances reaching c. 50 km downstream from the source region. Much of the coarsest fraction of the medium to smaller lahar deposits (i.e., volume <5 × 10 <jats:sup>7</jats:sup> m <jats:sup>3</jats:sup> ) was emplaced close to the source on the Whangaehu Fan, where the river debouches onto the ring plain. Mainly thin, fine grained, pebbly overbank debris‐flow deposits are preserved downstream of the fan, with increasing proportions of sandy hyperconcentrated‐flow deposits. The largest Onetapu lahars are inferred to have been generated by partial collapse of the wall of Crater Lake as a result of powerful explosive eruptions. In contrast, smaller lahars were the consequence of phreatic/phreatomagmatic activity within Crater Lake, expelling lake waters onto the flanks of the volcano. </jats:p>