 View from the north-east, 2003 | |
 Macauley Island in the Kermadec Islands | |
| Geography | |
|---|---|
| Location | South Pacific Ocean |
| Coordinates | 30°14′S178°26′W / 30.233°S 178.433°W /-30.233; -178.433 |
| Archipelago | Kermadec Islands |
| Area | 3.06 km2 (1.18 sq mi) |
| Length | 2.5 km (1.55 mi) |
| Width | 1.8 km (1.12 mi) |
| Highest elevation | 238 m (781 ft) |
| Highest point | Mount Haszard |
| Administration | |
New Zealand | |
| Demographics | |
| Population | 0 |
| Additional information | |
| Nature Reserve | |
Macauley Island is avolcanic island inNew Zealand'sKermadec Islands, approximately halfway between New Zealand'sNorth Island andTonga in the southwestPacific Ocean. It is part of a largersubmarine volcano that features a 10.5 by 7 kilometres (6.5 mi × 4.3 mi) wide underwatercaldera northwest of Macauley Island. Two islets, Haszard Island and Newcome Rock, lie east offshore of Macauley Island. The island is mostly surrounded by highcliffs that make accessing it difficult; the inland parts are mostly gently sloping terrain covered withferns andgrasses.
The island was formed during several volcanic episodes that produced mainlybasaltic rocks aslava flows. During theHolocene, a largeexplosive eruption produced the Sandy Bay Tephra; this eruption may have had a volume of more than 100 cubic kilometres (24 cu mi) and the Macauley caldera might have formed during that occasion. Later, the Haszard Formation built most of the current surface of Macauley Island. Two uncertain eruptions took place during the 19th century offshore Macauley Island; ahydrothermal vent system is active on Macauley Cone in the caldera.
Macauley Island is an important breeding place for numerousseabirds, which come on land only to reproduce. While the island is uninhabited,Polynesians, and during the 19th centurywhalers, introduced goats, pigs and rats which damaged the island's ecosystem. During the 20th and 21st century theseinvasive species were largely eradicated, leading to a recovery of the previous vegetation. The island is part of severalprotected areas.
Macauley Island was first seen by Westerners on 30 June 1788 aboard theLady Penrhyn,[1] but it is likely thatPolynesians visited the island during the last 700 years despite the lack of directarchaeological evidence[2] other than anobsidianflake discovered in 2015.[3] They may have obtained obsidian from the island.[4] The island was originally named Macaulay,[5] after George Mackenzie McCaulay,alderman of theCity of London who had contracted the voyage of theLady Penrhyn.[6]
Macauley Island and other Kermadec islands are part of New Zealand's territory since the 19th century;[7] early explorers envisaged planting trees on Macauley Island and using the Kermadec islands as places to settle from New Zealand,[8] and in 1957 were briefly considered as a potential testing ground for theBritish nuclear weapons programme.[9]Archaeological excavations were conducted in 1990.[10]
Macauley Island is in theKermadec Islands, 109 kilometres (68 mi) south-southwest ofRaoul Island[11] and roughly halfway betweenTonga andNew Zealand.[12]Cheeseman Island andCurtis Island lie south-southwest of Macauley Island, and Havre and L'Esperance Rock are even farther south;[13] together these islands form the southern group of the Kermadec Islands,[14] of which Macauley Island is the largest.[14] Under good conditions Raoul Island can be seen from Macauley Island.[15] Located within theexclusive economic zone of New Zealand,[16] the islands are since 1990 within theKermadec Islands Marine Reserve[17] and theKermadec Benthic Protected Area[16] administered by theNew Zealand Department of Conservation.[18] The islands are, with the exception of theweather station on Raoul Island, uninhabited.[19]
The volcano rises from a depth of 1,700 metres (5,600 ft) where it is 23–30 kilometres (14–19 mi) wide;[20] it is elongated in east-southeast direction[21] and features a 10.5 by 7 kilometres (6.5 mi × 4.3 mi) wide submarinecaldera northwest of Macauley Island. The caldera floor lies at 1,100 metres (3,600 ft) depth and its rim at 600 metres (2,000 ft).[22] The caldera is elongated in east-northeast direction[13] and features north-northeast trendinglineaments that extend to Macauley Island. Jumbled blocks, presumably fromlandslides orslumps, cover the portions of the caldera adjacent to Macauley Island,[21] and there is evidence of collapses on the western caldera margin.[23] Afault runs inside the caldera next to its southeastern margin.[24] The caldera floor is covered withpumice,[24] and thick pumice deposits occur on the flanks of Macauley volcano.[25]
North-northwest of the caldera is a 9 kilometres (5.6 mi) long structure called Lloyd dome[26] or Curtis Ridge;[27] it is surmounted by a lineament of cones that rise to 80 metres (260 ft) below sea level.[26] 300 metres (980 ft) high and 700 metres (2,300 ft) wide[23] Macauley Cone is located on the southeastern caldera margin and rises to 250 metres (820 ft) depth.[22] It is capped by an 80 metres (260 ft) wide and 45 metres (148 ft) deep crater[28] with a floor covered with ash andsulfur, while its slopes are covered withtalus.[29]Parasitic vents and structures have been identified on the caldera rim[23] and submarine slopes of Macauley Island volcano duringbathymetric analysis,[30] some are the source oflava flows[22] that extend into the caldera and down the flanks.[23]
The seafloor on the slopes of Macauley volcano is covered with sand, rock,breccia andbacterial mats.[31] "Sediment waves" up to 1 kilometre (0.62 mi) long and 100 metres (330 ft) wide occur on the submarine slopes[32] and extend over 55 kilometres (34 mi) away from the volcano.[33] These waves appear to have formed in part duringsector collapses and in part duringdensity flows triggered by eruptions;[34] the size of the latter waves may be indicative of the large size of eruptions.[35] The submarine slopes are dissected by canyons and channels.[26] The total volume of the volcano is about 269 cubic kilometres (65 cu mi),[20] of which less than five percent are emergent.[24]
| External image | |
|---|---|
 Bathymetric map of Macauley Island and Giggenbach |
Parts of the volcano emerge above sea level, forming Macauley Island, Haszard Island and Newcombe Rock. Together they have an area of about 3 square kilometres (1.2 sq mi), making it the second-largest island in the Kermadec Islands.[36] Macauley Island was also known as Green Island.[37]
Macauley Island is about 2.1 kilometres (1.3 mi) wide with a roughly circular[11] to rectangular shape[38] and rises from an average elevation of about 100 metres (330 ft) to the 238 metres (781 ft) high Mount Haszard in the northern part of the island.[36] Seen from the north the island has the shape of a wedge, while it has a more rounded shape when seen from the east.[39]
The island has a surface area of 3.06 square kilometres (306 ha).[40] 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.[41] The deepest of these gullies is 45 metres (148 ft) deep Grand Canyon on the eastern side of Macauley Island;[40] there is evidence that the gullies have become deeper in historical times. Flowing water only occurs after rainfall.[42] The island is geologically unstable, with beaches and landforms frequently shifting due to erosion[43] during rainfall andtropical cyclones, but also due to earthquakes.[15] Acastaway depot was established on the northeastern side of[42] Macauley Island in 1888.[44]
Cliffs with heights of over 61 metres (200 ft) surround most of Macauley Island[11] and allow landing only at Sandy Bay,[45] a 140 metres (150 yd) longbeach.[38] Boulder, gravel and sandbeaches occur at some places, while others have steep rocky slopes at subtidal depths with crevices, caves and overhangs.[46] At its northwestern end[47] west of Mount Haszard, the steep Perpendicular Cliff drops into the southern part of the caldera. The structure of theshield volcano withlava flows,tephra and twovolcanic craters crop out in the cliff.[36] South and southwest of Mount Haszard are two more craters[47] known as Haszard Crater and Macauley Crater.[14]
Haszard Island lies 0.23 kilometres (250 yd)[11] east of Macauley Island, next to Sandy Bay.[47] Its name is derived fromHenry Douglas Morpeth Haszard (at first, it was named Roaches' Isle),[48] and, like that of Mount Haszard, is often spelled as Hazard.[49] It has a surface of about 0.032 square kilometres (8 acres)[11] and is entirely surrounded by cliffs, making access difficult.[50] Newcombe Rock - also known as Haszardette - is located northeast of Haszard Island and may be part of the same edifice, separate from the Macauley Island one.[51][14] Three more emergent rocks are found northeast and southwest of Haszard Island and south of Macauley Island,[42] and a shallow rock named Mac Donald lies reportedly a few kilometres off Mount Haszard.[52]
In theSouthwest Pacific, thePacific Platesubducts beneath theAustralian Plate[53] and a set ofmicroplates[54] in the 2,550 kilometres (1,580 mi) long[55] and maximally 10,800 metres (35,400 ft) deep[56]Kermadec Trench.[57] This subduction has given rise to theTonga-Kermadec volcanic arc,[53] which is subdivided at 25.6° latitude (where theLouisville seamount chain subducts in the trench) in the northern Tonga and the southern Kermadec arc.[55] In the Kermadec arc, subduction proceeds at a rate of 50–70 millimetres per year (2.0–2.8 in/year).[58]
The Kermadec arc consists mainly of 33[58]submarine volcanoes andcalderas[53] and extends fromWhite Island next to New Zealand to Raoul Island; most of these volcanoes however aresubmarine.[59] The volcanoes are formed principally bybasaltic andandesitic magmas[53] and are lined up in a 40 kilometres (25 mi) wide zone west of the trench.[60] The volcanoes are located 15–25 kilometres (9.3–15.5 mi) west of the ridge,[22] except for Macauley, Raoul and Curtis Islands which are on it,[58] and rise on an oceanic crust ofEocene age.[61] Volcanism began in thePliocene and has occurred at the present-day location of theTonga-Kermadec Ridge for about one million years.[57] Many of the volcanoes were discovered usingbathymetry,[62] and about 80% showhydrothermal activity.[28] Because of its mostly submarine location, volcanism in the Kermadec arc is poorly understood.[63]
TheKermadec Islands are the emergent part of the Kermadec arc,[36] and most of the ridge lies at over 500 metres (1,600 ft) depth.[12] The islands are usually found on ridges perpendicular to the main ridge, implying that local lineaments control the position of volcanoes;[36] they are separated from each other by water depths of over 900 metres (3,000 ft).[12] twenty-four kilometres (15 mi)[22] west-northwest of Macauley Island liesGiggenbach, a submarine volcano, and even farther west ares the Havre Trough[57] and a caldera.[64] This trough separates the Kermadec microplate from the Australian plate[54] and it and the Lau Basin began to form through crustal spreading 6 million years ago; the lineaments of the Macauley caldera appear to match these of the Havre Trough.[65] The Kermadec-Havre system is considered to be an archetype of abackarc-volcanic arc system.[63]
Most of the rocks on Macauley Island have abasaltic composition,[36] which define analumina-rich totholeiitic suite[66] with intermediatepotassium contents.[67] The rocks containolivine andpyroxenephenocrysts with rare glass,[68] and the Sandy Bay Tephra containsaugite,hypersthene,ilmenite,magnetite andplagioclase.[69] There is a single instance ofdacite[13] andrhyolite in Sandy Bay Tephra,[70] and evidence for earlierfelsic eruptions;[62] with the exception of that Macauley Island rocks have a largely uniform composition.[71][70] Magma mingling processes appear to have occurred prior to the eruption of the Sandy Bay Tephra.[72]
Gabbro and basalts with different compositions occur asxenoliths[70] and resembleRaoul Island rocks.[73] Chemical alteration has given rise togypsum,hematite,kaolinite,montmorillonite,natroalunite andtridymite. The altered rocks have pink and red colours[47] and there are occurrences ofpalagonite.[74]Hyaloclastite is found in the sea at shallow depths[75] andferromanganese crusts have been dredged from the submarine flanks of Macauley Island.[76]
The volcano is believed to consist mostly of basaltic rocks.[77] The occurrence offelsic rocks at Macauley Island[55] and elsewhere in the Tonga-Kermadec volcanic arc is unexpected.[78] Processes wheremagma ascending into the lowercrust heats it until it melts (anatexis) and dehydrates have been invoked to explain the felsic volcanism in the Kermadec arc.[65] In contrast, an origin throughfractional crystallization of basaltic magmas is unlikely for a number of reasons.[79][80] Thebackarc 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.[81]
The climate of Macauley Island is expected to resemble that of Raoul Island, where temperatures range between 12–25 °C (54–77 °F) and about 1,500 millimetres (60 in) of precipitation fall each year. In summer, winds blow from east and southeast and the rest of the year from northwest. Mean sunshine duration per year is about 2100 hours.[42]
The Kermadec Islands are largely exposed tooceanic swells coming from all directions.[16]Ocean current regimes in this area of the Pacific Ocean are poorly known and appear to be seasonal, with northerly currents during summer and southeasterly ones the rest of the year.[82]Sea surface temperatures at Raoul Island to the north range between 16–26 °C (61–79 °F) and these at L'Esperance Rock south between 14–26 °C (57–79 °F),[83] thus they are considered too cold to be tropical.[56] Waters are salty and clear.[82]
Most of the island is covered bybeadfern andsedgeland;[84] trees are rare.[85] Several vegetation associations occur, such asturf along the coasts,lichens andmosses on the northwestern cliffs, sedgelands mostly at the periphery of the island,shrubs at various sites,[86] anddwarf ngaio[87] andHomalanthus forests.[88]Moisture-loving plants occur in the canyons and gullies.[89] 68 plant species were recorded on Macauley Island in 2008,[41] and there arebryophytes,lichens,lycophytes,pteridophytes,seaweeds andspermatophytes. One fungus was recorded in 2015, theartist's conk.[90] Changes in vegetation occurred due to the introduction and eradication of pigs and goats and a spread of ferns that may be due to climatic changes.[91]
Macauley Island has the largestseabird population of New Zealand.[92]Seabirds live on the sea and come to land only to breed;[93] birds found breeding on Macauley Island includeblack noddies,black-winged petrels,grey noddies,Kermadec parakeets,Kermadec petrels,little shearwaters,masked boobies,red-tailed tropicbirds,sooty terns,wedge-tailed shearwaters,white-bellied storm petrels andwhite-naped petrels.[94] Of these, the Kermadec parakeet is the only land bird on Macauley Island.[95] Catastrophic volcanic eruptions that periodically wipe the animals out may be the reason why Macauley Island and other Kermadec islands lackendemic birds.[96] In 1988, 5.2 million breeding birds were reported.[37] They nest mainly in the sedgelands[97] and cliffs,[98] and both Macauley[15] and Haszard Island are heavily burrowed by nesting seabirds.[99] Birds may fly from the colonies on Macauley Island to islands farther west, such asBalls Pyramid andLord Howe Island.[100]
Invertebrates reported from the islands includeants,beetles,butterflies,centipedes,crickets,flies,moths,orthopterans,snails andspiders[101] although centipedes were not recorded in 2011 andmillipedes andearthworms were absent as well.[102]
Unlike the land-based fauna,[18] the marine flora and fauna of the Kermadec Islands is mostlytropical andsubtropical. It resembles that ofLord Howe Island more than that of New Zealand[12] and there arereef-buildingcorals.[103] Reefs have been reported from boulders and rocks[82] at depths of over 7 metres (23 ft).[104] These reefs are dominated byalgae with proper corals covering less than one percent of the ground;[105] there are no propercoral reefs at Macauley Island[103] presumably owing to the only marginally suitable water temperatures, as has been observed at other subtropical sites.[106]Spiny kelp occur at Macauley Island, the northernmost occurrence of this species.[107]
Squids,[17]sharks,dolphins[108][109] andcrown-of-thorns starfish occur at Macauley Island.[110] Other animal taxa includeascidians,bivalves,bony fish,cartilaginous fish,crabs,[111]crinoids,[108]echinoids,eels,gastropods,hermit crabs,hydrozoans,jellyfish,nudibranchs,ophiuroids,polychelida,porifers,sea anemones,sea pens,sea stars,soft corals,stony corals,tube worms andtunicates.[112][111] Animal density is higher in the areas of hydrothermal venting.[113] About 105mollusc taxa[114] and numerousbryozoan species have been identified in the surrounding sea.[115] Species discovered at Macauley Island and other Kermadec volcanoes are the crabsGandalfus puia (Macauley Island and submarine volcanoes farther south)[116] andXenograpsus ngatama (Macauley Island andBrothers volcano),[117] thecrustaceanMunidopsis maunga (Macauley caldera),[118] and the musselVulcanidas isolatus (at Macauley Island and Giggenbach volcano).[119]
ThePolynesian rat was presumably introduced by Polynesians, whilegoats andpigs were introduced presumably bywhalers in the 19th century[2] probably as an emergency food supply for shipwrecked sailors;[120] whalers also burned the shrubs of the island.[121] The island was reported in 1789 to be heavily populated by rats and mice.[48] While not as severe as at Raoul Island,[122] theseintroduced animals altered the ecosystem of the island,[2] preying on birds and confining many plant species to inaccessible cliffs;[48] they might be responsible for the absence of several birds known from Raoul Island on Macauley Island[123] and for changes in the vegetation of Macauley Island between human visits during the 20th century.[124] Additionally, plants may have been imported by later visitors[125] or birds[126] andflotsam/marine debris arriving from New Zealand is frequently reported.[82]
In the late 20th century, there have been efforts to eradicate introduced species from the Kermadec Islands.[127] Pigs had died out by 1910 and goats were removed in the 1960s[2] by theNew Zealand Wildlife Service;[124] about 3,200 goats were killed during that occasion.[128] After an initial delay due to concerns that toxic baits used for rat removal could impact parakeet populations,[129] in 2006 theNew Zealand Department of Conservation began an effort to eradicate the Polynesian rat.[2] This eradication programme was probably successful, leading to the recovery of a more diverse fern-sedge vegetation that may be still underway as of 2015[update][88] and could lead to a future reduction of fern occurrences.[130] Rats and other rodents have never been reported from Haszard Island.[48] Since the establishment of the Kermadec Islands Marine Reserve, certain activities such as discharging wastewaters, fishing, mining and layingsubmarine cables are prohibited around Macauley Island.[17]
The history of Macauley Island is easily[131] recognizable from the cliffs on its northern side,[13] where fivegeological formations crop out; from bottom to top these are the North Cliff Lavas, the Boulder Beach Formation, the Annexation Lavas, the Sandy Bay Tephra and the Haszard Formation.[132] An additional formation, the Grand Canyon Formation, crops out in the east of the island.[47]Tephra layers dating to 130,000, over 40,000, 30,000, 8,400 and 5,600 years ago identified inmarine cores around Macauley Island may originate from eruptions there.[26][133][134]
Rocks dip away from the northwestern side of the island[51] and all the rocks appear to have been emplaced above sea level;[135] there is no evidence fororogenic deformation although the presence of subaqueous lava flows indicates that recent eruptions occurred during a time of low sea level.[73] During the sea level lowstand of thelast glacial period, a much larger area of the island was exposed above sea level.[26]
Pumices dredged from Macauley Island bear evidence of having formed through a unique process ("Tangaroan eruption"), where expanding magma forms a foam-like structure that fragments into numerous spherical pieces. These pieces upon contacting water solidify on the outside but remain molten on the inside. These pumice deposits are distinct from the Sandy Bay Tephra deposits and probably formed during additional eruptions.[136] The chemistry and density of Macauley Island pumices indicate a complex volcanic history.[137]
The lava flows of the North Cliff Lavas are the oldest formation that crops out,[13] and are part of ashield volcano[138] with at least one crater.[47] Little erosion took place beforephreatomagmatic eruptions emplaced the tephras and lavas of the Boulder Beach Formation,[74] presumably after water had entered the vent.[139]Dykes, most of which are correlated to the Annexation Lavas, are intruded into the Boulder Beach Formation.[74]
The Annexation Lavas are widespread on Macauley Island and also occur at Haszard Island and Newcombe Rock.Hawaii-like eruptions of vents located northwest of present-day Macauley Island produced lava flows with average thicknesses of 1 metre (3 ft 3 in) or less intercalated with brown tephra. Dykes fed lava to additional vents, including the Newcombe Rockvolcanic plug. Lava also ponded in a crater that crops out in Perpendicular Cliffs; the ponded lava which was originally interpreted as avolcanic intrusion. The total volume of the Annexation Lavas is about 1 cubic kilometre (0.24 cu mi), they reach total thicknesses of about 115 metres (377 ft). Along with volcanic activity, tectonic activity increased during the Annexation Lavas stage, giving rise tonormal faulting andsubsidence; at the end of the stage asummit crater was left.[140] At this time, Macauley Island may have had a diameter of 4 kilometres (2.5 mi) and maximum elevation of 150 metres (490 ft).[71]
The Sandy Bay Tephra was erupted 7,200[57] or 6,310 years ago[49] during the formation of the Macauley caldera[13] from a shallow submarine[141] vent close to Macauley Island,[142] and is named after Sandy Bay.[143] Its total volume is poorly known,[25] an estimate of 100 cubic kilometres (24 cu mi) inferred from the caldera volume would make the eruption that gave rise to Sandy Bay Tephra one of the largest volcanic eruptions during theHolocene[144] but it is possible that the Macauley caldera formed during multiple eruptions.[145] A submarine eruption jet breached the sea surface,[146] producing at least thirty successivepyroclastic flows,pyroclastic surges and tephra fallout episodes on Macauley Island. The eruption products buried and sometimes eroded earlier deposits. The flows were cold, most likely from interaction with seawater. The flows buried thescrubby vegetation on Macauley Island, leaving wood casts in the rocks.[147]
The Sandy Bay Tephra has a conspicuous white colour, contrasting with the dark colours of the rest of Macauley Island. It consists of dacitictephra,[140] which forms layered deposits containinglapilli,pumice, sand and finevolcanic ash. The total thickness of the Sandy Bay Tephra ranges from about 100 metres (330 ft) in the south to 15 metres (49 ft) in the north,[142] with evidence of thicker deposits in topographic depressions.[147]Basaltic andplutonic rocks are found embedded in the Sandy Bay rocks[140] and reach sizes of 1.5 metres (4 ft 11 in);[142] they indicate that older rocks were integrated into the erupting magma.[69] The total volume of Sandy Bay Tephra on Macauley Island is about 0.1–0.2 cubic kilometres (0.024–0.048 cu mi) but it is likely that the total volume of the tephra was considerably larger.[144] Erosion has affected the Sandy Bay Tephra,[148] leaving cliffs around Haszard Islet.[143]
Tephra from the Sandy Bay eruption has been identified insediment cores taken around the island[149] and formed concentric ridges on the western flank of submarine Macauley.[23] It is likely that the eruption produced large amounts ofpumice, which would have been transported byocean currents to other islands in theSouthwest Pacific.[144] Caldera collapse and collapses of caldera flanks perhaps producedtsunamis which may have hit theBay of Plenty region of New Zealand.[150] The Sandy Bay Tephra is the only demonstratedfelsic eruption at Macauley volcano;[151] the presence ofobsidian andpitchstone in the Sandy Bay Tephra indicate that earlierfelsic eruptions took place,[131] but their dates are unknown.[152]
The Haszard Formation makes up the bulk of exposed Macauley Island rocks.[47] It includes the Parakeet Tuff, Haszard Scoria and Cascade LavaMembers,[74] which probably were produced by the same eruptive episode.[144] The 1 cubic kilometre (0.24 cu mi) Cascade Lava was produced from the craters at Mount Haszard and other vents; the lava flows reach thicknesses of 1–2 metres (3 ft 3 in – 6 ft 7 in) and their eventual course was strongly influenced by topography. The lavas were overlaid by the bedded Haszard Scoria.[153] Submarinephreatomagmatic activity generated the Parakeet Tuff, which was erupted along with the Haszard Scoria and consists oflapilli andvolcanic ash.[154] Both the Haszard Scoria[153] and the Parakeet Tuff include rafted blocks with diameters of 2 metres (6 ft 7 in).[154]
The Parakeet Tuff and Haszard Scoria[154] are thought to have originated in the southeastern sector of the caldera,[13] 0.75 kilometres (0.47 mi) northwest of Macauley Island.[21] Their emplacement may have begun decades or centuries after the Sandy Bay Tephra eruption, during which rainfall eroded the Sandy Bay Tephra and formed valleys later filled by the Haszard Formation.[144] The eruption was centred at the crater of the Annexation Lavas and on flank vents and reached sub-Plinian dimensions; the crater eventually collapsed below sea level, perhaps during the slumping of the southeastern flank of Macauley Caldera, but the eruption continued as aSurtseyan eruption. Several smallphreatic craters on southern Macauley Island probably relate to the Haszard Formation, as does the Grand Canyon Formation formed in a lake formed through the damming of a valley at the eastern end of the island.[21][155]
Macauley Island is considered to be adormant volcano.[156] An eruption supposedly took place in 1825 at a "Brimstone Island" 45 kilometres (28 mi) west of Macauley Island, while another was reported 1 December 1887 north-northeast. These locations may be erroneous given the absence of bathymetric structures coinciding with their location, but they may be historical eruptions of the Macauley Island volcano.[71] There are anecdotal reports ofearthquakes,[15][50] and a faint smell ofsulfur was reported at the northern cliffs, next to the oldest rocks of Macauley Island.[47]
Hydrothermal activity occurs in the submarine Macauley Cone, where white fluids and occasional bubbles emanate from rocks[28] and chimney-shaped vents.[157] Elementalsulfur occurs around the vents,[28] which release warm (112 °C (234 °F)) acidic mineral-rich waters[158] with abrine-like composition[159] and intense hydrothermal plumes.[29] These waters may be derived from magmatic fluids[159] and theirheliumisotope ratios appear to vary between observations.[160] The influence of the hydrothermal emanations extends 7 kilometres (4.3 mi) from the volcano. One or two additional vent sites are suspected to exist in the Macauley Caldera.[161] There is evidence that a lake of molten sulfur once filled the Macauley Cone crater[162] and left sulfur deposits with thicknesses exceeding 1 metre (3 ft 3 in).[163]
Avent biota has become established around the hydrothermal vents[23] consisting ofmussels:Vulcanidas isolatus at shallow depths andGigantidas gladius at both shallow and intermediate depths.[164]Sea stars prey on them whilecrabs andtonguefish graze at the sulfurous crater walls.[165] The hydrothermal activity occurs at shallow depths,[166] thus fluids can enter thephotic zone where biological productivity is highest.[167]
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