Rapid melting of the terminal section of the Hooker Glacier (Mt Cook National Park, New Zealand)

Abstract

A 3 km long section of the Hooker Glacier near its terminus was studied in 1996 using GPS, tacheometric, and bathymetric surveys, as well as ground penetrating radar and gravity surveys. With reference to sparse surface levels and oblique photos dating back to 1889, the studies indicate that between c. 1915 and 1964 downwasting of an axial strip along the terminal section occurred at a rate of c. 0.7 m/yr. Between 1964 and 1986 the rate increased to 1.0 m/yr. Marginal segments of the glacier near the terminus experienced positive buoyancy from 1982 onwards, which promoted rapid melting. Apparent subaqueous melting rates of c. 9 m/yr occurred between 1986 and 1996 over large stretches of the downmelting terminal area. By 1996, a 1 4 km long sector of the glacier had melted down forming a melt lake (Hooker Lake) with a volume of c. 40 × 106 m3 covering an area of 0.78 × 106 m2. A maximum water depth of 135 m was measured near the retreating glacier front where the ice wall descends as a vertical cliff to the lake bottom and temperatures of 0.5°C prevail. Ice thickness measurements by radar surveys along profiles 1.7 and 3.0 km upstream from the terminus indicate a maximum thickness of 165 and 260 m, respectively; the results have been confirmed by the interpretation of residual gravity anomalies. The lake level is controlled by the outlet level (875 m a.s.l.) of the Hooker River, which has remained almost constant since 1889. Melting of the glacier front at a rate of c. 40 m/yr will cause the glacier to retreat at a rather uniform rate. The lake will continue to grow until it reaches the glacier bed c. 5 km upstream from the terminus.