Seamounts can form in a wide variety of tectonic settings, resulting in a very diverse structural bank. Seamounts come in a wide variety of structural shapes, from conical to flat-topped to complexly shaped. Some are built very large and very low, such as Koko Guyot and Detroit Seamount; others are built more steeply, such as Loihi Seamount and Bowie Seamount. Some seamounts also have a carbonate or sediment cap. Many seamounts show signs of intrusive activity, which is likely to lead to inflation, steepening of volcanic slopes, and ultimately, flank collapse. There are also several sub-classes of seamounts. The first are guyots, seamounts with a flat top.
Structure
Seamount
Seamount | Structure
Seamounts can form in a wide variety of tectonic settings, resulting in a very diverse structural bank. Seamounts come in a wide variety of structural shapes, from conical to flat-topped to complexly shaped. Some are built very large and very low They have various structural shapes; from conical to flat-topped to complexly shaped Seamounts come in a wide variety of structural shapes, from conical to flat-topped to complexly shaped. They come in a wide variety of structural shapes, from conical to flat-topped to complexly shaped.
what is a chain of underwater mountains called
user
Seamount
assistant
what can you tell about its geographic features?
user
Seamounts can be found in every ocean basin in the world, distributed extremely widely both in space and in age. A seamount is technically defined as an isolated rise in elevation of or more from the surrounding seafloor, and with a limited summit area, of conical form.
assistant
how are these chain of mountains formed?
user
Formed millions of years ago by volcanism, they have since subsided far below sea level. This long chain of islands and seamounts extends thousands of kilometers northwest from the island of Hawaii.
assistant
what can you tell about its structure?
user
All the seamounts in the group are complex northeast-southwest trending structures, consisting of parallel ridges separated by sediment-filled troughs. The ridges constructed run parallel to an ancient spreading center which has since been replaced in its role by the San Andreas Fault system. They are unique in this origin, as they are formed from the remnants of an old ocean-ridge spreading center. A series of "knobs" are aligned with the ridges; however the distinctive summit crater, evident in many oceanic volcanoes, is absent. This lack of a collapse crater suggests that magma was never stored in a chamber within the structure, as with most other volcanoes.
Davidson Seamount | Geology
These tops must be or more below the surface of the sea; the diameters of these flat summits can be over . Knolls are isolated elevation spikes measuring less than . Lastly, pinnacles are small pillar-like seamounts.
Seamount | Structure
The Foundation chain eventually becomes a set of three ridges close to the Pacific-Antarctic Ridge, the northern two of which are the more voluminous ones. These northern ridges are probably the main ones; the southern one may have formed through cracks in the crust induced by the northern ridge, but it might also be a "paired" expression of a hotspot similar to the Kea and Loa trends in Hawaii. The seamounts reach depths of beneath sea level (the highest seamount is located at ), and contain typical volcanic features such as smaller volcanic cones, calderas and individual rift zones. Some of the seamounts close to the ridge have flat tops and show evidence of having formed islands above sea level in the past.
Foundation Seamounts | Geography
A further sampling cruise by the United States Geological Survey (USGS) recovered pillow lava and hyaloclastite from 4 of the 8 volcanoes. The samples seemed to be of more primitive origin then those erupted elsewhere on the ridge. In 1998, the Monterey Bay Aquarium Research Institute conducted two dives on a cone cluster located southeast of the main structure. The chain consists of eight volcanoes. Of these, four are self-dependent, whereas the other four form twin fused, morphalogically complex, flat-topped structures. The volcanoes form a linear chain, and are relatively small, measuring less than in diameter, and are nearly circular in shape.
President Jackson Seamounts | Introduction
Seamounts can form in a wide variety of tectonic settings, resulting in a very diverse structural bank. Seamounts come in a wide variety of structural shapes, from conical to flat-topped to complexly shaped. Some are built very large and very low, such as Koko Guyot and Detroit Seamount; others are built more steeply, such as Loihi Seamount and Bowie Seamount. Some seamounts also have a carbonate or sediment cap. Many seamounts show signs of intrusive activity, which is likely to lead to inflation, steepening of volcanic slopes, and ultimately, flank collapse. There are also several sub-classes of seamounts. The first are guyots, seamounts with a flat top.
Seamount | Structure
The Hawaiian–Emperor seamount chain is remarkable for its length and its number of volcanoes. The chain is split into two subsections across a break, separating the older Emperor Seamount Chain from the younger Hawaiian Ridge; the V-shaped bend of the chain is easily noticeable on maps. The volcanoes are progressively younger to the southeast; the oldest dated volcano, located at the northern end, is 81 million years old. The break between the two subchains is 43 million years; in comparison, the oldest of the principal islands, Kauaʻi, is little more than 5 million years. The "assembly line" that forms the volcanoes is driven by a hotspot, a plume of magma deep within the Earth producing lava at the surface.
Evolution of Hawaiian volcanoes | Background
The mountain comprises two principal components: a voluminous basal shield volcano and an eroded stratovolcano cap. The lower but more extensive basal shield volcano rises from an elevation of above the surrounding forested lowlands much like an inverted dishware plate. It consists of four distinctive stratigraphic units comprising thin mafic lava flows. Individual flows have an average thickness of but can range from less than to more than thick. They are separated by thin discontinuous breccias, sporadic tuff horizons and local lenses of fluvial, lacustrine and glacial sediment. This volcanic edifice forms a broad, oval-shaped, north-south trending lava plateau that local streams flow on.
Level Mountain | Structure
In-depth studies have given scientists a detailed view of the seamount's internal structure. The model developed by the Jasper Seamount studies closely resembles that of the Hawaiian islands, especially the eruption, in stages, of increasingly alkalic rocks. More than 90% of the volcano is tholeiitic to alkalic basalt in a wide variety of forms, including pillow lava and vesicular lapilli, evidence of shallow submarine eruptions in the past, and a wide range of xenoliths.
Jasper Seamount | Introduction
The Hawaiian–Emperor seamount chain is a mostly undersea mountain range in the Pacific Ocean that reaches above sea level in Hawaii. It is composed of the Hawaiian ridge, consisting of the islands of the Hawaiian chain northwest to Kure Atoll, and the Emperor Seamounts: together they form a vast underwater mountain region of islands and intervening seamounts, atolls, shallows, banks and reefs along a line trending southeast to northwest beneath the northern Pacific Ocean. The seamount chain, containing over 80 identified undersea volcanoes, stretches about from the Aleutian Trench in the far northwest Pacific to the Loʻihi seamount, the youngest volcano in the chain, which lies about southeast of the Island of Hawaiʻi.
Hawaiian–Emperor seamount chain | Introduction
The structure appears to consist of lagoonal sediments surrounded by a reef, and the shallowest point of Allison Guyot lies at less than depth below sea level. Volcanic cones dot the eastern side of the summit plateau. The seamount bears traces of slumping, which on the southeastern side of Allison Guyot has removed part of the platform perimeter. The seamount rises 1.5 kilometres above the seafloor. Underneath Allison Guyot, the seafloor is about 130–119 million years old, and a 128-million year-old magnetic lineation is located nearby. The Molokai Fracture Zone forms a ridge which passes close to Allison Guyot and intersects with another ridge at the seamount.
Allison Guyot | Local setting
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 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 to the north, and Jingu Seamount, about to south, by abyssal depths.
Nintoku Seamount | Introduction
A seamount is a large geologic landform that rises from the ocean floor but 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 abruptly and are usually found rising from the seafloor to in height. They are defined by oceanographers as independent features that rise to at least above the seafloor, characteristically of conical form. The peaks are often found hundreds to thousands of meters below the surface, and are therefore considered to be within the deep sea. During their evolution over geologic time, the largest seamounts may reach the sea surface where wave action erodes the summit to form a flat surface.
Seamount | Introduction
Alternatively, the site may be of volcanic origin.
Gatun structure | Introduction
However, some geologists have interpreted that these eclogites were subducted through a SW-dipping subduction zone. The two culminations are separated by the Semail Gap. This is a prominent linear structure, trending NNE—SSW. However, it is still debated as to what this structure is. Different geologists claim that it is a left-lateral (sinistral) strike-slip fault, a normal fault, a lateral ramp, a monocline due to a blind thrust, or a fault with multiple phases of deformation.
Al Hajar Mountains | Structures
A detailed topographic analysis of the Hawaiian – Emperor seamount chain reveals the hotspot as the center of a topographic high, and that elevation falls with distance from the hotspot. The most rapid decrease in elevation and the highest ratio between the topography and geoid height are over the southeastern part of the chain, falling with distance from the hotspot, particularly at the intersection of the Molokai and Murray fracture zones. The most likely explanation is that the region between the two zones is more susceptible to reheating than most of the chain. Another possible explanation is that the hotspot strength swells and subsides over time.
Hawaii hotspot | Topography and geoid
The rim of the summit plateau is formed by volcanic rocks on the eastern part of the seamount and by reefal limestones on the western; the reef deposits form a long ring around the summit platform of Ioah Guyot and cover an area of over . The lower slopes are gentle but steepen between depth. Only the lower slopes are covered with thick sediment layers; the upper slopes have sediment accumulations only in sheltered areas and also feature step-like structures. A number of volcanic cones grew on Ioah and form morphostructures, with a density of about 11.1 vents per .
Ioah Guyot | Geography and geomorphology
The ridges may be volcanotectonic features, composed of extrusives along fissures. However, they are mapped only as ridges because we cannot determine if they are volcanic material that should be mapped as a separate unit or uplifted intercrater plains. These same structures may have been the source of older plains units.
Bach quadrangle | Structure
The volcano is of Barremian age, the limestones are of Aptian to Albian age. Magnetic traits in the seamount suggest that it formed 140 - 120 million years ago close to a spreading center, which is older than the age of 100 million years inferred from fossils but comparable to ages inferred from radiometric dating. Radiometric dating has yielded ages of 120.4 ± 2.7 million years ago for the eastern and of 117.8 ± 8.4 million years ago for the western side of the seamount. The seamount is thus considered to be 120 - 100 million years old, while the underlying crust is about 20 million years older.
Daiichi-Kashima Seamount | Geological history
The Foundation Seamounts form a long and wide band of seamounts that extend northwestward away from the Pacific-Antarctic Ridge towards the Resolution () and Del Cano () fracture zones. A small ridge continues from there to the Macdonald seamount, but the relation of this ridge to the Foundation Seamounts appears to be questionable at best, and there are no other clear bathymetric features in between Foundation and Macdonald. Close to the western end of the Foundation Seamounts lie the "Old Pacific Seamounts", which may have the same origin. On the other end of the chain, the seamounts become short ridges decorated by volcanic cones; the Pacific-Antarctic Ridge is migrating northwestward and approaching the hotspot, and the younger crust close to the ridge results in the morphology of developing volcanoes being altered.
Foundation Seamounts | Geography
Hyaloclastic rocks have also been observed.
Monowai (seamount) | Composition
They average ± in volume, but range from to . In addition to the eight main seamounts, there is a scattered disk of minor volcanic cones and flows about southeast of the chain.
President Jackson Seamounts | Introduction
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 to the northwest and Kimmei Seamount to the southeast. The seamount is located just 2.3 degrees north of the bend.
Koko Guyot | Geology and characteristics
The seamount lies in the Mid-Pacific Mountains on their southern side and is a submarine mountain with a flat top that rises over to a depth of -. The flat top has an oval shape and a surface area of ; it is characterized by a limestone dome on the summit, buried beneath sediments; in turn a volcanic hill is buried within the limestone dome. The top of the seamount has a hummocky appearance which has been interpreted as a sediment cover and its southeastern sector has a bank-like shape that resembles that of an atoll. Cape Johnson Guyot is considered to be of Middle Cretaceous age with an age of 120 million years reported and shallow-water fossils were emplaced on it at that time.
Cape Johnson Guyot | Geography and geology
This flat summit is covered by high volcanic cones and features terraces at the margin of the summit as well as ridges that point due north, east, south and west. Volcanic cones are concentrated in the southern and eastern parts of the summit platform. The seamount has a volume of about , similar to other Atlantic Ocean volcanoes. Raised beaches have been reported from Tropic Seamount. The outer slopes of the seamount become steeper to the summit perhaps due to geochemical differences between the rocks that form the lower slopes and these of the upper slopes. They are cut by curved flank collapses that open to the northeast, southeast, southwest and northwest; these collapses have cut into the volcanic edifice and have left ridges between the individual collapses which give the seamount its four-point star form and deposited debris around the seamount, although the deposits are not recognizable probably because they are old and buried beneath sediments.
Tropic Seamount | Local
Takuyo-Daisan is a guyot that rises to a minimum depth of and is capped by an approximately square-shaped surface platform at about depth which has a surface area of about . Depressions up to deep cover the flat top. The top is surrounded by a wide and up to high ridge, especially on the northeastern and southern sides. It may be either a bioherm or a karst landform. A - high structure may be part of the old volcanic edifice; the interface between the carbonate platform and the volcanic basement has a sloping aspect. Away from the platform, the seamount has steep slopes probably formed by short lava flows that rise above the seafloor.
Takuyo-Daisan | Local
They are formed from mud volcanism. The composition for the seamounts differ in the Izu-Ogasawara Trench and Mariana systems which indicates regional changes in geology.
Mariana Plate | Defining Features
The Tuscaloosa Seamount is an undersea mountain in the Hawaiian archipelago. It is located about northeast of the island Oʻahu. In contrast to the overwhelming majority of seamounts, the Tuscaloosa Seamount is not a submarine volcano. It is a huge block of rocks that broke off about two million years ago at the Nuʻuanu submarine landslide when the volcano Koʻolau collapsed. The Tuscaloosa Seamount is long and wide. Its shallow summit rises across the sea bottom but is below sea level.
Tuscaloosa Seamount | Introduction
A look at the USGS map on the origin of the Hawaiian Islands clearly shows this "spearpoint". In a more recent study, Sharp and Clague (2006) interpret the bend as starting at about 50 million years ago. They also conclude that the bend formed from a "traditional" cause—a change in the direction of motion of the Pacific plate.
Hawaiian–Emperor seamount chain | Formation
Basins around the volcano increase its irregularity, making it unusually complex (most seamounts of roughly the same size are circular or flattened in shape.) Axial Seamount's summit is marked by an unusual rectangular caldera, in area, ~3° in slope, and breached on the southeast side. The area is offset by the two rift zones and defined on three sides by boundary faults up to deep. The caldera is roughly deeper at the north side then it is in the south. Flows within the caldera consist mostly of sheet flows pocketed by lava ponds and pit craters. Less common are pillow lavas; their arrangement along the caldera walls suggests that they were an important component in the volcano's early growth.
Axial Seamount | Structure
Identical chains run parallel to the "c" axis that are linked only by one weak I-O bond. The inner layer of the slab is composed of one Mg octahedron that shares corners with two Cr tetrahedra. This forms a [MTΦ] cluster. The other two anions of the Mg octahedron link by corner-sharing to two (IO) groups. These [Mg(CrO)(IO)O] clusters link together two (CaΦ) polyhedra. This forms chains parallel to the "b" axis. Weak I-O bonds link these chains to form the central layer of the slab.
George-ericksenite | Structure topology
The seamount is very close to the "V"-shaped bend in the Hawaiian-Emperor seamount chain, and thus would be useful in understanding the exact age of the bend. Although few dredge samples are available, they have all been reliably dated at 47 million years (Sharp and Clague, Science, 313, 1281-84, 2006), during the Eocene epoch of the Paleogene period. During the cruise SO112 of the "R/V SONNE", high resolution bathymetric mapping was conducted, showing that Daikakuji is nearly in diameter and nearly in height, with a summit lying underwater. Because of its flat capped top, Daikakuji is considered a guyot. A smaller, younger, secondary guyot just east of the main mass overlaps its slope.
Daikakuji Guyot | Geology
Side-scan surveys of South Chamorro Seamount show a major southeastern sector edifice collapse, with debris flows of serpentine material (dredged in 1981 and observed by submersibles in 1995) that blankets the slope of the trench from summit to axis. The true summit of the volcano sits above this collapse; its formation was likely tied to the collapse. Submarine observation of the summit show that the summit knoll is broken up into slabs of serpentine mud, with meter-deep fissures arranged in a crosscutting orientation.
South Chamorro Seamount | Structure
They appear to have been formed under the influence both of fracture zones and of a melt anomaly that also formed a topographic anomaly west of the Lamont seamount chain along with elongated volcanic structures; a bathymetric swell is also noted around the seamounts as well as a deep depression that may be an isostatic moat. The volcanism may be ultimately the consequence of hotspot activity.
Lamont seamount chain | Geology
Huaynaputina lies at an elevation of about . It consists of an outer composite volcano (or stratovolcano) and three younger volcanic vents nested within a wide and deep amphitheatre. This horseshoe-shaped structure is set in the older volcano at an elevation of and opens eastwards. The amphitheatre lies at the margin of a rectangular high plateau that is covered by about ash over an area of . In general, the volcano has modest dimensions and rises less than above the surrounding terrain, but the products of the volcano's 1600 eruption cover much of the region to this day especially west, north and south from the amphitheatre; these include pyroclastic flow dunes that crop out from underneath the tephra.
Huaynaputina | Structure
Guide Seamount is a seamount in the eastern Pacific Ocean, about 16.6±0.5 million years old. It is similar in shape and orientation to the nearby Davidson, Pioneer, Rodriguez, and Gumdrop seamounts. It is named for the United States Coast and Geodetic Survey survey ship "USC&GS Guide". Guide Seamount is constructed of four nearly parallel volcanic ridges, separated by sediment-filled throughs. These are aligned parallel to magnetic anomalies in the underlying oceanic crust. It is very similar in shape and structure to the nearby Davidson Seamount, except that it is smaller, at approximately by . It rises about above the seafloor and sits at depth of .
Guide Seamount | Introduction
Allison Guyot is located in the equatorial Pacific Ocean, part of the western Mid-Pacific Mountains. The Mid-Pacific Mountains contain seamounts which were covered by limestones during the Barremian and Albian (circa 129.4 – circa 125 million years ago and circa 113–100.5 million years ago, respectively). Hawaii lies due east and the Marshall Islands southwest; Resolution Guyot is 716 kilometres northwest. The guyot (also known as tablemount) has an outline resembling a trapezoid and consists of two connected volcanic ridges facing north-northwest to east-northeast. Its western parts may be a distinct volcano. The surface platform has dimensions of 35 by 70 kilometres, with an upwards-doming form 0.3–0.5 kilometres high, and is covered by large sediment mounds; the rim surrounding the platform lies at a depth of about 1,650 metres and there is evidence of former reefs.
Allison Guyot | Local setting
In addition there are long gullies formed either by erosional or volcanic processes, as well as parasitic vents and volcanic ridges.
Tropic Seamount | Local
Seamounts are often found in groupings or submerged archipelagos, a classic example being the Emperor Seamounts, an extension of the Hawaiian Islands. Formed millions of years ago by volcanism, they have since subsided far below sea level. This long chain of islands and seamounts extends thousands of kilometers northwest from the island of Hawaii. There are more seamounts in the Pacific Ocean than in the Atlantic, and their distribution can be described as comprising several elongate chains of seamounts superimposed on a more or less random background distribution. Seamount chains occur in all three major ocean basins, with the Pacific having the most number and most extensive seamount chains.
Seamount | Grouping
Kohala had two active rift zones, a characteristic common to Hawaiian volcanoes. The double ridge, shaped axially, was active in both the shield and postshield stages. The southeast rift zone passes under the nearby Mauna Kea and reappears further southeast in the Hilo Ridge, as indicated by the relationship between collected lava samples. In addition, the two features align to one another more closely then does the ridge to Mauna Kea, of which it was once thought to constitute a part. The bulk of the ridge is built of volcanic rock with reverse magnetic polarity, evidence that the volcano is at least 780,000 years old.
Kohala (mountain) | Structure
The internal structure of a volcano can be visualized with a technique known as seismic wave tomography. An inverted funnel-shaped low velocity anomaly with a width of extends to a depth of beneath the volcano and appears to be associated with areas of high fumarolic activity; it may be the hydrothermal system. An even stronger anomaly at depths of may be the magma chamber of the volcano and an associated fluid-filled system. Magnetotelluric imaging showed structures similar to those revealed using seismic imaging.
Lastarria | Internal structure
The seamount is a volcano between 10.9 and 11.1 million years old. It is about long as well as about wide, and has a volume of about . It rises about above the surrounding ocean floor, and its peak is a minimum of below the oceans surface. Samples from the seamount consist of highly vesicular alkalic basalt, hawaiite, and mugearite. The seamount and its volcano once extended above the sea surface, but eroded and sank as the seamount and the seabed at its base were carried further away from the spreading center from which it presumably originated.
Pioneer Seamount | Physical characteristics
The Vance Seamounts are a group of seven submarine volcanoes located west of the Juan de Fuca Ridge. Most of the seamounts contain a caldera. They are the southernmost of several near-ridge chains located on the Pacific Plate, stemming from the Juan de Fuca Ridge. The easternmost five of the seven were surveyed using SeaBeam bathymetry. A chemical analysis of the lavas of the volcanoes indicated that the materials erupted from the volcanoes are relatively primitive. With the exception of one, the volcanoes form a nearly linear chain of circular volcanic cones. The second volcano from the northwest is not discretely shaped like the rest, rather it is a rough structure of lava cones and flows, which cover an area of at least .
Vance Seamounts | Introduction
Observations show that both the north and south rift zones lack sediment cover, indicating recent activity. A bulge in the western part of the north rift zone contains three cone-shaped prominences.
Lōʻihi Seamount | Geology
There are several dome-like structures within the caldera with heights of . There are several small craters within the region, the largest of which, nicknamed the "D.D. Cone", is in diameter and in relief. However, most of the features do not range over deep and across. The northern rift zone of Axial Seamount is a long ridge running 10 to 20 degrees northeast of the main caldera. The rift is pocketed by multiple fissures, in length, as far as from Axial Volcano's center, and reaching up to long and deep. The area contains high amounts of volcanic glass; a major eruption is still visible in the form of an elongated glassy lava flow extending off the caldera wall, east of the main rift line.
Axial Seamount | Structure
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. 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.
Macdonald seamount | Regional setting
The chain-like arrangement of summits and the formation of long, jagged mountain crests – known in Spanish as sierras ("saws") – is a consequence of their collective formation by mountain building forces. The often linear structure is linked to the direction of these thrust forces and the resulting mountain folding which in turn relates to the fault lines in the upper part of the earth's crust, that run between the individual mountain chains. In these fault zones, the rock, which has sometimes been pulverised, is easily eroded, so that large river valleys are carved out. These, so called longitudinal valleys reinforce the trend, during the early mountain building phase, towards the formation of parallel chains of mountains.
Mountain chain | Formation of parallel mountain chains
The Cobb-Eickelberg seamount chain is a range of undersea mountains formed by volcanic activity of the Cobb hotspot located in the Pacific Ocean. The seamount chain extends to the southeast on the Pacific Plate, beginning at the Aleutian Trench and terminating at Axial Seamount, located on the Juan de Fuca Ridge.The seamount chain is spread over a vast length of approximately 1800 km. The location of the Cobb hotspot that gives rise to these seamounts is 46° N -130° W. The Pacific plate is moving to the northwest over the hotspot, causing the seamounts in the chain to decrease in age to the southeast.
Cobb–Eickelberg Seamount chain | Introduction
Structurally the Massif Central consists of stacked metamorphic basement nappes that have been overthrust onto their southern foreland (Aquitania). The following structural units can be discerned (from structurally higher to structurally lower):
Geology of the Massif Central | Structural organisation
Mauna Loa is a typical shield volcano in form, taking the shape of a long, broad dome extending down to the ocean floor whose slopes are about 12° at their steepest, a consequence of its extremely fluid lava. The shield-stage lavas that built the enormous main mass of the mountain are tholeiitic basalts, like those of Mauna Kea, created through the mixing of primary magma and subducted oceanic crust. Mauna Loa's summit hosts three overlapping pit craters arranged northeast-southwest, the first and last roughly in diameter and the second an oblong feature; together these three craters make up the summit caldera Mokuʻāweoweo, so named for the Hawaiian ʻāweoweo fish ("Priacanthus meeki"), purportedly due to the resemblance of its eruptive fires to the coloration of the fish.
Mauna Loa | Structure
The Foundation hotspot is considerably weaker than many other hotspots such as the Society hotspot or the Hawaii hotspot, and its volcanoes were constructed in shorter timespans than the Hawaiian ones. Because an age progression was not found at first (Macdonald seamount is active in its own right), a "hot line" origin was proposed at first; argon-argon dating performed later on rocks dredged from the seamounts has demonstrated clearly age progressive volcanism. The plume may be located either directly underneath the spreading ridge, or about west of it; geoid anomalies are centered on the volcanic ridges and indicate a distance of but with some large uncertainty.
Foundation Seamounts | Geology
These structures resemble large, flat-topped mesas with rugged surfaces. Other mountains appear to be tilted crustal blocks, with a shallow slope from the formerly flat surface and a steep slope consisting of formerly sub-surface materials uplifted by compressive stresses. Both types of mountains often have steep scarps along one or more margins. Only a handful of mountains on Io appear to have a volcanic origin. These mountains resemble small shield volcanoes, with steep slopes (6–7°) near a small, central caldera and shallow slopes along their margins. These volcanic mountains are often smaller than the average mountain on Io, averaging only in height and wide.
Io (moon) | Mountains
The crust underneath these seamounts tends to subside as it cools, and thus the islands and seamounts sink. Continued subsidence balanced by upward growth of the reefs led to the formation of thick carbonate platforms. Sometimes volcanic activity continued even after the formation of the atoll or atoll-like structure, and during episodes where the platforms rose above sea level erosional features such as channels and blue holes developed. The formation of many seamounts has been explained by the hotspot theory, which suggests that the chains of volcanoes become progressively older along the length of the chain, with an erupting volcano only at one end of the system.
Resolution Guyot | Regional setting
Most seamounts are built by one of two volcanic processes, although some, such as the Christmas Island Seamount Province near Australia, are more enigmatic. Volcanoes near plate boundaries and mid-ocean ridges are built by decompression melting of rock in the upper mantle. The lower density magma rises through the crust to the surface. Volcanoes formed near or above subducting zones are created because the subducting tectonic plate adds volatiles to the overriding plate that lowers its melting point. Which of these two process involved in the formation of a seamount has a profound effect on its eruptive materials. Lava flows from mid-ocean ridge and plate boundary seamounts are mostly basaltic (both tholeiitic and alkalic), whereas flows from subducting ridge volcanoes are mostly calc-alkaline lavas.
Seamount | Geochemistry and evolution
Meanwhile, the East Fissural Zone comprises several alignments of Hawaiian/Strombolian cinder cones and associated lava flows, which overflowed many of the cliffs, cut in older units and originated lava deltas (). The tectonic structure is characterized by two fault systems. The main WNW-ESE structures are the dextral faults of Lagoa do Capitão and Topo, that merge to the east, forming a narrow shallow graben (or trench). To the west, the graben is completely covered by the Pico stratovolcano occurring less than 10,000 years ago, and infilled by lava flows and cones of the Eastern Fissural zone, that includes many of the volcanic alignments and scarps.
Mount Pico | Geology
The seamount is elongated in about northeast-south-southwest direction, with ridges radiating from its northern sector and the lower southern sector spreading southwestward like a tongue. This elongation matches the trend of magnetic lineations on the surrounding seafloor, implying that it may be controlled by the surrounding tectonic environment. The submarine slopes of the seamount are quite steep and show two benches at and depth. The seamount has dimensions of on the seafloor. About 15–20 parasitic vents dot the deeper flanks of the seamount and reach height, and small ridges en echelon to Vesteris are found on the southeastern flank. The total volume of the seamount is about .
Vesteris Seamount | Geography and geology
The chain can be divided into three subsections. The first, the Hawaiian archipelago (also known as the "Windward isles"), consists of the islands comprising the U.S. state of Hawaii. As it is the closest to the hotspot, this volcanically active region is the youngest part of the chain, with ages ranging from 400,000 years to 5.1 million years. The island of Hawaii is composed of five volcanoes, of which three (Kilauea, Mauna Loa, and Hualalai) are still active. Haleakalā on the island of Maui is dormant. Lōihi Seamount continues to grow offshore of Hawaii island, and is the only known volcano in the chain in the submarine pre-shield stage.
Hawaiian–Emperor seamount chain | Regions
The chain has been produced by the movement of the ocean crust over the Hawaii hotspot, an upwelling of hot rock from the Earth's mantle. As the oceanic crust moves the volcanoes farther away from their source of magma, their eruptions become less frequent and less powerful until they eventually cease to erupt altogether. At that point erosion of the volcano and subsidence of the seafloor cause the volcano to gradually diminish. As the volcano sinks and erodes, it first becomes an atoll island and then an atoll. Further subsidence causes the volcano to sink below the sea surface, becoming a seamount and/or a guyot.
Hawaiian–Emperor seamount chain | Aging
Ascraeus Mons was built by many thousands of fluid basaltic lava flows. Except for its great size, it resembles terrestrial shield volcanoes like those that form the Hawaiian Islands. The flanks of Ascraeus Mons are covered with narrow, lobate lava flows and lava channels. Many of the lava flows have levees along their margins. Levees are parallel ridges formed at the edges of lava flows. The cooler, outer margins of the flow solidify, leaving a central trough of molten, flowing lava. Partially collapsed lava tubes are visible as chains of pit craters. By examining the morphologies of lava flow structures on Ascraeus Mons, geologists are able to calculate the rheological properties of the lava and estimate the rate at which it poured out during eruption (effusion rate).
Ascraeus Mons | Geology
Older lava dated from the toe of the ridge was estimated to be roughly between 1.1 and 1.2 million years old. Geologists have classified the volcano's lava flows into two units. The Hawi Volcanic layers were deposited in the postshield stage of the volcano's life, and the younger Pololu Volcanics were deposited in the volcano's shield-building stage. The rock in the younger Hawi section, which overlies the older Pololu flows, is mostly 260,000 to 140,000 years old, and composed mainly of hawaiite and trachyte. The separation between the two layers is not clear; the lowest layers may actually be in the Pololu section, based on their depositional patterns and low phosphorus content.
Kohala (mountain) | Structure
In the middle of the chain lies Mount Steere with a summit elevation of and a rectangular summit caldera. Lie Cliff is a volcanic outcrop on the northeastern flank. Mount Steere is heavily dissected, bears evidence of former glaciation in the form of moraines and cirques have been eroded into its northern and northeastern flanks. As with Mount Rees, volcanic rocks alternate between these that formed subglacially and these formed subaerially. Outcrops feature breccias and lava with numerous intruded dikes.
Crary Mountains | Geography and geomorphology
Between 200,000 and 700,000 years ago, the hotspot was encroached by the tectonic spreading center, displacing it by as much as and building up the long Juan de Fuca Ridge. At least 7 spreading centers have been recognized, and plate measurements near Axial show that the ridge is separating at a rate of per year, producing a complex system of oceanic basins and ridges. However some scientists have questioned this theory, pointing out that the high density of the chain's overlapping seamounts is incompatible with such an origin, as a hotspot would form a well organized, widely spaced chain. Although the exact nature of Axial Seamount remains unknown, its complex origins makes it one of the most geologically interesting features in the North Pacific.
Axial Seamount | Tectonic setting
Volcanic activity at Echo and Tropic seamounts was probably focused and generated a circular volcanic structure, while at Drago and The Paps it was controlled by lineaments and thus formed elongated edifices.
Tropic Seamount | Geology
Whether there was Holocene activity 5,000–6,000 years ago or present-day hydrothermal activity is unclear. A variety of lifeforms are found both on Vesteris Seamount and within its rocks. Layers of sponges and bryozoans cover the upper parts of the seamount and form structures like mounds. This ecosystem has been compared to coral reefs.
Vesteris Seamount | Introduction
Similar structures are throughout the United States
Delmar Calaboose | History
The lavas from Guide are mostly alkalic basalt, hawaiite, mugearite with some pyroclastic flows near the top of the summit.
Guide Seamount | Introduction
Axial Seamount is the most active volcanic site in the North Pacific. Study of magnetic delineations along the seamount have modeled the ridge's history up to 30 million years ago, and shown that growth has progressed mostly in the north, with some southward progression dating back 3.5 million years. The base of Axial Seamount is a long, low-lying plateau, and the eastern part of the seamount is defined by a series of linear scarps. Axial Seamount has two major volcanic rifts extending approximately north and south of its main summit, as well as several much smaller, ill-defined ones aligned in a roughly similar pattern.
Axial Seamount | Structure
Though the remaining ridge appears to be a steep and weathered fault scarp evidence suggests the geographic features are from wind and water rather than the landslide event. The volcano collapsed nearly in half at the caldera with the eastern half falling into the ocean cataclysmically with enough force to send debris including the massive Tuscaloosa seamount across the ocean floor and up a ridge. This landslide created a massive tsunami which ravaged the islands and possibly the Western coast of ancient North America.
Nuʻuanu Slide | Introduction
Jingū Seamount, also called Jingū Guyot, is a guyot of the Hawaiian-Emperor seamount chain in the Pacific Ocean. It erupted 55 million years ago. The seamount is elongated in structure, running north–south, and has an oval shaped crater in the center, which is evidence of collapse when above sea level. The seamount was named in 1954 by Robert S. Dietz, after Japanese Empress Jingū.
Jingū Seamount | Introduction
The Lamont seamount chain is a group of five seamounts in the Pacific Ocean; from southeast to northwest they are known as "Sasha", "MIB", "MOK", "DTD" and "NEW" and there is an additional unnamed seamount southwest from "NEW". The seamounts were discovered in 1983 and named later. The seamounts reach heights of and have summit craters/calderas, that on "DTD", "MOK" and "NEW" form nested, complex calderas. Horseshoe-shaped volcanic ridges accompany the calderas and lava effusion appears to have preferentially occurred on the margins of the calderas, along with mass wasting. The Lamont seamount chain forms a long chain and the individual seamounts surrounded by lava cones high.
Lamont seamount chain | Geography and geomorphology
Several faults that occur west of the volcano relate to the developing graben and additional faults dissect the volcano, including ring faults that surround the caldera. The faults that dissect the Monowai volcano have strikes and trends comparable to those of other volcanoes in the central Kermadec arc.
Monowai (seamount) | Geology
The Musicians Seamounts lie in the north-central Pacific, north of the Hawaiian Ridge north and northwest of Necker Island, extending over a length of . The seamounts were formerly known as the North Hawaiian Seamount Range and were among the first submarine mountains to be thoroughly researched. The seamounts consist of two separate chains (one trending in north-south direction parallel to the Emperor Seamounts and the other in northwest-southeast direction parallel to the northern Line Islands) of ridges that extend east-west and seamounts. Seamounts have usually an elliptical cross section and heights of and no flat tops like other seamounts south of the Hawaiian Ridge; the highest is Liszt Seamount whose summit has a depth of .
Musicians Seamounts | Geography and geomorphology
Cross Seamount, a landform arising from the ocean floor, is one of the more distant lesser seamounts surrounding the island of Hawaii. The NOAA has taken advantage of Cross Seamount’s position, and planted several weather buoys on the seamount. Very little is known about Cross Seamount's geology, as all of its studies as of March 2009 were of its ecology; however, it was mapped in 1996 by a four-year effort to map all of Hawaii's seafloor in detail.
Cross Seamount | Geology
Images of Pan show a structure similar to that of Atlas.
Equatorial ridge | Introduction
Many of the volcanoes are named after former emperors of Japan. The seamount chain extends to the West Pacific, and terminates at the Kuril–Kamchatka Trench, a subduction zone at the border of Russia.
Hawaiian–Emperor seamount chain | Regions
In 2015, researchers found that the volcano's structure bore patterns of magnetic striping on either side, indicating that the volcano is likely a hybrid of a mid-ocean ridge and a shield volcano. Geologic data also indicated that Tamu Massif formed at the junction of three mid-ocean ridges, which was a highly unusual occurrence. A study found that the Moho line, the boundary between the Earth's crust and mantle, extends more than beneath the base of Tamu Massif, meaning that the volcano is unlikely to ever erupt again, since magma is almost unable to penetrate a barrier that thick.
Tamu Massif | Geology
Guyots show evidence of having once been above the surface, with gradual subsidence through stages from fringed reefed mountain, coral atoll, and finally a flat-topped submerged mountain. Seamounts are made by extrusion of lavas piped upward in stages from sources within the Earth's mantle, usually hotspots, to vents on the seafloor. The volcanism invariably ceases after a time, and other processes dominate. When an undersea volcano grows high enough to be near or breach the ocean surface, wave action and/or coral reef growth tend to create a flat-topped edifice. However, all ocean crust and guyots form from hot magma and/or rock, which cools over time.
Guyot | Formation
Irnini Mons is composed of continuous flows of basaltic lava, with younger pyroclastic material within the flows near the summit. The younger flows resurface and superimpose the older flows as seen by the intensity of observable wrinkle ridge deformation decreasing from older to younger flows. Irini Mons is characterized by various fracture, ridge, and graben complexes. This includes north-south trending rift and ridge structures that are parallel to the Badb Linea rift zone. Furthermore, many graben radiate from the central magma chamber resulting in a symmetrical web of structures. The radial graben are highly reflective in radar imaging and, therefore, easily noticeable.
Irnini Mons | Volcanic-Tectonic Structures
The Pacific plate seafloor east of the IBM arc system can be subdivided into a northern portion that is bathymetrically ‘smooth’ and a southern portion that is bathymetrically rugged, separated by the Ogasawara Plateau. These large-scale variations mark distinct geologic histories to the north and south. The featureless north is dominated by the Nadezhda Basin. In the south, crude alignments of seamounts, atolls, and islands define three great, WNW-ESE trending chains : the Marcus Island-Wake Island-Ogasawara Plateau, the Magellan Seamounts Chain, and the Caroline Islands Ridge. The first two chains formed by off-ridge volcanism during Cretaceous time, whereas the Caroline Islands chain formed over the past 20 million years.
Izu–Bonin–Mariana Arc | Geology and composition of the westernmost Pacific plate
Volcanic activity of the Foundation hotspot appears to have been steady throughout 23-5 million years ago, but may have weakened dramatically since then. Radiometric dating performed on the Foundation Seamounts shows that their age decreases from 21 million years on their western end to present on their eastern end. On average, age progression along the chain is about , comparable to that of the Hawaii hotspot. Close to the intersection between the Foundation chain and the Antarctic-Pacific Ridge, a lava flow field was emplaced between 1997 and 2001 on a bathymetric high. Hydrothermal communities and thermal anomalies have been observed in the same area, suggesting ongoing hydrothermal venting there.
Foundation Seamounts | Activity
The spurs appear to be rift zones, similar to these formed on Hawaii by dyke injection although some of the ridges at Wōdejebato may have a different origin. Wōdejebato appears to have four such ridges, which is more than is observed at Hawaii. One explanation is that the northwestern ridge is another seamount; another that Wōdejebato consists of more than one volcano although the relatively small size of the seamount would argue against this view. Wōdejebato's slopes descend rather steeply until, at depth, where they become more gentle, they are decorated with forms resembling cones and channels. Part of its southern flank, where there is a downdropped terrace, seems to have collapsed in the past.
Wōdejebato | Local setting
Channels, debris flow fronts and levees decorate its slopes. Noticeable ridges emanate from the east-southeastern, southwestern, west-northwestern and north-northeastern corners of the seamount; they appear to be rift zones, and flank cones are found on the eastern slopes. The southern slopes feature slump deposits, and the entire seamount is surrounded by a sediment fan. The volume of the entire seamount is about . Two large seamounts lie west and southwest of Takuyo-Daisan with a smaller seamount between the two; Takuyo-Daini is the western one and Jensen seamount the south-western one. The seafloor underneath Takuyo-Daisan has an age of 143 million years.
Takuyo-Daisan | Local
The Monmouth Creek complex on the west side of the Squamish River mouth is a prominent and enigmatic edifice composed of basaltic andesite and dacite of unknown age. It may represent a group of dikes and lava domes that formed subglacially. At least four dikes protrude its summit. These form the ribs of high lava spines, the tallest being The Castle, which contains horizontal and radiating columnar joints. The spines are covered by welded breccia close to their bases and columnar jointing extends into the welded sequence. The most elevated lava flows and spines are composed of dacite.
Canadian Cascade Arc | Southern segment
The search was difficult because the salty water had altered the volcanic rock over time, but they determined in lab tests that the Denson Seamount was about 18 million years old. The team used bathymetric mapping to render three-dimensional images of the Denson Seamount and its counterparts.
Denson Seamount | Introduction
Linear chains of volcanoes in the Pacific have been attributed to deep mantle plumes, but a number of such chains appear to correspond to short lasting volcanism, such as processes triggered by tension in the Pacific Plate. The South Pacific Superswell is a large structure in the southern Pacific Ocean, where there are many volcanoes and the seafloor is unusually shallow. Asthenospheric processes may be responsible for its existence.
Tarava seamounts | Regional
The oldest volcano in the chain, Meiji Seamount, perched on the edge of the Aleutian Trench, formed 85 million years ago. At its current velocity, the seamount will be destroyed within a few million years, as the Pacific Plate slides under the Eurasian Plate. It is unknown whether the seamount chain has been subducting under the Eurasian Plate, and whether the hotspot is older than Meiji Seamount, as any older seamounts have since been destroyed by the plate margin. It is also possible that a collision near the Aleutian Trench had changed the velocity of the Pacific Plate, explaining the hotspot chain's bend; the relationship between these features is still being investigated.
Hawaii hotspot | Movement
As the seamount slowly grows, it goes through the shield stages. It forms many mature features, such as a caldera, while submerged. The summit eventually breaches the surface, and the lava and ocean water "battle" for control as the volcano enters the explosive subphase. This stage of development is exemplified by explosive steam vents. This stage produces mostly volcanic ash, a result of the waves dampening the lava. This conflict between lava and sea influences Hawaiian mythology.
Hawaii hotspot | Evolution and construction
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 track. Dredged rocks are weakly tholeiitic basalts of Oligocene age. They are about high and wide at their base. Despite being smaller in size than the Macdonald seamounts however the Ngatemato seamounts have larger volumes which is masked by stronger plate deformation; the magma output that created the Ngatemato and Taukina chains when summed up is about .
Ngatemato seamounts | Introduction
The composition of the volcanoes' magma has changed significantly according to analysis of the strontium–niobium–palladium elemental ratios. The Emperor Seamounts were active for at least 46 million years, with the oldest lava dated to the Cretaceous Period, followed by another 39 million years of activity along the Hawaiian segment of the chain, totaling 85 million years. Data demonstrate vertical variability in the amount of strontium present in both the alkalic (early stages) and tholeiitic (later stages) lavas. The systematic increase slows drastically at the time of the bend. Almost all magma created by the hotspot is igneous basalt; the volcanoes are constructed almost entirely of this or the similar in composition but coarser-grained gabbro and diabase.
Hawaii hotspot | Magma
This fault, which appears to be split in two or three subsidiary faults separated by grabens, extends past the Daiichi-Kashima seamount and covers a length of ; evidently Daiichi-Kashima has been split in half by the fault, which is much younger than the ocean floor and moved at a rate of but does not appear to be presently active in light of the sediment cover on the scarp. Aside from a normal motion, the western half of the seamount has also been moved away from the eastern half and is tilted west. The seamount appears to be part of a seamount chain called Joban Seamount Chain or Kashima-Ryofu No.1 that formed during the Cretaceous in the Equatorial Pacific and about 30° south of their present-day position.
Daiichi-Kashima Seamount | Local
The subhorizontal dish structure consists of two parts, the dish itself and the [sediment] contained within the dish plus the region stretching up to the bounding surface of the overlying dish(or dishes) above. The bounding surface of the dish can take on variable shapes, from substantially flat to bowl-like and to strongly concave up. The bounding surfaces are thin, (and usually) dark(er) laminae; they are richer in clay, silt or organic material than the surrounding sediment. The individual dishes are arranged "en echelon". Their width can vary from 2 centimeters to over 50 centimeters, the vertical spacing ranges usually from less than 1 centimeter to about 8 centimeters.
Dish structure | Description
Magnetic anomalies at Rodriguez indicate that it is located on a 19-million-year-old crustal surface. Rocks recovered from Rodriguez Seamount are largely composed of alkaline basalt and Hawaiite. Ar-Ar dating techniques indicate that the volcano is between 10 and 12 million years of age. Rodriguez Seamount rises about above the surrounding seafloor, to a minimum depth of . Its calculated volume is greater than ; however this is likely an understatement because the survey did not include its lowermost slope. The slope is composed mostly of layered volcanic rock, mostly coarse sandstone, with a few scattered large lava boulders. They were likely formed from fragments of volcanic glass formed in the steam explosions of lava touching down against water, similar to the process that is happening today on Kilauea.
Rodriguez Seamount | Geology
Compared to mid-ocean ridge seamounts, subduction zone seamounts generally have more sodium, alkali, and volatile abundances, and less magnesium, resulting in more explosive, viscous eruptions. All volcanic seamounts follow a particular pattern of growth, activity, subsidence and eventual extinction. The first stage of a seamount's evolution is its early activity, building its flanks and core up from the sea floor. This is followed by a period of intense volcanism, during which the new volcano erupts almost all (e.g. 98%) of its total magmatic volume. The seamount may even grow above sea level to become an oceanic island (for example, the 2009 eruption of Hunga Tonga).
Seamount | Geochemistry and evolution
Hualālai stands at with a prominence of . It is the westernmost of the five major volcanoes which form the island of Hawaii. Being in the post-shield stage of development, Hualālai is overall much rougher in shape and structure than the more youthful Mauna Loa and Kīlauea. Hualālai's structure is denoted by three rift zones: a well-developed one approximately 50° to the northwest, a moderately developed one to the southeast, and a poorly developed one trending northwards about east of the summit. There are over 100 cinder and spatter cones arranged along these rift zones. Hualālai has no summit caldera, although there is a collapse crater about across atop a small lava shield.
Hualālai | Structural features
The large guyot rises over from the eastern flank of the Tonga Trench to a depth of . It features a wide flat top at depth, which tilts gently westwards. A north-northeast trending scarp separates the flat top from another, shallower flat-topped knoll on the eastern side of the flat top which also tilts westwards; the shallowest point of the seamount lies on this knoll. Additional volcanic cones dot the slopes of Capricorn Seamount. At its foot the seamount is wide. The crust underneath the seamount is 75-95 million years old and is thickened, perhaps by the seamount's lava flows.
Capricorn Seamount | Geomorphology and geology
The Western Pacific Ocean features many seamounts with flat tops at depths of below sea level which are known as guyots. They were considered to be remnants of eroded islands before Cretaceous carbonate platforms were discovered on them which resemble present-day atolls although they do not have exactly the same properties. Later similar submarine mountains were identified elsewhere in the Pacific Ocean as well. Menard 1958 suggested that the Western Pacific guyots once were part of an elevated region known as the Darwin Rise, later compared to present day French Polynesia. Many of these seamounts originally formed in the South Pacific, such as in French Polynesia, where a number of recent volcanoes and volcanic chains can be found.
Takuyo-Daisan | Regional
That the mountain is mostly ice covered makes it difficult to determine its composition, the origin of the elongated shape of the volcano and the volcanological relation between the parasitic cinder cones and the main volcanic pile. Its volume may be about . The plateau and parasitic cones are formed by hawaiite and the few outcrops on the main volcano by benmoreite and comendite. They contain phenocrysts of olivine, plagioclase, pyroxene and titanaugite in the former and of clinopyroxene, feldspar and olivine. An age of 9.1 million years ago has been obtained on a basaltic lava flow beneath the volcano, and it has been inferred that the basal plateau formed between 10.1 and 9.1 million years ago.
Toney Mountain | Introduction
In seismic profile, the main body of the seamount rises steeply over in predominantly unsedimented volcanic slope to the thinly sedimented ( to ), from an Emperor point of view, gently domed summit region between to 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.
Nintoku Seamount | Geology and characteristics
Volcanic structures are aligned along the eastern moat of this structure, which is filled by lacustrine deposits and welded ignimbrites. Another eastern collapse was generated by the Tara Ignimbrite eruption, with dimensions of . The margins of the caldera-graben structure are about high while the caldera floors are about lower. Probably dacitic lava domes are found on the northern caldera rim, with the caldera floor possibly containing lava flows.
Cerro Guacha | Geography and structure
Dives in 1983 found extensive low-temperature venting at the northern half of the fissure. The shorter, newer southern rift zone consists of a topographically plunging rift, surrounding by subtle, discontinuous faults. Camera tows along the southern flank reveal that the area is built of delineated sheet flows, small lava ponds, and lava channels.
Axial Seamount | Structure
Seamounts are volcanic mountains which rise from the seafloor. The unlimited supply of water surrounding these volcanoes can cause them to behave differently from volcanoes on land. The lava emitted in eruptions at Bowie Seamount is made of basalt, a common gray to black or dark brown volcanic rock low in silica content (the lava is mafic). When basaltic lava makes contact with the cold sea water, it may cool very rapidly to form pillow lava, through which the hot lava breaks to form another pillow. Pillow lava is typically fine-grained, due to rapid cooling, with a glassy crust, and has radial jointing.
Bowie Seamount | Structure