Nintoku Seamount
Encyclopedia
Nintoku Seamount or Nintoku Guyot is a seamount
(underwater volcano) and guyot
(flat top) in the Hawaiian-Emperor seamount chain
. It is a large, irregularly shaped volcano that last erupted 65 million years ago. Three lava flows have been sampled at Nintoku Seamount; the flows are almost all alkalic (subaerial) lava
. It is 56.2 million years old.
Nintoku is positioned a roughly 41 degrees north latitude
, approximately two-thirds the way southward along the north-northeast-south-southeast Emperor seamounts extending from Meiji Seamount (about 53°N) in the north to Kammu Seamount (about 32°N) at the chain's southern terminus. Nintoku Seamount was named after the 16th emperor of Japan, Emperor Nintoku
, by geologist Robert Dietz
in 1954.
The seamount occupies a central position in the Emperor Seamount chain and is thus an important point in the paleolatitude history of the Hawaiian hotspot, instrumental to proving the scientific hunch that the Hawaii hotspot
was a mobile entity. The structure of the seamount is elongate, aligned north-northwest along the Emperor trend, with two prominent ridges trending southwest and south-southwest as far as 100 km (62 mi) from the main crater. Nintoku Seamount is a plexus of coalesced volcanoes, much like many of the larger seamounts in this chain. The Nintoku system is, however, clearly isolated from Yomei Seamount
, about 100 km (62 mi) to the north, and Jingu Seamount, about 200 km (124 mi) to south, by abyssal depths.
From analysis of seismic reflection survey data and core material recovered by drillings at Site 432, the shipboard party of Deep Sea Drilling Project (DSDP) Leg 55 proposed that Nintoku Seamount was in an intermediate atoll stage (no lagoon but fringing reefs and banks and extensive carbonate bank interiors) before subsidence removed the island below the wave base. It was further thought that a few small remnant volcanic peaks and domes still pierce the sedimentary deposits.
Nintoku Seamount apparently remained at or above sea level
long enough to be almost completely devastated by subaerial erosion and wave action. Reefs were not indicated in the seismic studies, but fragmented pieces of coral were recovered and documented, showing a shallow-water sediment-rich condition. The rock records indicate deposition in waters cooler than the present tropical condition. Shallow-water sedimentary deposition ceased in Paleogene times
.
The seamount was first drilled by Site 432, located on the northwestern edge of the summit region of Nintoku Seamount, in a gently sloping area mapped as terrace deposits. Although the sediment cover was estimated, based on other seamount covers, to be 80 metres (262 ft) thick, bedrock was hit after only 42 m (138 ft). Poorly recovered and preserved sedimentary deposits indicated a shallow-reef bed typical of terraced flanking reefs and banks, as well as volcanic sand
. Drilling at Site 432 penetrated 32 m (105 ft) of volcanic rock (74 m (243 ft) total) before terminating because of hole caving and damage to the drill assembly.
The site was drilled as part of Leg 197 by the Ocean Drilling Program
, at site number 1205. A short bathymetric acoustic survey was conducted to find the best site for the location and structure of a core sampling. The locale chosen was about 100 m (328 ft) southwest of Site 432, the location of a previous drilling by the ODP.
Site 1205 (41°20.00′N 170°22.70′E) was located in 1310 m (4,298 ft) deep water, where previous drilling had reached volcanic rock beneath Paleocene
deposits. It was elected to return to the site for a number of reasons. First, drilling at the nearby site 432 had hit reasonably unaltered and unchanged basalt with good remnant magnetic properties, key to finding the latitude of origin; but insufficient sampling caused a lack of data, and determining the age accurately was not possible. Hence, deeper drilling was promised to achieve that goal, providing a time-averaged (as the seamount is in the center of the chain) movement ratio. Second, a survey of the region showed a rock structure suitable for deep drilling, and nearby sites met low levels of sedimentary cover. Thirdly, the composition of previously drilled volcanic rock seemed to match the volcano's "average" type, erupted during the post-shield stage of it life. This helped another project goal, to recover a suitable and datable chunk of tholeiitic lava, which appeared to be rare on the seamount.
The hole was drilled in what appeared to be a large, broad sedimentary cover (estimated 70 m (230 ft) in thickness) covering a swath of the ancient volcano's main slope. The coring encountered volcanic rock at 42 m (138 ft) below the sea floor, and continued to a final depth of 326 m (1,070 ft) below the sea floor. The sedimentary cover, an element commonly found on many of the Emperor seamounts, was found to be largely a stack interlaced lava flows (about 95%). The drilling penetrated 283 m (928 ft) into the seamount's volcanic rock, and recovered at least 25 different hardened lava flows.
It was established that Nintoku Seamount's sedimentary cap consists of sandstone
and siltstone
containing well-rounded to subrounded basalt
blocks, volcanic ash
, fossil fragments of mollusks, benthic foraminifers
, bryozoans, and coralline
red algae
. These observations indicate a shallow-water, high-rate depositional setting. Little variation was found in the density
, grain size, or porosity
of the volcanic rock, and it was stable in composition, except for the volcanic-sedimentary cover. It is believed that this is the underlying cause of aucousically recorded layering of the upper 200 m (656 ft)-230 m (755 ft) of rock, after which the effect of soil layering fades away.
The age of the youngest volcanic rocks was constrained by nano-fossils in the sediment to be older than 53.6–54.7 million years, an age that is just younger than the radiometric age
of 56.2 ± 0.6 million obtained for the basalt drilled from nearby Hole 432A. The thickness and vesicularity of the lava flows, as well as the presence of oxidized flow tops and soil horizons and a lack of pillow lava
, indicate that all of the obtained samples erupted subaerially. The volcanic rock ranges from aphyric to highly plagioclase and olivine-phyric basalt. At 230 to 255 meters below ground, two flows of tholeiitic basalt were found interlaced with the alkalic basalt flows. Above these flows the degree of alkalinity skyrockets. There is evidence suggesting that the eruption rates must have been lower at the period during which the two flows were deposited, which is consistent with the model of Hawaiian volcanic growth, which increases in activity slowly over time before ceasing altogether. Internal lava flows have also created course-grained rocks. Lavas from Nintoku Seamount have similar composition to lava erupted during the post-shield stage of Hawaiian volcanoes such as Mauna Kea
. Slight differences in trace element composition between lavas from Nintoku Seamount and active Hawaiian volcanoes probably result from differences in source composition or variations in the degree of
melting.
All of the recovered lava flows had been altered very little by erosion or other lava flows, except for thin flow tops. Sparse veining
indicates that there is only small-scale fluid circulation within the rocks, in contrast with some of the data collected from Detroit Seamount
.
Rock magnetic data obtained suggest that the lava flows from Site 1205 carry a remnant magnetization suitable for scientific analysis. Although some of the rocks needed a more complex and thorough analysis, most samples yielded data suitable to make a preliminary determination of magnetic inclinations. Twenty-two independent magnetic group were identified, yielding a mean inclination of -45.7° (+10.5°/-6.3°). The mean inclination suggests a latitude of formation on an early Eocene
Nintoku Seamount at 27.1° (+5.5°/-7.7°). This value, together with paleo-latitudes from analyses of rocks at Site 433 (1980), Site 884 (1997), and Sites 1203 and 1204 (Leg 197; Detroit Seamount
), form a consistent data set indicating southward motion of the Hawaiian hotspot from Late Cretaceous to early Tertiary time, a hunch that many scientists had harbored for a long time.
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...
(underwater volcano) and guyot
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 ....
(flat top) in the 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...
. It is a large, irregularly shaped volcano that last erupted 65 million years ago. Three lava flows have been sampled at Nintoku Seamount; the flows are almost all alkalic (subaerial) lava
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...
. It is 56.2 million years old.
Nintoku is positioned a roughly 41 degrees north latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
, approximately two-thirds the way southward along the north-northeast-south-southeast Emperor seamounts extending from Meiji Seamount (about 53°N) in the north to Kammu Seamount (about 32°N) at the chain's southern terminus. Nintoku Seamount was named after the 16th emperor of Japan, Emperor Nintoku
Emperor Nintoku
was the 16th emperor of Japan, according to the traditional order of succession.No firm dates can be assigned to this emperor's life or reign, but he is conventionally considered to have reigned from 313–399.-Legendary narrative:...
, by geologist Robert Dietz
Robert S. Dietz
Robert Sinclair Dietz was Professor of Geology at Arizona State University. Dietz was a marine geologist, geophysicist and oceanographer who conducted pioneering research along with Harry Hammond Hess concerning seafloor spreading, published as early as 1960–1961...
in 1954.
The seamount occupies a central position in the Emperor Seamount chain and is thus an important point in the paleolatitude history of the Hawaiian hotspot, instrumental to proving the scientific hunch that 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....
was a mobile entity. The structure of the seamount is elongate, aligned north-northwest along the Emperor trend, with two prominent ridges trending southwest and south-southwest as far as 100 km (62 mi) from the main crater. Nintoku Seamount is a plexus of coalesced volcanoes, much like many of the larger seamounts in this chain. The Nintoku system is, however, clearly isolated from Yomei Seamount
Yomei Seamount
Yomei Seamount is a seamount of the Hawaiian-Emperor seamount chain in the northern Pacific Ocean. Its eruption ages are unknown.-References:...
, about 100 km (62 mi) to the north, and Jingu Seamount, about 200 km (124 mi) to south, by abyssal depths.
Geology and characteristics
In seismic profile, the main body of the seamount rises steeply over 5000 m (16,404 ft) in predominantly unsedimented volcanic slope to the thinly sedimented (10 m (33 ft) to 200 m (656.2 ft)), from an Emperor point of view, gently domed summit region between 1200 m (3,937 ft) to 1400 m (4,593 ft) high peak profile, which covers about 3400 square kilometers of area.From analysis of seismic reflection survey data and core material recovered by drillings at Site 432, the shipboard party of Deep Sea Drilling Project (DSDP) Leg 55 proposed that Nintoku Seamount was in an intermediate atoll stage (no lagoon but fringing reefs and banks and extensive carbonate bank interiors) before subsidence removed the island below the wave base. It was further thought that a few small remnant volcanic peaks and domes still pierce the sedimentary deposits.
Nintoku Seamount apparently remained at or above sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
long enough to be almost completely devastated by subaerial erosion and wave action. Reefs were not indicated in the seismic studies, but fragmented pieces of coral were recovered and documented, showing a shallow-water sediment-rich condition. The rock records indicate deposition in waters cooler than the present tropical condition. Shallow-water sedimentary deposition ceased in Paleogene times
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
.
The seamount was first drilled by Site 432, located on the northwestern edge of the summit region of Nintoku Seamount, in a gently sloping area mapped as terrace deposits. Although the sediment cover was estimated, based on other seamount covers, to be 80 metres (262 ft) thick, bedrock was hit after only 42 m (138 ft). Poorly recovered and preserved sedimentary deposits indicated a shallow-reef bed typical of terraced flanking reefs and banks, as well as volcanic sand
Black sand
Black sand is sand that is black in color. One type of black sand is a heavy, glossy, partly magnetic mixture of usually fine sands, found as part of a placer deposit. Another type of black sand, found on beaches near a volcano, consists of tiny fragments of lava.While some beaches are...
. Drilling at Site 432 penetrated 32 m (105 ft) of volcanic rock (74 m (243 ft) total) before terminating because of hole caving and damage to the drill assembly.
The site was drilled as part of Leg 197 by the Ocean Drilling Program
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...
, at site number 1205. A short bathymetric acoustic survey was conducted to find the best site for the location and structure of a core sampling. The locale chosen was about 100 m (328 ft) southwest of Site 432, the location of a previous drilling by the ODP.
Site 1205 (41°20.00′N 170°22.70′E) was located in 1310 m (4,298 ft) deep water, where previous drilling had reached volcanic rock beneath Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
deposits. It was elected to return to the site for a number of reasons. First, drilling at the nearby site 432 had hit reasonably unaltered and unchanged basalt with good remnant magnetic properties, key to finding the latitude of origin; but insufficient sampling caused a lack of data, and determining the age accurately was not possible. Hence, deeper drilling was promised to achieve that goal, providing a time-averaged (as the seamount is in the center of the chain) movement ratio. Second, a survey of the region showed a rock structure suitable for deep drilling, and nearby sites met low levels of sedimentary cover. Thirdly, the composition of previously drilled volcanic rock seemed to match the volcano's "average" type, erupted during the post-shield stage of it life. This helped another project goal, to recover a suitable and datable chunk of tholeiitic lava, which appeared to be rare on the seamount.
The hole was drilled in what appeared to be a large, broad sedimentary cover (estimated 70 m (230 ft) in thickness) covering a swath of the ancient volcano's main slope. The coring encountered volcanic rock at 42 m (138 ft) below the sea floor, and continued to a final depth of 326 m (1,070 ft) below the sea floor. The sedimentary cover, an element commonly found on many of the Emperor seamounts, was found to be largely a stack interlaced lava flows (about 95%). The drilling penetrated 283 m (928 ft) into the seamount's volcanic rock, and recovered at least 25 different hardened lava flows.
It was established that Nintoku Seamount's sedimentary cap consists of sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
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...
containing well-rounded to subrounded 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...
blocks, volcanic ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
, fossil fragments of mollusks, benthic foraminifers
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
, bryozoans, and coralline
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...
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...
. These observations indicate a shallow-water, high-rate depositional setting. Little variation was found in the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
, grain size, or porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
of the volcanic rock, and it was stable in composition, except for the volcanic-sedimentary cover. It is believed that this is the underlying cause of aucousically recorded layering of the upper 200 m (656 ft)-230 m (755 ft) of rock, after which the effect of soil layering fades away.
The age of the youngest volcanic rocks was constrained by nano-fossils in the sediment to be older than 53.6–54.7 million years, an age that is just younger than the radiometric age
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
of 56.2 ± 0.6 million obtained for the basalt drilled from nearby Hole 432A. The thickness and vesicularity of the lava flows, as well as the presence of oxidized flow tops and soil horizons and a lack of 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...
, indicate that all of the obtained samples erupted subaerially. The volcanic rock ranges from aphyric to highly plagioclase and olivine-phyric basalt. At 230 to 255 meters below ground, two flows of tholeiitic basalt were found interlaced with the alkalic basalt flows. Above these flows the degree of alkalinity skyrockets. There is evidence suggesting that the eruption rates must have been lower at the period during which the two flows were deposited, which is consistent with the model of Hawaiian volcanic growth, which increases in activity slowly over time before ceasing altogether. Internal lava flows have also created course-grained rocks. Lavas from Nintoku Seamount have similar composition to lava erupted during the post-shield stage of Hawaiian volcanoes such as Mauna Kea
Mauna Kea
Mauna Kea is a volcano on the island of Hawaii. Standing above sea level, its peak is the highest point in the state of Hawaii. However, much of the mountain is under water; when measured from its oceanic base, Mauna Kea is over tall—significantly taller than Mount Everest...
. Slight differences in trace element composition between lavas from Nintoku Seamount and active Hawaiian volcanoes probably result from differences in source composition or variations in the degree of
melting.
All of the recovered lava flows had been altered very little by erosion or other lava flows, except for thin flow tops. Sparse veining
Vein (geology)
In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation...
indicates that there is only small-scale fluid circulation within the rocks, in contrast with some of the data collected from 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...
.
Rock magnetic data obtained suggest that the lava flows from Site 1205 carry a remnant magnetization suitable for scientific analysis. Although some of the rocks needed a more complex and thorough analysis, most samples yielded data suitable to make a preliminary determination of magnetic inclinations. Twenty-two independent magnetic group were identified, yielding a mean inclination of -45.7° (+10.5°/-6.3°). The mean inclination suggests a latitude of formation on an early 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...
Nintoku Seamount at 27.1° (+5.5°/-7.7°). This value, together with paleo-latitudes from analyses of rocks at Site 433 (1980), Site 884 (1997), and Sites 1203 and 1204 (Leg 197; 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...
), form a consistent data set indicating southward motion of the Hawaiian hotspot from Late Cretaceous to early Tertiary time, a hunch that many scientists had harbored for a long time.