Engineering geology model of the Crater Lake outlet, Mt. Ruapehu, New Zealand, to inform rim breakout hazard

Abstract

Mt. Ruapehu, in the central North Island of New Zealand, hosts a hot acidic Crater Lake over the active volcanic vent with a surface elevation of c. 2530m.a.s.l. Volcanic activity and other montane processes have previously resulted in catastrophic releases of some or all of the c. 10Mm3 of water retained in the lake, creating serious hazards downstream. A major lahar in March 2007 exposed a 10m high face representative of the rock units impounding the lake, providing an opportunity to conduct both field and laboratory analysis to characterise the rock mass conditions at the outlet to assess the stability of the outlet area. This paper presents an engineering geology model of Crater Lake outlet. Our model shows three andesitic geological units at the outlet, each with different geological histories and physical and mechanical properties, which impact its stability.Geotechnical methods used to characterise the outlet include laboratory testing of the strength, stiffness, porosity and unit weight, and field-based rock mass characterisation using the geological strength index (GSI) and rock mass rating (RMR). Field observations, geomorphology mapping, historic and contemporary photographs, and laboratory results are combined to create cross sections that provide key information for establishing the engineering geology model. The units are recognised in what is informally termed the Crater Lake Formation: i) The upper surface layer is a c. 2m thick sub-horizontal dark grey lava unit (Armoured Lava Ledge) with sub-horizontal cooling joints spaced at 0.2m to 2.0m intervals. The intact rock has a porosity range of 15–27%, density range of 1723–2101kg/m3, GSI range of 45–75, and unconfined compressive strength (UCS) range of 19–48MPa. ii) Below this, and outcropping down the majority of the outlet waterfall is a poorly sorted breccia unit composed of block and matrix material (Lava Breccia). The blocks range from 0.1m to 0.8m in diameter with an average porosity of 21%, a density of 1956kg/m3 and strength of 85MPa. The matrix has soil-like properties with an estimated UCS of 1.5MPa. iii) At the base of the waterfall, the material sharply transitions into a light grey, slightly weathered unit (Lower Grey Member). This lower unit has an irregular surface expression with sub-vertical discontinuities. Porosity is 6%, density is 2569kg/m3, the GSI range is 65–75, and the UCS is 98MPa.The engineering geology model portrays the relationships between the units in three dimensions, highlights key structures and takes into consideration the material source, transportation and depositional processes. Historical outlet photographs suggest past eruptive and glacial activities are both significant factors controlling the deposition and erosion of material at the outlet. The Lower Grey Member appears to be a sound material for the outlet and water fall to be founded on. The upper aa Armoured Lava Ledge currently has moderate strength and GSI, and is resistive, providing protection for the underlying weaker block and matrix unit, however, continued incision by the outlet stream will eventually expose the weaker block and matrix material of the Lava Breccia. Once exposed, the Lava Breccia could rapidly erode or fail down to the Lower Grey Member and could potentially release 1Mm3 of hot, acidic Crater Lake water. We recommend that erosion rates for the upper Armoured Lava Ledge be established to aid in preparation for eventual rim breakout.