Macauley Island is a volcanic island in New Zealand's Kermadec Islands, approximately halfway between North Island of New Zealand and Tonga, in the south-west Pacific Ocean. It is part of a larger submarine volcano that features a wide underwater caldera northwest of Macauley Island. Two islets, Haszard Island and Newcome Rock, lie east offshore of Macauley Island. The island is mostly surrounded by high cliffs that make it difficult to access; the inland parts are mostly gently sloping terrain covered with ferns and grasses.
The island formed during several volcanic episodes that produced mainly basaltic rocks as lava flows. During the Holocene, a large explosive eruption produced the Sandy Bay Tephra; this eruption may have had a volume of more than and the Macauley caldera might have formed during that occasion. Later, most of the current surface of Macauley Island formed as the Haszard Formation. Two possible eruptions took place during the 19th century offshore Macauley Island; a hydrothermal vent system is active at Macauley Cone in the caldera.
Macauley Island is an important breeding place for many seabirds, which come on land only to reproduce. The island is uninhabited but Polynesians and, during the 19th century, whalers introduced goats, pigs and rats which damaged the island's ecosystem. During the 20th and 21st centuries these invasive species were largely eradicated, leading to a recovery of the previous vegetation. The island is part of several protected areas.
History
Macauley Island was first seen by Westerners on 30 June 1788 aboard the Lady Penrhyn. It is likely that Polynesians visited the island during or after 1300 despite the lack of direct archaeological evidence other than a flake made out of obsidian discovered in 2015. They may have obtained obsidian from the island. The island was originally named Macaulay after George Mackenzie McCaulay, an alderman of the City of London who had contracted the voyage of the Lady Penrhyn. A castaway depot was established on the north-eastern side of Macauley Island in 1888.
Macauley Island and other Kermadec Islands have been part of New Zealand's territory since the 19th century. Early explorers envisaged planting trees on Macauley Island and using the Kermadec islands as places to settle from New Zealand. In 1957, the Kermadec Islands were briefly considered as a potential testing ground for the British nuclear weapons programme but were ruled out for logistical and political reasons. Archaeological excavations were conducted in 1990 on Macauley and Raoul Island.
Geography and geomorphology
Macauley Island is in the Kermadec Islands, south-south-west of Raoul Island and roughly halfway between Tonga and New Zealand. Cheeseman Island and Curtis Island lie south-south-west of Macauley Island, and Havre and L'Esperance Rock are even farther south; together these islands form the southern group of the Kermadec Islands, of which Macauley Island is the largest. Raoul is visible from Macauley Island when the weather is clear. Located within the exclusive economic zone of New Zealand, the islands are since 1990 within the Kermadec Islands Marine Reserve and the Kermadec Benthic Protected Area administered by the New Zealand Department of Conservation. The islands are, with the exception of the weather station on Raoul Island, uninhabited. The volcano rises from a depth of where it is wide; it is elongated in east-southeast direction and features a wide submarine caldera northwest of Macauley Island. The caldera floor and rim are and below sea level, respectively. The caldera is elongated in east-north-east direction and features north-north-east trending lineaments that extend to Macauley Island. Jumbled blocks, presumably from landslides or slumps, cover the portions of the caldera adjacent to Macauley Island, and there is evidence of collapses on the western caldera margin. A fault runs inside the caldera next to its southeastern margin. The caldera floor is covered with pumice, and thick pumice deposits exist on the flanks of Macauley volcano.North-northwest of the caldera is a long structure called Lloyd dome or Curtis Ridge; it is surmounted by a lineament of cones that rise to below sea level. high and wide Macauley Cone is located on the southeastern caldera margin and rises to depth. It is capped by an wide and deep crater with a floor covered with ash and sulfur, and its slopes are covered with talus. Parasitic vents and structures have been identified on the caldera rim and submarine slopes of Macauley Island volcano during bathymetric analysis, some of which are the source of lava flows that extend into the caldera and down the flanks.
The seafloor on the slopes of Macauley volcano is covered with sand, rock, breccia and bacterial mats. "Sediment waves" up to long and wide occur on the submarine slopes and extend over away from the volcano. These waves appear to have formed in part during sector collapses and in part during density flows triggered by eruptions; the size of the latter waves may be indicative of the large size of eruptions. The submarine slopes are dissected by canyons and channels. The total volume of the volcano is about , of which less than five per cent are above sea level.
Macauley Island
Parts of the volcano emerge above sea level, forming Macauley Island, Haszard Island and Newcombe Rock. Together they have an area of about , making it the second-largest island in the Kermadec Islands. Macauley Island was also known as Green Island.
Macauley Island is about wide with a roughly circular to rectangular shape and rises from an average elevation of about to the high Mount Haszard in the northern part of the island. Seen from the north the island has the shape of a wedge; it has a more rounded shape when seen from the east.
The island has a surface area of . Most of the island is a gently tilted plateau, cut by gullies and ravines which are the only way to reach the inside of the island. The deepest of these gullies is the deep Grand Canyon on the eastern side of Macauley Island; there is evidence that the gullies have become deeper in historical times. Flowing water occurs only after rainfall. The island is geologically unstable; beaches and landforms frequently shift owing to erosion during rainfall and tropical cyclones, but also due to earthquakes.
Cliffs with heights of over surround most of Macauley Island and allow landing only at Sandy Bay, a long beach. Boulder, gravel and sand beaches occur in some places; others have steep rocky slopes at subtidal depths with crevices, caves and overhangs. At its northwestern end west of Mount Haszard, the steep Perpendicular Cliff drops into the southern part of the caldera. The structure of the shield volcano with lava flows, tephra and two volcanic craters crop out in the cliff. South and south-west of Mount Haszard are two more craters known as Haszard Crater and Macauley Crater.
Haszard Island lies east of Macauley Island, next to Sandy Bay. Its name is derived from Henry Douglas Morpeth Haszard (at first, it was named Roaches' Isle) and, like that of Mount Haszard, is often spelled as Hazard. It has a surface of about and is entirely surrounded by cliffs, making access difficult. Newcombe Rock, also known as Haszardette, is located north-east of Haszard Island and may be part of the same edifice, separate from the Macauley Island one. Three more emergent rocks are found north-east and south-west of Haszard Island and south of Macauley Island, and a shallow rock named Mac Donald lies reportedly a few kilometres off Mount Haszard.
Geology
In the south-west Pacific, the Pacific Plate subducts beneath the Australian Plate and a set of microplates in the long and maximally deep Kermadec Trench. This subduction has given rise to the Tonga-Kermadec volcanic arc, which is subdivided at 25.6° latitude (where the Louisville seamount chain subducts in the trench) into the Tonga arc to the north and the Kermadec arc to the south. In the Kermadec arc, subduction proceeds at a rate of .
The Kermadec arc consists mainly of 33 mostly submarine volcanoes and calderas and extends from White Island next to New Zealand to Raoul Island. The volcanoes are formed principally by basaltic and andesitic magmas and are lined up in a wide zone west of the trench. The volcanoes are located west of the Kermadec Ridge, except for Macauley, Raoul and Curtis Islands which are on it, and rise on oceanic crust of Eocene age. Volcanism began in the Pliocene and has occurred at the present-day location of the Tonga-Kermadec Ridge for about one million years. Many of the volcanoes were discovered using bathymetry and about 80% show hydrothermal activity. Because of its mostly submarine location, volcanism in the Kermadec arc is poorly understood.
The Kermadec Islands are the emergent part of the Kermadec arc, most of which is more than below sea level. The islands are usually found on ridges perpendicular to the main ridge, implying that local lineaments control the position of volcanoes; they are separated from each other by water depths of over . west-northwest of Macauley Island lies Giggenbach, a submarine volcano, and even farther west are the Havre Trough and a caldera. This trough separates the Kermadec microplate from the Australian plate and it, along with the Lau Basin, began to form through crustal spreading 6 million years ago; the lineaments of the Macauley caldera appear to match those of the Havre Trough. The Kermadec–Havre system is considered to be an archetype of a backarc-volcanic arc system.
Composition
Most of the rocks on Macauley Island have a basaltic composition, which defines an alumina-rich to tholeiitic suite with intermediate potassium contents. The rocks contain olivine and pyroxene phenocrysts with rare glass, and the Sandy Bay Tephra contains augite, hypersthene, ilmenite, magnetite and plagioclase. There is a single instance of dacite and rhyolite in Sandy Bay Tephra, and evidence for earlier felsic eruptions; apart from that Macauley Island rocks have a largely uniform composition. Mixing of chemically distinct magma appears to have occurred before the eruption of the Sandy Bay Tephra.
Gabbro and basalts with different compositions occur as xenoliths and resemble similar rocks of Raoul Island. Chemical alteration has given rise to gypsum, hematite, kaolinite, montmorillonite, natroalunite and tridymite. The altered rocks have pink and red colours and there are occurrences of palagonite. Hyaloclastite is found in the sea at shallow depths and ferromanganese crusts have been dredged from the submarine flanks of Macauley Island.
The volcano is believed to consist mostly of basaltic rocks. The occurrence of felsic rocks at Macauley Island and elsewhere in the Tonga-Kermadec volcanic arc is unexpected. Processes where magma ascending into the lower crust heats it until it melts (anatexis) and dehydrates have been invoked to explain the felsic volcanism in the Kermadec arc. An origin through fractional crystallization of basaltic magmas is unlikely for several reasons. The backarc extension in the Havre Trough, where tectonic lineaments match the trend of the Macauley caldera and of Denham Caldera on Raoul Island, may also have influenced the explosive activity at both calderas.
Climate and oceanic conditions
The climate of Macauley Island is presumed to resemble that of Raoul Island, where temperatures range and about of precipitation falls each year. In summer, winds blow from east and southeast and the rest of the year from northwest. The island undergoes approximately 2,100 hours of sunlight per year.