A dynamic interpretation of Tristan da Cunha volcano, South Atlantic Ocean

Abstract

New observations on the morphostructure, eruptive characteristics and intrusive system of Tristan da Cunha, an active intraplate volcano of predominantly trachybasaltic composition, are combined with previously published data to arrive at a model of its internal structure. Volcanic activity on the island of Tristan da Cunha is characterized by a dual (central and lateral) feeding system. The central activity occurred at the summit, presumably on top of an axial magma column. The eruptions produced small flows as well as a high percentage of pyroclastic deposits partly of hydrovolcanic origin. The basaltic magma probably encountered a shallow water table or ephemeral crater lakes. These pyroclastic products contributed in large part to the building up of a well-developed central cone. From the column a dense swarm of radial dykes were generated by hydraulic fracturing. In contrast, the lateral activity occurred on a much lower level on the flanks of the volcano. The style of eruption was strombolian resulting in scoria cones and flows. There is a concentration of strombolian vents around the base of the central cone. This concentric pattern is ascribed to conical shear planes in depth created by uplift of the roof of the magma chamber. The uplift may have contributed to the elevation of the central cone. In addition, a number of lateral vents occur along radial fractures that appear to be concentrated along two tectonic axes oriented N170°E and N80°E. These directions are parallel to the compressive axes of the stress field in the ocean floor. The most prominent axis (N170°E) connects the two most recent eruptions of rather unusual (trachyandesitic, entirely effusive) character. Segmented en échelon dykes and fissures observed in plan and in vertical cliffs are related to slope instability. When a dyke propagates to the surface it enters a zone of bimodal stress: extension and shearing as a result of downslope movement. Under such conditions en échelon segmentation of the dyke takes place.