The Geology of Tofua Island, Tonga

Abstract

Tofua Island is an oval, steep-sided composite volcano, 5 miles by 6 miles in diameter, the summit of which has collapsed to form a caldera. Within the caldera, Lofia cone is still active. Four unit s have been mapped: (1) the Hamatua Formation, of precaldera age, includes basaltic andesites, pyroxene andesit es, and pyroxene dacites; (2 ) the Hokula Froth Lava, a microvesiculated lava flow of andesite; ( 3) the Kolo Formation, composed of air-laid lapilli tuff-breccia, tuff, unconsolidated ash and cinder, small basaltic andesite lava flows, and one thick pyroxene andesite lava flow; and ( 4) the Lofia Formation, composed of air-laid tuff, ash, basaltic andesite lava flows, and andesite lava flows. An erosional unconformity separates the Hamatua Formation from the Hokula Froth Lava, and another lies between the froth lava and the Kolo Formation. The rocks are typical orogenic andesites and related types, unusually high in CaO, which is reflected in the calcic nature of the plagioclase. The pyroxenes of the groundmass are usually pigeonite and pigeonitic augite, whereas the phenocrysts are augite. Hypersthene is less common and occurs only as phenocrysts surrounded by reaction rims of pigeonite and/or pigeonitic augite . Concentric normal faults associated with caldera collapse are common on the northern, eastern, and southern rims of the caldera. Some of the faults have served as conduits for rising magma of the Kolo Formation; others , on which caldera collapse is continuing, do not exhibit associated volcanism. Tensional cracks are abundant along the southern rim.