Koko Guyot
Encyclopedia
Koko Guyot is a 48.1 million year old guyot
, a type of underwater volcano with a flat top, which lies near the southern end of the Emperor seamounts, about 200 km (124 mi) north of the "bend" in the volcanic Hawaiian-Emperor seamount chain
. Pillow lava
has been sampled on the north west flank of Koko Seamount, and the oldest dated lava is 40 million years old. Seismic studies indicate that it is built on a 9 km (6 mi) thick potion of the Pacific Plate
. The oldest rock from the north side of Koko Seamount is dated at 52.6 and the south side of Koko at 50.4 million years ago. To the southeast of the bend is Kimmei Seamount at 47.9 million years ago and southeast of it, Daikakuji at 46.7.
(A.D. 885-887) by geologist Thomas Davies
and his colleagues in 1972, based on the results from a bathymetric expedition
and contents of two dredge huals, led by Thomas Washington and undertaken with the ship Aries-7. The seamount
is elongate in shape, aligned northwest-southeast (the same direction as the chain), and has a gentle slope and a large, flat top. Koko Seamount also has a lot of small reefal bodies on its slopes. It rises from the abyssal floor
about 5000 m (16,404.2 ft) in height.
A prominent south-trending ridge extends about 50 km (31 mi) from the summit area in the direction of Kimmei Seamount
, to the southeast. The base of the guyot is similar to a "pedestal," and is composed of consolidated lavas
and extinct volcanic centers of the volcano's formally active history; it is similar to structure to the pedestal found at the base of most of the other, usually larger Emperor seamounts. However, a thick carbonate cap, similar to the one covering Detroit Seamount
, makes it difficult to find the exact eruptive centers. The volcano is clearly isolated, even in comparison to other seamounts in the spread-out Emperor chain, with Ojin Seamount about 200 km (124 mi) to the northwest and Kimmei Seamount 100 km (62 mi) to the southeast. The seamount is located just 2.3 degrees north of the bend.
Much of what we know about Koko comes from early dredgings and the Ocean Drilling Program's
core samples
, collected as part of Leg 197, at Site 1206, which aimed to supply information on the relatively obscure Emperor seamounts and study their relation to the Hawaiian chain. Site 1206 was the last and southernmost drilling site during Leg 197, and was located on the southeastern side of the lower summit terrace of Koko Seamount. A seismic survey of the region was utilized to locate a suitable place for the drill site, initially targeted near Site 308, drilled in 1973 during Leg 32. Weather conditions during the drilling had prevented it from reaching 68.5 m (225 ft) in depth, the approximate depth of the sediment cover in the region. Due to a shortage of time, priority was placed on finding a region with a thin sedimentary cover. The site eventually chosen was located at a water depth of 1545 m (5,069 ft), 6.2 km (4 mi) south of Site 308, at coordinates 34°55.55′N 172°8.75′E. The sediment cover at this site was less than half that at the 1973 drill site, and rock was hit at a subsurface depth of 57 m (187 ft). Drilling continued to 278 m (912 ft) into the slopes.
The top 57 m (187 ft) of sediment included fossil
-rich calcarenite
and calcium
-rich mudstone
and siltstone
, indicating a shallow-water setting at the time of deposition. The lower part of the core sample recovered a 15 cm (6 in) to 20 cm (8 in) section of shell-bearing mudstone
containing many microfossils
typical of the early to middle Eocene
(43.5-49.7 Ma). This age range fits well with a radiometric analysis (48.1 Ma) reported for a dredged rock from Koko Seamount from the 1973 expedition. Although shell fragments had been recovered from the sediment cover in 1973, none of these deposits contained microfossils.
Lava flows dominate the lithology
of the main body, with a small proportion of calcarenite
. Many lavas were pahoehoe flows laced with a'a, evidence of subaerial eruption
s. There was a large amount of variation in the density, structure, porosity, and grain size of the recovered volcanic rock, varying widely with depth. The bulk of the volcanic rock is basalt
of aphyric to olivine
-phyric lava, and tholeiitic or alkalic in composition. The basaltic lavas from Koko Seamount resemble those drilled during Leg 55, at Suiko Seamount
.
Studies suggested that the magnetic arrangement of the rock, used to determine its latitude at formation (magnets align to the North pole
; also, the drift and position of the Hawaii hotspot
at various times is important to hotspot
studies), were relatively stable. 14 magnetic groupings were found on the seamount, yielding a mean latitude of 38.5 degrees south of the seamount's present location (the percent of error
is +8.4°/-10.9°). That would put the seamount at 21.7° N in latitude during its early history, before the Pacific Plate
moved it to its current position relative to Earth.
and several other corals, covered by coralline algae
at shallow to medium depth. Also present were Amphistegina, red algae
(mainly Lithothamnion
and Sporolithon), lepidocyclines, bryozoans, and coralline
at deeper depths. The recorded lepidocyclinids indicate an Early Miocene
age for the drowned carbonate platforms found on the seamount, at about 500 m (1,640 ft).
Guyot
A guyot , also known as a tablemount, is an isolated underwater volcanic mountain , with a flat top over 200 meters below the surface of the sea. The diameters of these flat summits can exceed ....
, a type of underwater volcano with a flat top, which lies near the southern end of the Emperor seamounts, about 200 km (124 mi) north of the "bend" in the volcanic Hawaiian-Emperor seamount chain
Hawaiian-Emperor seamount chain
The Hawaiian–Emperor seamount chain is composed of the Hawaiian ridge, consisting of the islands of the Hawaiian chain northwest to Kure Atoll, and the Emperor Seamounts, a vast underwater mountain region of islands and intervening seamounts, atolls, shallows, banks and reefs along a line trending...
. Pillow lava
Pillow lava
Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in...
has been sampled on the north west flank of Koko Seamount, and the oldest dated lava is 40 million years old. Seismic studies indicate that it is built on a 9 km (6 mi) thick potion of the Pacific Plate
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....
. The oldest rock from the north side of Koko Seamount is dated at 52.6 and the south side of Koko at 50.4 million years ago. To the southeast of the bend is Kimmei Seamount at 47.9 million years ago and southeast of it, Daikakuji at 46.7.
Geology and characteristics
The seamount was named for the 58th emperor of Japan, Emperor KokoEmperor Koko
was the 58th emperor of Japan, according to the traditional order of succession.Kōkō reigned from 884 to 887.- Traditional narrative :Before his ascension to the Chrysanthemum Throne, his personal name was or Komatsu-tei. He would later be identified sometimes as "the Emperor of...
(A.D. 885-887) by geologist Thomas Davies
Thomas Davies
Thomas Davies may refer to:*Thomas Davies , Welsh clergyman, Bishop of St Asaph, 1561–1573*Thomas Davies English physician*Thomas Davies Thomas Davies may refer to:*Thomas Davies (bishop) (c. 1511–1573), Welsh clergyman, Bishop of St Asaph, 1561–1573*Thomas Davies (physician) (died 1615)...
and his colleagues in 1972, based on the results from a bathymetric expedition
Bathymetry
Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθύς , "deep", and μέτρον , "measure"...
and contents of two dredge huals, led by Thomas Washington and undertaken with the ship Aries-7. The seamount
Seamount
A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface , and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of depth. They are defined by oceanographers as...
is elongate in shape, aligned northwest-southeast (the same direction as the chain), and has a gentle slope and a large, flat top. Koko Seamount also has a lot of small reefal bodies on its slopes. It rises from the abyssal floor
Abyssal plain
An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 3000 and 6000 metres. Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth’s surface. They are among the flattest, smoothest...
about 5000 m (16,404.2 ft) in height.
A prominent south-trending ridge extends about 50 km (31 mi) from the summit area in the direction of Kimmei Seamount
Kimmei Seamount
Kimmei Seamount is a seamount of the Hawaiian-Emperor seamount chain in the northern Pacific Ocean. It last erupted about 40 million years ago.-References:...
, to the southeast. The base of the guyot is similar to a "pedestal," and is composed of consolidated lavas
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
and extinct volcanic centers of the volcano's formally active history; it is similar to structure to the pedestal found at the base of most of the other, usually larger Emperor seamounts. However, a thick carbonate cap, similar to the one covering Detroit Seamount
Detroit Seamount
Detroit Seamount, which was formed around 76 million years ago, is one of the oldest seamounts of the Hawaiian-Emperor seamount chain...
, makes it difficult to find the exact eruptive centers. The volcano is clearly isolated, even in comparison to other seamounts in the spread-out Emperor chain, with Ojin Seamount about 200 km (124 mi) to the northwest and Kimmei Seamount 100 km (62 mi) to the southeast. The seamount is located just 2.3 degrees north of the bend.
Much of what we know about Koko comes from early dredgings and the Ocean Drilling Program's
Ocean Drilling Program
The Ocean Drilling Program was an international cooperative effort to explore and study the composition and structure of the Earth's ocean basins. ODP, which began in 1985, was the direct successor to the highly successful Deep Sea Drilling Project initiated in 1968 by the United States...
core samples
Core sample
A core sample is a cylindrical section of a naturally occurring substance. Most core samples are obtained by drilling with special drills into the substance, for example sediment or rock, with a hollow steel tube called a core drill. The hole made for the core sample is called the "core hole". A...
, collected as part of Leg 197, at Site 1206, which aimed to supply information on the relatively obscure Emperor seamounts and study their relation to the Hawaiian chain. Site 1206 was the last and southernmost drilling site during Leg 197, and was located on the southeastern side of the lower summit terrace of Koko Seamount. A seismic survey of the region was utilized to locate a suitable place for the drill site, initially targeted near Site 308, drilled in 1973 during Leg 32. Weather conditions during the drilling had prevented it from reaching 68.5 m (225 ft) in depth, the approximate depth of the sediment cover in the region. Due to a shortage of time, priority was placed on finding a region with a thin sedimentary cover. The site eventually chosen was located at a water depth of 1545 m (5,069 ft), 6.2 km (4 mi) south of Site 308, at coordinates 34°55.55′N 172°8.75′E. The sediment cover at this site was less than half that at the 1973 drill site, and rock was hit at a subsurface depth of 57 m (187 ft). Drilling continued to 278 m (912 ft) into the slopes.
The top 57 m (187 ft) of sediment included fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
-rich calcarenite
Calcarenite
thumb|250px|The [[Pietra di Bismantova]] in central [[Italy]] is an example of calcarenite formation.Calcarenite is a type of limestone that is composed predominately, more than 50 percent, of detrital sand-size , carbonate grains...
and calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
-rich mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
and siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
, indicating a shallow-water setting at the time of deposition. The lower part of the core sample recovered a 15 cm (6 in) to 20 cm (8 in) section of shell-bearing mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
containing many microfossils
Micropaleontology
Micropaleontology is the branch of paleontology that studies microfossils.-Microfossils:...
typical of the early to middle Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
(43.5-49.7 Ma). This age range fits well with a radiometric analysis (48.1 Ma) reported for a dredged rock from Koko Seamount from the 1973 expedition. Although shell fragments had been recovered from the sediment cover in 1973, none of these deposits contained microfossils.
Lava flows dominate the lithology
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
of the main body, with a small proportion of calcarenite
Calcarenite
thumb|250px|The [[Pietra di Bismantova]] in central [[Italy]] is an example of calcarenite formation.Calcarenite is a type of limestone that is composed predominately, more than 50 percent, of detrital sand-size , carbonate grains...
. Many lavas were pahoehoe flows laced with a'a, evidence of subaerial eruption
Subaerial eruption
A subaerial eruption is a volcanic eruption that has occurred on the surface. They generally produce pyroclastic flows, lava fountains, and lava flows, which are commonly classified in different subearial eruption types, including Plinian, Peléan, and Hawaiian eruptions. Subaerial eruptions...
s. There was a large amount of variation in the density, structure, porosity, and grain size of the recovered volcanic rock, varying widely with depth. The bulk of the volcanic rock is basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
of aphyric to olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
-phyric lava, and tholeiitic or alkalic in composition. The basaltic lavas from Koko Seamount resemble those drilled during Leg 55, at Suiko Seamount
Suiko Seamount
Suiko Seamount, also called Suiko Guyot, is a guyot of the Hawaiian-Emperor seamount chain in the Pacific Ocean. The last eruption from Suiko Seamount occurred 60 million years ago.-References:...
.
Studies suggested that the magnetic arrangement of the rock, used to determine its latitude at formation (magnets align to the North pole
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
; also, the drift and position of the Hawaii hotspot
Hawaii hotspot
The Hawaii hotspot is the volcanic hotspot that created the Hawaiian Islands in the central Pacific Ocean, and is one of Earth's best-known and most heavily-studied hotspots....
at various times is important to hotspot
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
studies), were relatively stable. 14 magnetic groupings were found on the seamount, yielding a mean latitude of 38.5 degrees south of the seamount's present location (the percent of error
Percent difference
The relative difference, percent difference, relative percent difference, or percentage difference between two quantities is the difference between them, expressed as a comparison to the size of one or both of them. Such measures are unitless numbers...
is +8.4°/-10.9°). That would put the seamount at 21.7° N in latitude during its early history, before the Pacific Plate
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....
moved it to its current position relative to Earth.
Ancient ecology
Dredged carbonate samples from the top of the seamount contained poritesPorites
Porites is a genus of scleractinian coral which is characterised by a finger-like morphology. Members of this genus have widely spaced calices, a well-developed wall reticulum and are bilaterally symmetrical...
and several other corals, covered by coralline algae
Coralline algae
Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls...
at shallow to medium depth. Also present were Amphistegina, red algae
Red algae
The red algae are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species of mostly multicellular, marine algae, including many notable seaweeds...
(mainly Lithothamnion
Lithothamnion
Lithothamnion is a genus of thalloid red alga comprising 103 species. Its members are known by a number of common names.Recorded common names are griuán, maërl, punalevä-suku, stenhinna and maerl. The monomerous, crustose thalli are composed of a single system of filaments which grow close to the...
and Sporolithon), lepidocyclines, bryozoans, and coralline
Coralline
Coralline means "resembling coral" and may refer to:* Coralline algae, whose fronds are covered with calcareous deposits.* Coralline rock, formed by the death of layers of coralline algae.* Sclerosponges, sometimes called "coralline sponges"....
at deeper depths. The recorded lepidocyclinids indicate an Early Miocene
Early Miocene
The Early Miocene is a sub-epoch of the Miocene Epoch made up of two stages: the Aquitanian and Burdigalian stages....
age for the drowned carbonate platforms found on the seamount, at about 500 m (1,640 ft).