Coastal boulder production controlled by columnar joints of ignimbrite and extreme waves: insights from the high-energy coast of Pantelleria Island (Sicily Channel, Mediterranean Sea)

Abstract

We examined the relationship between coastal boulder production and the volcanic rock features on the high-energy rocky coast of Pantelleria Island (Sicily Channel, Mediterranean Sea). We used geological data for 125 boulders and 21 rupture surfaces, integrated with fracture analysis, radiocarbon dating, numerical hydrodynamic analysis, and hindcast numerical model for wave characterization. The boulder field consists of isolated and grouped blocks detected along the shore platform and on the cliff top of Punta Sidere (northwestern coast of Pantelleria). Boulder production occurred through two mechanisms: (1) boulders were quarried from the platform edge of the low coast and cliff top as suggested by the same lithology of boulders and outcropping rocks, and by the similar shape of the boulders and rupture surfaces carved out into the rocky platform; (2) waves eroded the cliff foot, forming a notch, and boulders detached from cliff face and fell down to the shore platform. The morphometric analysis of the characteristics of boulders and outcropping rocks demonstrates that the columnar fractures controlled the boulders' polygonal surfaces and size. Whereas, the thickness of horizontal planes within the ignimbrite controlled the boulders' thickness. The numerical hydrodynamic analysis indicates that the quarrying, transport and deposition of boulders were mainly the result of storm waves up to 8 m high. The storm events occurred at different times, even recently, as suggested by two radiocarbon dating of biogenic encrustations collected from the boulders' surface and the presence of wood and plastic embedded within the boulder deposits.