Soil depth-age relationship of landslides on deforested hillslopes,taranaki, New Zealand |
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| Institution: | 1. Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Turitea Campus, Palmerston North, New Zealand;2. Earth Science Institute, Slovak Academy of Sciences, Bratislava, Slovak Republic |
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| Abstract: | A soil chronosequence was examined on landslide scars of different ages in the Taranaki hill country. This area, underlain by Tertiary silty sandstone, was deforested 90 years ago. Sequential aerial photographs, historical terrestrial photographs and documented erosion events were used to date landslide scars formed since forest removal and establish age control for the chronosequence.Soil depth increased with landslide age and was used as an index of soil formation. Mean soil depth increased from 5 cm on 15 year old scars to 20 cm on 82 year old scars. Measures soil depths were attributed to rafted soil, colluvium and bedrock weathering. A chronofunction was derived by regressing mean soil depth against the logarithm of known scar age. Similarly, a second chronofunction was derived by excluding depths of rafted soil from the calculation of mean soil depth to describe soil accumulation on exposed bedrock within the landslide scar. This chronofunction showed a better correlation (r2 = 0.92 compared with r2 = 0.79) and can give an estimate of the age of other landslide scars to within ±27% for ages up to 90 years.The rate of soil depth increase averaged 3.5 mm yr?1 over the first 40 years after slipping but dropped to 1.2 mm yr?1 over the following 50 years. The logarithmic chronofunction suggests that the rate of soil formation further decreases beyond 90 years. Soil formation is primarily a result of bedrock weathering and accumulation of colluvium derived from surface fragmentation of exposed bedrock and crumbling scar margins. |
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