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Tonga Volcanoes and Volcanics

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Major Volcanoes of Tonga

The Kingdom of Tonga is an archipelago of two parallel island belts: that to the east is low, fertile, coralline, and well populated, while that to the west is young, volcanic, and ranges from steep cones to ephemeral islands. The unbroken line of subduction, from New Zealand through the Kermadecs and Tonga, ends abruptly and the trench swings west toward the remarkable, donut-shaped island of Niuafo'ou, a basaltic shield volcano with a large, lake-filled caldera. European contact with Tonga began in the 17th century and Captain Cook named them the Friendly Isles in 1773; the first recorded eruption was in the next year.

The Tonga Archipelago stretches 250 kilometers along the western margin of the Tonga Trench in the southern Pacific Ocean. Along the volcanic chain there are several active subaerial and submaring volcanoes, Tofua being one of the more prominent. More than 35 eruptions in the Tongan Islands were recorded in the 200 years ending in 1970 (Ewart and others, 1977).

A fumarolically active crater, breached to the SW with a fresh lava flow extending to the sea, is located on the small, less than 2-kilometer-wide island of Fonualei. In contrast to other islands of the Tonga arc, Fonualei lavas are of dacitic composition. Eruptions have been recorded since 1791, with major eruptions taking place in 1846 and 1847. Lava flows occurred in both years; in 1846 explosive eruptions produced large pumice rafts, and the following year ashfall damaged crops on the island of Vavua, 56 kilometers away and fell on vessels up to 950 kilometers distant. In 1939 explosive and effusive activity occurred from summit and flank vents, and water spouts were reported 1.6 kilometers SE of the island.

The small, 6-kilometer-wide circular island of Late contains a large summit crater. The 1,500-meter-high volcano is largely submerged, with its conical summit rising to 518 meters above sea level. Only two eruptions have occurred in historical time, both from E-flank craters, which produced explosive activity and lava flows in 1790 and 1854.

The Kingdom of Tonga is an archipelago of two parallel island belts: that to the east is low, fertile, coralline, and well populated, while that to the west is young, volcanic, and ranges from steep cones to ephemeral islands. The unbroken line of subduction, from New Zealand through the Kermadecs and Tonga, ends abruptly and the trench swings west toward the remarkable, donut-shaped island of Niuafo'ou, a basaltic shield volcano with a large, lake-filled caldera. European contact with Tonga began in the 17th century and Captain Cook named them the Friendly Isles in 1773; the first recorded eruption was in the next year.

Niuafo`ou ("Tin Can Island") is a low, 8-kilometer-wide island that forms the summit of a largely submerged shield volcano. The circular island encloses a 5-kilometer-wide caldera that is mostly filled by a lake whose bottom extends to below sea level. The inner walls of the caldera drop sharply to the caldera lake, named Big Lake (or Vai Lahi), which contains several small islands and pyroclastic cones on its NE shore. Historical eruptions, mostly from circumferential fissures on the west-to-south side of the island, have been recorded since 1814 and have often damaged villages on this small ring-shaped island. A major eruption at Niuafo`ou in 1946 forced evacuation of most of its 1,200 inhabitants. Additional eruptions may have occurred in 1947 and 1959.

Niuafo'ou Island ("Tin Can Island") lies at the northern end of the Tonga Archipelago. It is geographically within the Tongan group but consists of more mafic lavas (dominantly olivine tholeiites) than most Tnagan volcanoes (Ewart and others, 1977).

Niuafo'ou Island is a broad lava shield capped by a composite cone (Taylor, 1986). The cone has been modified by development of a summit caldera, now filled by a large lake. Several cones have erupted within the caldera, and lava flows have emanated from fissures along the western outer slopes of the caldera, which are roughly concentric with the caldera rim. The latter erupted on five occasions from 1857 to 1946 (Eward and others, 1977).

Historical eruptions at Niuafo'ou have occurred from multiple vents, possibly along the caldera ring-fracture system. In this regard, activity has been similar to that at Deception Island. At these two mafic centers, frequent historical leaks along ring fractures have yet to be followed by a catastrophic repeat of caldera formation.

The low, forested Tofua Island is the emergent summit of a large stratovolcano that was seen in eruption by Captain Cook in 1774. The first Caucasian to set foot on the island was Capt. William Bligh in 1789, just after the renowned mutiny on the "Bounty." The volcano's summit contains a 5-kilometer-wide, steep-walled caldera that is 500 meters deep. Three post-caldera cones were constructed at the northern end of a caldera lake, whose surface is at about sea level. The largest and westernmost of the cones, Lofia, has a 120-meter-deep crater, and has been the source of historical eruptions, most recently in 1958 or 1959.

The Tonga Archipelago stretches 250 kilometers along the western margin of the Tonga Trench in the southern Pacific Ocean. Along the volcanic chain there are several active subaerial and submarine volcanoes, Tofua being one of the more prominent. More than 35 eruptions in the Tongan Islands were recorded in the 200 years ending in 1970 (Ewart and others, 1977).

Tofua Caldera is the summit caldera of a steep-sided composite cone that forms Tofua Island. Pre-caldera activity is recorded by a sequence of pyroclastic deposits and lavas constituting the older cone, followed on the northern part of the island by froth lavas (Bauer, 1970) or welded and unwelded ignimbrite (Baker and others, 1971). Following caldera collapse, lavas were erupted from the northern part of the island and the caldera-rim fissure zone, scoria and lavas from the caldera-wall fissure zones, pyroclastics and lavas from intracaldera cones, and recent pyroclastic fall deposits on the outer cone. Eruptive products are mainly basaltic andesites and andesites, plus occasional dacite flows within the older cone (Ewart and others, 1977). A postcaldera cone with fumarolic activity (Lofia) is situated in the northern part of the caldera; a lake of unknown depth occupies most of the remainder. Bauer (1970) states that "the caldera is continuing to collapse along relatively minor faults ont he northern, easter, and southeastern sides. The fault displacements have given this part of the rim a hummocky topography, with small grabens and steps with escarpments averaging from 50 to 100 feet high." Such movements could be related to the original caldera collapse, or perhaps more likely to gravitational slumping of the caldera walls.

Most historical eruptions have been small explosions from Lofia cone along the northern caldera rim. The eruptions of 1958-59 caused most of all islanders to evacuate for a year or more (Baker and others, 1971).

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