Macdonald seamount (named after

Gordon A. Macdonald Gordon Andrew Macdonald (Boston, 15 October 1911 – Lanikai, 20 June 1978) was a notable American volcanologist. Macdonald was a Fellow of the Geological Society of America, the American Geophysical Union, the Mineralogical Society of America, a ...

) is a

seamount A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abru ...

in Polynesia, southeast of the Austral Islands and in the neighbourhood of a system of seamounts that include the

Ngatemato seamounts Ngatemato seamounts (Name derived from a ruling family in Rapa) are a series of seamounts in the southern Pacific Ocean. These seamounts have the shape of ridges and display calderas. The Aureka () and Make () seamounts are part of the Ngatemato ...

and the

Taukina seamounts Taukina seamounts are a series of seamounts on the Pacific Plate. The Macdonald hotspot and the Ngatemato seamounts are located nearby. The Taukina and Ngatemato seamounts were discovered in 1996 by the RV Maurice Ewing and both are named after fa ...

. It rises from the seafloor to a depth of about and has a flat top, but the height of its top appears to vary with volcanic activity. There are some subsidiary cones such as Macdocald seamount. The seamount was discovered in 1967 and has been periodically active with gas release and seismic activity since then. There is hydrothermal activity on Macdonald, and the vents are populated by hyperthermophilic bacteria. Macdonald seamount is the currently active volcano of the

Macdonald hotspot The Macdonald hotspot (also known as "Tubuai" or "Old Rurutu") is a volcanic hotspot in the southern Pacific Ocean. The hotspot was responsible for the formation of the Macdonald Seamount, and possibly the Austral-Cook Islands chain. It probably ...

, a volcanic hotspot that has formed this seamount and some other volcanoes. Eruptions occurred in 1967, 1977, 1979–1983 and 1987–1989, and earthquakes were recorded in 2007. The activity, which has produced basaltic rocks, has modified the shape of the volcano and may lead to the formation of an island in the future.

Discovery and name

Macdonald seamount was discovered in 1967, when hydrophones noted earthquake activity in the area. The seamount was named in 1970 after

. It is also known as Tamarii, while MacDonald appears to be an incorrect capitalization.

Geography and geology

Regional setting

The Pacific Ocean is characterized by long island chains, which typically extend from the southeast to the northwest in direction of the motion of the

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

. Often, such chains begin in the southeast with volcanoes such as Hawaii that become progressively more eroded northwestward and eventually end as series of atolls. This has led to the suggestion that they are formed by deep sources over which the Pacific Plate drifts and eventually carries the volcano away from its magma source. These sources are known as " hotspots", and their total number has been estimated to be between 42 and 117. Hotspots may also be formed by cracks propagating in the crust, and such hotspots would not necessarily show an age progression.

Local setting

Macdonald seamount is located off the southeastern end of the Austral Islands. The Austral Islands extend away from the southern Cook Islands to Îles Maria and eventually Marotiri southeastward, including the islands Rimatara,

Rurutu Rūrutu is the northernmost island in the Austral archipelago of French Polynesia, and the name of a commune consisting solely of that island. It is situated south of Tahiti. Its land area is .Tubuai, Raivavae and Rapa. A relatively large gap separates Marotiri from the Macdonald volcano. The

and

lie north of Macdonald, they are considerably older and appear to have a very different origin. Even farther southeast lies the Foundation seamount chain, and the associated hotspot may have generated some of the seamounts close to Macdonald. The seamount lies close to the southeastern end of an area of shallower ocean, which extends northwestward towards Marotiri, and includes Annie seamount, Simone seamount and President Thiers Bank. The high Ra seamount (named after Polynesian term for " sun") rises northwest of Macdonald to a depth of ; it is apparently an extinct volcano and may have once emerged above sea level. A smaller seamount, Macdocald, rises from the southern foot of Macdonald to depths of . Additional small seamounts that appear to have formed at the East Pacific Rise are also found in the area. The crust beneath Macdonald is of Eocene age, and away from the area of shallower ocean it is covered with hills and sediment. Macdonald seamount rises from the seafloor to a depth of about below sea level; surveys in 1979 found a pinnacle reaching to a depth of below sea level and a wide summit plateau with small ( high and wide) spatter cones. Other sources indicate a surface area of for the summit plateau. Ongoing volcanic activity may have modified the topography of the summit of Macdonald between surveys in 1975 and 1982, forming another elliptical pinnacle reaching a depth of at the northwestern margin of the plateau and raising the summit plateau to depths of . By the time of a new survey in 1986, the pinnacle had been replaced by a pile of rocks which only reached a depth of . The upper parts of the edifice are covered by thick

lapilli Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range f ...

with

lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...

s underneath. Some hydrothermal alteration products are also found, and a thick ash cover occurs to depths of . Aside from these lapilli deposits, scoriaceous lava flows are exposed on the edifice as well. Farther down,

fronts form scarps which become particularly noticeable at depths of , except on the northern flank. Even deeper, pillow lavas predominate. Below the summit area, the slopes fall down steeply to a depth of and then flatten out. Save for a debris-covered ridge to the northwest, Macdonald has a circular shape, with a width of at a depth of . The slopes of Macdonald display radial ridges which may reflect tectonically-controlled rift zones, as well as isolated parasitic cones. The volume of the whole edifice has been estimated to be . Macdonald seamount bears traces of

landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated grade (slope), slope failures, mudflows, and debris flows. Landslides occur in a variety of ...

s, including collapse scars up on the edifice and smooth terrain formed by debris on its lower slopes; collapses have been inferred on the eastern, southern, western and northwestern flank. The seafloor further shows evidence of turbidity currents, including ripples. Geomagnetic analysis of the edifice has demonstrated the existence of a normally magnetized structure at the base of the volcano and an additional anomaly which seems to be the magma chamber at a depth of within the edifice, close to the northern flank. Data obtained in gabbroic rocks expelled by the volcano during its eruptions also suggest that another magma reservoir exists at depths of , that is within the crust beneath Macdonald.

Composition

Macdonald has principally erupted basalt. This basalt contains phenocrysts of clinopyroxene, olivine and especially plagioclase. Additional rocks are

basanite Basanite () is an igneous, volcanic (extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoids, pyroxenes, olivine, and plagioclase and forms from magma low in silica and enriched in alkali metal oxides that s ...

, mugearite, picrite and tephrite. The overall composition is alkaline and nephelinic. Rock debris found on Macdonald seamount includes

intrusive rock Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form ''Igneous intrusion, intrusions'', such as batholiths, dike (geology), dikes, Sill (geology), sills, laccoliths, and volcanic necks.I ...

s such as gabbro, metadolerite, picrite and pyroxenite; the gabbros appear to originate from slow crystallization of basaltic magma within a magma reservoir, followed by low temperature alteration. Such rocks were uprooted by explosive activity. In addition, hydrothermal and thermal alteration has formed

amphibole Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is A ...

chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...

, epidote, phyllosilicates, pyrite, quartz and smectite, with additional components including albite,

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

, labradorite, leucodiorite and

orthopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

. The vulcanites are typical ocean island basalts, whose alkaline nature is unlike the

tholeiite The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma i ...

that is found on other hotspot volcanoes such as Hawaii, Iceland and Reunion. In these volcanoes alkaline lavas are erupted in the post-shield stage but Macdonald is clearly a developing volcano, and further research is needed to explain the chemical history of Macdonald. These magmas in the case of Macdonald were derived from the partial melting of

spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...

- lherzolite and further influenced by

fractional crystallization Fractional crystallization may refer to: * Fractional crystallization (chemistry), a process to separate different solutes from a solution * Fractional crystallization (geology) Fractional crystallization, or crystal fractionation, is one of the ...

and carbon dioxide, but with no influence of the overlying plate.

Eruptions

Macdonald is the only known active volcano in the Cook Islands and Austral Islands, unlike in the Society Islands where active volcanism is spread over several volcanoes. The first recorded eruptions at Macdonald occurred in 1967 and was followed by additional activity in 1977, although pumice rafts observed in 1928 and 1936 could have been formed by the seamount as well. These eruptions were recorded with hydrophones; further such activity occurred 1979–1983. Some eruptions, especially eruptions on the southern flank or within a crater, would have passed unnoticed. Additional eruptions at Macdonald occurred between June 1987 – December 1988, and a

seismic swarm In seismology, an earthquake swarm is a sequence of seismic events occurring in a local area within a relatively short period. The time span used to define a swarm varies, but may be days, months, or years. Such an energy release is different f ...

probably unassociated with eruptions occurred in 2007. Eruptions at Macdonald include phreatic and phreatomagmatic activity which led to the formation of lapilli and lava bombs and also to the hydrophone signals, but also

effusive In physics and chemistry, effusion is the process in which a gas escapes from a container through a hole of diameter considerably smaller than the mean free path of the molecules. Such a hole is often described as a ''pinhole'' and the escape ...

eruptions forming

s. Volcanic activity is not steady, with prolonged pauses observed between eruptions. Macdonald seamount is among the most active submarine volcanoes in the world, and the most active on the floor of the Pacific Ocean. Radiometric dating of rocks dredged from Macdonald has yielded two separate clusters of ages, one less than two million years old and the second about 30 million years.

1989 events

Several eruptions occurred in 1989 when a scientific expedition was underway on the seamount. These eruptions were accompanied by the discolouration of the water over of length, the release of burning hydrogen and

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...

accompanied by the formation of a plume of hydrothermally altered water. The

submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

Cyana ''Cyana'' is a genus of moths in the family Erebidae. Species are well distributed in Africa, Madagascar, China, India, Sri Lanka, Myanmar, Sumatra, Java and Borneo. The genus was erected by Francis Walker in 1854. Description Palpi slender a ...

'' observed activity directly in one summit crater in the form of intense bubbling, while steam and water fountains were seen on the ocean surface. Grey-coloured slicks developed on the ocean surface, which were formed by pyrite,

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

and volcanic glass plus smaller amounts of cinnabar, cubatine and quenstedtite. The events caused changes in the pH of the water on the seamount and increased methane concentrations.

Future birth of an island

Macdonald likely formed an island during the

last glacial maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eur ...

when sea level was lower, and future eruptions at Macdonald may lead to the birth of an island even with present-day sea levels. Such an eruption would have to be fairly large and continuous, otherwise the resulting island will likely be eroded away quickly. Depending on how quickly erosion and other factors reduce its size, such an island will likely be temporary.

Hydrothermal system

Macdonald seamount is hydrothermally active, with several hydrothermal vents inferred to exist on the western flank. A wide

eruption fissure A fissure vent, also known as a volcanic fissure, eruption fissure or simply a fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is often a few metres wide and may be many kilom ...

was observed to be hydrothermally active in 1989. Further, the volcano releases gases including carbon dioxide, methane and

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...

. Such release occurs in the summit area in the so-called "Champagne Field", but also from a second crater at depth in the southeastern flank. Macdonald volcano may be a major source of heavy metals for the area. The methane appears to be partially of biological origin and partly abiogenic.

Biology

Hyperthermophilic bacteria have been found on Macdonald, including '' Archaeoglobus'', ''

Pyrococcus ''Pyrococcus'' is a genus (biology), genus of Thermococcaceaen archaean.See the National Center for Biotechnology Information, NCBI]webpage on Pyrococcus Data extracted from the Description and significance ''Pyrococcus'' has similar charac ...

'', ''

Pyrodictium ''Pyrodictium'' is a genus in the family Pyrodictiaceae. It is a genus of submarine hyperthermophilic Archaea whose optimal growth temperature range is 80 to 105°C. They have a unique cell structure involving a network of cannulae and flat, di ...

'' and ''

Thermococcus In taxonomy, ''Thermococcus'' is a genus of thermophilic Archaea in the family the Thermococcaceae. Members of the genus ''Thermococcus'' are typically irregularly shaped coccoid species, ranging in size from 0.6 to 2.0 μm in diameter. Some ...

'' as well as previously undescribed species. These bacterial communities contain both hydrogen- or

- consuming autotrophs and heterotrophs and appear to be capable of long-range propagation, considering that relatives of the species found are known from Vulcano in Italy. Aside from hyperthermophiles, craniids, corals, polynoids and sponges have been found in the summit area of Macdonald.

References

Sources

* * * * * * * * * * * * * * * * * * {{refend Active volcanoes Volcanoes of the Pacific Ocean Seamounts of the Pacific Ocean