Uniaxial compression strength and erosive potential rate of rocks in strata on the coast of Miura Peninsula with specific relation to the algal communities of Eisenia bicyclis and Ecklonia cava

Abstract

To clarify the physical properties of the rock substratum on which algal communities of Eisenia bicyclis and Ecklonia cava are found, studies were conducted on: (i) strata on the coast of the Miura Peninsula; (ii) uniaxial compression strength and erosive potential rate of the substratum; and (iii) bond strength of Eisenia bicyclis classified by location. Sea beds and rock strata lying immediately off promontories and coves with lush algal growth were identified at Bokke-zaki (Misaki Formation: Ms), Jyogashima Island (Hatsuse Formation: Ht), Ara-saki (Ms), Kawajiribana (Zushi Formation: Zs), and Chojyaga-saki (Zs). These have all been verified as belonging to the late Miocene (Miura Group). The compression strengths (kgf/cm²) of rocks at Ms, Zs and Ht were 73.0, 86.2 and 148.9, respectively. Erosive potential rates (%/h) of Ms, Zs and Ht strata were 4.47×10^-2, 3.64×10^-2 and 2.09×10^-2, respectively. Bond strengths of Eisenia bicyclis to the substratum (kg) on Ms, Zs and Ht strata were 25.3, 27.4 and 23.1, respectively. Rocks of the Quaternary (Miyata Formation: My) with no algal community were found to be much more brittle than those in the Tertiary period. Conversely, concrete blocks were very rigid, but bond strength of Eisenia bicyclis was as low as 50% compared to Tertiary period rock. It can be seen from the observations above that the geological properties and the nature of the late Miocene rocks are the most favourable substratum for algal communities of Eisenia bicyclis and Ecklonia cava.