Surface swelling of coastal bedrock on inter-tidal shore platforms, Kaikoura Peninsula, South Island, New Zealand

Abstract

This paper reports the occurrence of swelling of coastal bedrock measured with a traversing micro-erosion meter (TMEM) on inter-tidal mudstone and limestone shore platforms. While numerous micro-erosion meter (MEM) studies have been made, only a small number have reported surface swelling. During this study, 24,055 measurements were made with a TMEM, of those 4047 indicated swelling or 17% of the total number of measurements. Vertical swelling of up to 8.9 mm was measured and values of 3 and 4 mm were common. Swelling displayed a complex pattern with minor swelling being superimposed on longer period episodes. Duration of swelling events in excess of 697 days were recorded, while other events were commonly in the order of 3 to 4 months. There is a distinct seasonal variation with greater and more frequent swelling occurring during summer months compared with winter. Explanation for swelling includes the growth of salt crystals in rock lattices and expansion from wetting and drying. Warm summer temperatures provide the best conditions for both processes. Swelling appears to be an important precursor to erosion of material from shore platform surfaces. The conclusion drawn is that subaerial weathering makes an important contribution to the lowering of the shore platforms on the Kaikoura Peninsula.