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The Geology of Tofua Island, Tonga! The Geology of Tofua Island, Tonga! GLENN R. BAUER2 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 units have been mapped: (1) the Hamatua Formation, of precaldera age, includes basaltic andesites, pyroxene andesites, 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 .associared volcanism. Tensional cracks are abundant along the southern rim. THE GEOLOGICAL INVESTIGATION of Tofua foot contour intervals was drawn up primarily Island is part of a larger study of the Tonga and for the gravimetric work, but it was also used as the Lau islands by the Hawaii Institute of Geo­ a base for the geologic mapping (Fig. 2 B) . physics. The investigation includes paleomag­ Elevations were obtained by aneroid altimeter. netic and gravity studies of many of the islands, Most of the geologic mapp ing was done con­ but only the areal geology and petrology of currently with the topographic mapping. Tofua are dealt with in this paper. Tofua's flanks are heavily wooded: rock out­ Field work on Tofua was performed during crops are especially difficult to locate on the the first three weeks of June 1968. A base map windward, eastern, and southern slopes. Pyro­ showing the triangulation stations established by clastic deposits of the Kolo Formation cover the the Tongan Government Survey in 1961 was northwestern flank and part of the western flank, obtained from the Lands Department of Tonga, in Nuku'alofa. Although the triangulation sta­ and the bedrock is concealed. Approximately a tions were only temporary, it was not difficult to week was spent working on the caldera rim and relocate them, since all were at prominent land­ within the caldera, and two weeks were devoted marks or villages. Using them as control, a to mapping the flanks of the cone. topographic map of Tofua (Fig. 2 A) with 200- PREVIOUS INVESTIGATIONS i Hawaii Insti tute of Geophysics Contr ibuti on N o. 336. Investigation supported by Grant GA-1 473 from Previous geolog ical studies in the Tonga the N ational Science Foundation. Manuscript received D ecember 18, 1969. Islands (Lister, 1891 ; Hoffmeister, 1932) were 2 P.O. Box 3415, Honolulu, Hawaii 96801. primarily concerned with detailed descriptions 333 334 PACI FIC SCIENCE, Vol. 24, July 1970 of the Tertiary limestones. Descriptions of the amounts of MnO, in the vicinity of 0.16 per­ igneous rocks were brief. cent, are shown. In 1643 Abel Tasman became the first Euro­ pean explorer to sight Tofua Island. He made GEOGRAP HICALDESCR IPTI ON note of sailing near two high islands (Tofua and Kao) and seemed surprised at their height, The Tonga Archipelago lies between latitudes for he had just left the low limestone islands 15° and 22°S and longitudes 173° and 177°W. that lie east of Tofua. He sketched its profile, Its northern end lies approximately 400 miles and called the island "Amatofua" (Woide, south of Samoa, and it stretches on southward 1776) . for approximately 160 miles. The northernmost The volcanic nature of Tofua was first noted islands are N iuafo 'ou and Niuatoputapu. Farther by Capt. James Cook in 1774 when he wrote of south these islands fall into three recognized "To fooa" in his journal. Cook sailed a mile groups, or island clusters: Vava'u, the most north of the island and noted that it was in northerly; Ha'apai, about 70 miles to the south; eruption. His crew reported that raindrops from and Tongatabu, 90 miles farth er south. Tofua a localized rainstorm, caused by the eruption is one of the westernmost islands of this Ha'apai cloud, created a burnin g sensation in their eyes. Group (Fig. 1). Cook also wrote that the rocks along the north­ Just west of the Tonga Trench a series of west coast were cavernous and appeared to form coralline islands lies in.a north-south alignm ent columns, and that the foliage just below the along the eastern side of the archipelago. The summit of the island on the northwest flank had volcanic islands form a line paralleling the been recently burnt. He concluded that the coralline islands on the western side. Along the summit of Tofua contained a crater (Cook and volcanic line there are several active terrestrial Forester, 1777, vol. 4, pp . 162-163) . and submarine volcanoes, Tofua being one of the The first Caucasian to set foot on Tofua was more prominent of the terrestrial ones. Accord­ Capt. William Bligh, just after the 1789 mutiny ing to Admiralty Chart 2421, the geographic aboard the "Bounty." He also described "Tofoa" center of Tofua is at 19°45'S and 175°05'W. as a volcanic mountain, and made reference to Th e island is 17 miles northwest of Kotu deep gull ies on the southwestern side (Dalyell, Island, from which it is separated by the north­ 1812, p. 153). south-trending Tofua trough. The volcanic Later, Lister, who visited the island in 1889, island of Kao lies 4 miles to the north. It is stated (1891 , p. 593) that "Tofua is a volcano joined to Tofua by a submarine ridge. The in a state of intermittent activity." He provided "andesite line," marking the boundary between Alfred Harker, of Cambridge University, with a continental- and oceanic-type volcanics, lies collection of rocks from Tonga, but rocks from along the Tonga Trench (Marshall, 1911 ; Mac­ Tofua were not among them (Harker, 1891). donald, 1949, p. 1590 ), and the rocks of Tofua Marshall (1911, p. 22) made collections on are all volcanic and of continental orogenic the island. He gave the first brief petrographic types. descriptions , stating that the rocks were "augite andesites." Daly ( 1916, p. 340) repeated GEOMOR PHOLOGY Marshall 's statement. No further work was done on Tofua until Tofua Island is oval in plan, and approxi­ 1959, when Richard (1962, p. 16) visited the mately 5 miles by 6 miles across (Fig. 2 A ). It island and collected two samples which were is a single composite volcano, the summit of chemically analyzed. The analyses indicated that which has collapsed to form a caldera. The the rocks are quartz basalts in Rittmann's lower slopes rise at an angle of about 35 0 , but (1952) classification. However, the MnO con­ the slope angle decreases near the summit. When tent of the rocks as reported in the analyses is viewed from the east, Tofua has an extremely nearly 2 percent, which is much too high for regular profile. Elevation of the caldera rim rocks of this type. In the new analyses of Tofua varies less than 300 feet-from 1,400 to almost rocks given in Table 1 of this paper, only normal 1,700 feet. Geology of Tofua Island-i-Bxusn 335 V a v a ' u I. ~ y>"", 0 fft• L ate I. 0 VAVA'U GROUP 175· 174· 19· N :> Met is Shoa l ~I HA 'APAI GROUP • Ka o I. C \ 0) c::J L i f uka I. Tofua I- Q L! '" t:> Kotu I. ! 20· • • .. , .ceJ N o mu k a I • .. 0 10 20 30 M I LES 2 1· ~ TONGATABU GROUP To ng o t abu I. ~ 'E u a I. F IG. 1. Map of the main part of the Tonga Archipelago, showing the location of Tofua Island. 336 PACIFIC SCIENCE, Vol. 24, July 1970 N ~ 19·43' 5 19·46'5 A o ~CALE IN MILES 160 0' .., --.---: FIG. 2. Topographic map (A) and geologic map (B) of Tofua Island . Geology by Glenn R. Bauer, June 1968. Geology of Tofua Island-i-Baunn 337 19°43 ' 5 Homc tuc B o Scale in Miles L ATE PL~T O CENE - _________________R:s-:,N_T " --?- EROSIONAL EROS IONAL ~ UNCONFORM ITY UNCONFORMIT Y ~ HOKULA FROTH LA VA KOLO FORMATI ON h - Basall ,pyroxene andesit e, f l -Porphyri tic andes ite fr oth ka - Consolidated lopilh l uff­ 10- Consolidate d l a p i lli ~ l u ff , pyroxene doci le lava flows lava f low brecc ia luff, spotte r, unconsolidated kb - Unconsolidated tephra lopilli c inde r, interbe dded - bosalt fa andesite lava flows kc - Andesile lava fl ow lap - Ponded lava flow Ib- Ash cone ic - Large cndesitic Java f low Faul l , defi nitel y lo cated , showing dow n - - - - - - - Geolog ic conta ct cppr oximutely loc ct ed th ro w n si de -....r Dip ond str ike of beds Fau ll , appr oxima te ly locat ed • Villoge Buried fault 15~63' Geolc qrc contact def initely located Trinnqufc t ton sta l ion showing eleva tion 338 PACIFIC SCIENCE, Vol.
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