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|Title:||The Geology of Tofua Island, Tonga|
|Authors:||Bauer, Glenn R.|
|Publisher:||University of Hawai'i Press|
|Citation:||Bauer GR. 1970. The geology of Tofua Island, Tonga. Pac Sci 24(3): 333-350.|
|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
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.
|Appears in Collections:||Pacific Science Volume 24, Number 3, 1970|
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