Volcanic influences on terrestrial sedimentation

et al. 1997

Observations of active flows, their deposits, and the effects of lahars generated during the 1995 eruptions of Ruapehu are compared to those of the last 2000 years. The 1995 lahars were generated by similar mechanisms, and had similar volumes and flow rheologies, to those of the last 135 years. However, the large number of lahars in the 1995 sequence and the eventual emptying of the entire Crater Lake on Mt Ruapehu by lahars is distinctive in the context of the historic record. The 1953 and 1975 lahars, although of smaller total volume than the largest 1995 lahar, had higher peak discharges and consequently greater effects on life and property within 57 km from the source. This implies that failure of a lake dam is the most efficient mechanism for generating a fast lahar with high peak discharge, and that eruptions through Crater Lake are more efficient at generating a lahar with high peak discharge when the lake is full.The scant 1995 lahar deposits preserved indicate that the geological record of lahars is likely to be incomplete and biased to events of greatest magnitude. Given this limitation, lahars which formed deposits of the prehistoric (>135 years old) Onetapu Formation were of larger magnitude in all respects, and often more concentrated with sediment, than those of the last 140 years (including 1995). In order to generate such events, catastrophic total emptying of Crater Lake is required. Alternatively, some of the events may have been the result of small flank collapses on the eastern slopes of Ruapehu. These larger events are less frequent than small eruption-triggered lahars on Ruapehu, but they may be preceded by little or no warning. Future lahar hazard mitigation strategies in the Whangaehu River should begin with improving the detection and early warning of such flows. G96042

Section: Lahar Terminologymentioning

confidence: 99%

1995 Ruapehu lahars in relation to the late Holocene lahars of Whangaehu River, New Zealand

Cronin

Hodgson

et al. 1997

“…3, 4). Smith & Fritz (1989) define a lahar as "a general term for a rapidly flowing mixture of rock debris and water (other than streamflow) from a volcano". This term encompasses a range of possible flow types, and as a result many terms are now widely used to classify lahars, or more specifically to define end-member processes and the range of flow transformations that occur between these, as well as the resulting deposits (Pierson & Scott 1985;Pierson & Costa 1987;Smith 1986;Costa 1988;Vallance & Scott 1997).…”

Section: Tephrostratigraphic and Tephrochronological Frameworkmentioning

confidence: 99%

Late Quaternary constructional history of the southeastern Ruapehu ring plain, New Zealand

Donoghue

2001

“…The word lahar is here used to describe "a rapidly flowing mixture of rock debris and water (other than normal streamflow) from a volcano" (Smith & Fritz 1989). Lahars are generally subdivided on the basis of differing sediment concentrations and resulting differences in rheology.…”

Section: Terminologymentioning

confidence: 99%

Lahar history and hazard of the Tongariro River, northeastern Tongariro Volcanic Centre, New Zealand

Cronin

Palmer

1997

Lahar deposits beside the Tongariro River have been mapped and dated using andesitic and rhyolitic marker tephras. Coupling the stratigraphic record obtained with that of the Tongariro and Ruapehu ring plains has enabled reconstruction of the history of lahars along the Tongariro River. This forms the basis of a lahar hazard map for the entire catchment. Six lahar hazard zones, with assigned recurrence intervals ranging from 1 in 35 yr to 1 in >15 000 yr, have been mapped. Lahar deposits between the ages of 14.7 and 9.8 ka cover the greatest areas, while younger lahar deposits are confined to lower surfaces closer to the present river channel. Holocene lahar deposits along the Tongariro River are not as well preserved as older units, probably because the Holocene lahars were confined to a more deeply incised channel and were more readily eroded following emplacement. All recorded post-11.85 ka lahars in the Tongariro catchment were derived from Ruapehu volcano. The Mangatoetoenui Stream has been the conduit for the greatest number of Holocene lahars and for all of the historic ones. Most of Turangi is built on a surface which has not been inundated by a lahar since c. 10 000 yr ago. However, the area identified where high risk affects the greatest population is a part of Turangi, which was inundated by a lahar less than 1850 yr ago. The infrastructures at greatest risk include the State Highway 1 bridge across the Mangatoetoenui Stream and the Rangipo Dam and power station.