Ice flow and surficial velocity inferred from satellite image and air photo analysis of Larsen Ice tongue, Hells Gate and Nansen ice shelves (Northern Victoria Land, Antarctica).

The analysis of aerial photographs TMA 325, 365 (1956, 1957) and TMA 2851 and 2852 (1985) and of the satellite images Landsat 1 MSS (Nov. 29, 1972) and Spot multispectral (Dec. 19, 1988) allows a first evaluation of ice surface velocity and of ice cliff variation of the Larsen Ice Tongue, Hells Gate and Nansen ice shelves. The Nansen Ice Sheet (shelf) covers an area of about 1800 km2. The mean ice surface velocity (in the central sector of the ice shelf between Inexpressible Island and Tarn Flat) ranges from 120 to 360 m/yr (in the period 1972-1988). The highest velocities are to be found close to the ice barrier where the ice shelf spreads out. Between 1972 and 1988 the ice barrier advanced causing an increase of the ice shelf of about 56 km2 at an average rate of 3.5 km2/yr. The surface velocity of the small Larsen Ice Tongue is comprised between 115 and 170 m/yr. The ice tongue surface increased about 19 km2 over the period 1957-1988. Hells Gate is a small ice shelf more than 70 km2 wide, extending between the Northern Foothills, Vegetation Island and Inexpressible Island. The comparison of the aerial photographs taken in 1956 and 1985 shows that the mean ice surface velocity close to the calving margin varies from 3.3 m/yr in the eastern sector, to 10.2 m/yr in the faster moving western sector. The ice cliff retreated between 1956 and 1988 with an areal reduction of 1. 15 km2. In the same period the ice lost at the calving has been evaluated between 40 . 106 m3 and 72 . 106 m3. A large amount of the ice was broken away in winter 1987. The ice shelf retreated after 1912 when a first topographic sketch map was drawn by Priestley (1923).