ERUPTION, INTRUSION, AND PRESERVATION OF A COASTAL SCORIA CONE: RED BAY, BANKS PENINSULA, NZ

Cross sections of scoria cones provide insights into eruptive processes and dynamics, and stages of intrusion. At Red Bay, Banks Peninsula, New Zealand, the ocean has incised the coastline, providing a cross section through lava flows, scoria deposits, and dykes.

Scoria deposits are primarily bedded scoriaceous units with partially-welded spindle and cow-pat bombs, indicating initial eruptions of Strombolian style. Thin lavas overlie the scoriaceous deposits, and dip away from the head of the bay. Lavas indicate transition from Strombolian to effusive Hawaiian style eruptions. Intruding through scoria deposits and lava flows are a series of randomly oriented dykes. Dykes at upper levels branch out or taper, with some dykes showing multiple intrusions. Intrusions through scoria deposits indicate co-intrusion deformation, deforming bedded scoriaceous clasts upwards along the profile of vertical intrusion. Overlying the units associated with the scoria cone are a series of lava flows, sourced from the main vent eruptives of the Akaroa Volcanic Complex.

Geochemical analysis indicates that scoriaceous eruptions and initial lava flows as picrite basalt, with later lava flows being more evolved (hawaiite). Later intrusive phases vary from hawaiite to mugearite. Thin-section analysis further suggests that there was an evolution of the magma source. Scoriaceous units are not highly evolved, made up of mostly plagioclase groundmass with a few larger plagioclase phenocrysts, clino-pyroxene, and olivine. Dykes have trachytic texture and felsic xenoliths suggesting they are the most evolved and late stage.

A reconstructed scoria cone has a minimum footprint of 800m and an approximate height of 225m. The exposure and expression observed in the landscape today is the result of 1) scoria cone eruption; 2) later lava flows of the Akaroa Volcanic Complex, flowing around and covering the flanks of the scoria cone, providing a protective screen; 3) millions of years of erosion, resulting in the progressive incision from the coast into the interior of the scoria cone, probably promoted by increased erosive tendencies of intrusions; 4) further hollowing out of the interior of the scoria cone, removing more erodible scoria deposits and leaving dyke remnants as reefs and cliff forming units.