- Email: [email protected]
SEAMOUNTS
SEAMOUNTS 475
SATURN See SOLAR SYSTEM: Jupiter, Saturn and Their Moons
SEAMOUNTS S M White, University of South Carolina, Columbia, SC, USA ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction Maps of the seafloor reveal a surface that is dotted with a vast number of cones and truncated cones, arranged in lines, grouped in clusters, or isolated. These are known as seamounts, the name given to any steep-sloped more-or-less conical feature on the seafloor. The overwhelming majority of seamounts are volcanoes. In fact, seamounts are the most common volcanic landform on Earth, but one of the least studied, owing to their wide dispersal and relative inaccessibility. As a consequence, much of the information presented here is derived from a small number of well-studied seamounts and applied to the general population of seamounts. Seamounts are officially defined as volcanic cones on the seafloor with at least 1 km of relief. This distinction arose during the days of seafloor mapping with wide-beam echosounders when only these relatively large features could be unambiguously identified. Technological advances in remote sensing have improved our ability to locate and image seamounts of increasingly smaller size. Modern shipboard multibeam echosounders provide high-resolution maps showing the morphology and distribution of small (>50 m high) seamounts. Deep-towed sonars reveal tiny (<20 m high) seamounts along the axes of midocean ridges. Satellite altimetry has enabled complete mapping of the ocean basins, but is capable of imaging only relatively large (>2 km high) seamounts. There is little scientific justification for a 1 km height cut-off for seamounts. All volcanoes start growing from the seafloor, regardless of their current size. For the purpose of this work, seamounts are defined broadly to include submarine volcanic edifices of any size. Seamounts, ocean islands, and guyots form a natural continuum in the process of submarine volcanic construction from submerged, through emergent, to
erosional. This article focuses on the processes that build and erode seamounts and their distribution in the oceans. The quasi-conical shape and volcanic origin of seamounts distinguish them from other relief-forming features on the seafloor, such as abyssal hills and carbonate reefs, which are not dealt with here. One exception is the process of serpentine mud volcanism, which builds seamounts in convergentmargin settings.
Geochemical and Geophysical Characteristics Nearly all seamounts, aside from some in island arcs, are composed of basalt. Geophysical studies also suggest that seamounts have densities, magnetizations, and seismic velocities that are consistent with porous basalt. Most seamounts consist of a base layer of alkali basalt that grades into voluminous edifice-building tholeiitic basalt, which is capped by alkali basalt. Occasionally differentiation products of alkali basalt (hawaiite, mugearite, benmoreite, and trachyte) are observed in the latter stages of growth. However, not all seamounts exhibit all these growth stages, and small seamounts in particular may cease activity before reaching the main tholeiite stage. Smaller seamounts located near mid-ocean ridges have a composition that strongly resembles that of the lava erupted at the mid-ocean ridge but have a tendency to be both more primitive and more depleted than ridge-axis basalt. Seismic, geodetic, and gravity studies have identified magma chambers beneath several ocean islands. Geophysical studies of the Kilauea volcano on Hawaii provide perhaps the clearest picture of a magma system anywhere. These studies define a partly molten conduit that rises to about 5 km beneath the summit caldera, with large blade-like dikes running laterally down the rift zones. Large positive gravity anomalies centred beneath many of the islands of French Polynesia suggest large (100 km3) frozen magma chambers at shallow levels (2–4 km). At Krafla on Iceland, attenuation of seismic shear waves suggests a large
476 SEAMOUNTS
magma body (10–100 km3) at a depth of 3 km below the surface. Small melt bodies that have been detected seismically in the uppermost mantle near the EPR (East Pacific Rise) may be evidence that even very small near-ridge seamounts have ephemeral magma chambers.
Distribution Global Distribution and Spatial Arrangement
The earliest studies of seamount distribution and abundance in the ocean basins relied on estimates of height and volume obtained from single-beam echosounder crossings of seamounts. Since single-beam data provide only a cross-sectional view of the topography, with no guarantee of crossing the summit, approximations were required to estimate true seamount height and shape. Nevertheless, these early studies provided valuable indications of the distribution of seamounts in the ocean basins. Modern multi-beam echosounders have revealed the complexity and variety of seamount shapes and distributions, but still only cover regional areas (a few hundreds of square kilometres). The development of satellite altimetry has allowed the complete mapping of the larger seamounts in the ocean basins
(Figure 1). Studies of seamounts suggest that satellite altimetry reliably detects seamounts that are at least 2 km high. The global distribution of seamounts is best approximated by power-law models. Power-law models predict that there are in the order of 105 seamounts over 1 km high in the ocean basins and in the order of 107 seamounts including seamounts that are much less than 1 km high. Over 50% of all seamounts are found in the Pacific Ocean. Statistical studies of seamount distribution find very clear differences in the distributions of seamounts in different oceans and in different tectonic settings (Table 1). Seamounts are distributed in several spatial patterns. The most widely known pattern is the seamount chain (Figure 2). In seamount chains, several very closely spaced individual volcanoes occur in a line. The Hawaiian–Emperor Seamounts are a classic example of a seamount chain. In this chain, as in many others, the bases of individual seamounts nearly touch or may completely overlap, but each volcano creates an individual peak. When seamounts are even more closely spaced, so that individual seamounts along the chain are difficult to recognize, the arrangement is called a volcanic or aseismic ridge. Aseismic ridges often occur along the track of a hotspot, for example the Ninety
Figure 1 Global distribution of seamounts (indicated by plus symbols scaled according to the size of the seamount edifice) detected by Geosat/ERS-1 altimetry. Approximately 14 675 seamounts ranging in size from approximately 1 km to 7 km were identified. Reproduced by permission of American Geophysical Union from Wessel P (2001) Global distribution of seamounts inferred from gridded Geosat/ERS-1 altimetry. Journal of Geophysical Research 106: 19 431–19 441. Copyright 2001 American Geophysical Union.
SEAMOUNTS 477
Table 1 Seamount abundances in intraplate and mid-ocean ridge settings
Region
Mid-Atlantic Ridge, 24 – 30 N Reykjanes Ridge, 57 – 62 N Mid-Atlantic Ridge, 25 – 27 N Galapagos Ridge, 91 – 97.6 W East Pacific Rise, 8 – 17 N East Pacific Rise, 15.5 – 20 S Easter Seamount Chain Pacific plate Global
Inclusive area (km2)
Lithospheric age (Ma)
Observed seamount abundance (103 km2 )
Minimum height cutoff (m)
6027
0–0.5
79.8
50
5075
0–0.6
106
50
73 000
0–28
18.35
70
Smith and Cann (1992) Magde and Smith (1995) Jaroslow et al. (2000)
14 100
0–0.5
18.4
50
Behn et al. (2003)
200 400
0–2.9
0.9
200
113 000
0–1.9
9.5
50
243 000 Pacific plate
0–9.8 0–169
1.6 Total ¼ 8882
200 1500
All ocean basins
0–180
Total ¼ 14 675
1500
Reference
Schierer and Macdonald (1995) White et al. (1998) Rappaport et al. (1997) Wessel and Lyons (1997) Wessel (2001)
Data based on raw seamount counts and seafloor survey areas selected on the basis of complete seafloor coverage with modern mapping technology in the following references: Behn MD, Sinton JM, and Detrick RS (2003) Effect of the Galapagos Hotspot on seafloor volcanism along the Galapagos Spreading Center (90.9 – 97.6 W). Earth and Planetary Science Letters, vol 217, pp. 331–347 (2004); Jaroslow GE, Smith DK, and Tucholke BE (2000) Record of seamount production and off-axis evolution in the western North Atlantic Ocean, 25 25 – 27 10 N. Journal of Geophysical Research 105: 2737–2760; Magde L and Smith DK (1995) Seamount volcanism at the Reykjanes Ridge: relationship to the Iceland Hotspot. Journal of Geophysical Research 100: 8449–8468; Scheirer DS and Macdonald KC (1995) Near-axis seamounts on the flanks of the East Pacific Rise, 8 – 17 N. Journal of Geophysical Research 100: 7871–7885; Smith DK and Cann JR (1992) The role of seamount volcanism in crustal construction at the Mid-Atlantic Ridge (24 30 N) Journal of Geophysical Research 97: 1645–1658; Rappaport Y, Naar DF, Barton CC, Liu ZJ, and Hey RN (1997) Morphology and distribution of seamounts surrounding Easter Island. Journal of Geophysical Research 102: 24 719–24 728; Wessel P (2001) Global distribution of seamounts inferred from gridded Geosat/ERS-1 altimetry. Journal of Geophysical Research 106: 19 431–19 441; Wessel P and Lyons S (1997) Distribution of large Pacific seamounts from Geosat/ERS-1: implications for the history of intraplate volcanism. Journal of Geophysical Research 102: 22 459–22 475; White SM, Macdonald KC, Scheirer DS, and Cormier M-H (1998) Distribution of isolated volcanoes on the flanks of the East Pacific Rise, 15.3 – 20 S. Journal of Geophysical Research 103: 30 371–30 384.
East Ridge in the Indian Ocean. However, some aseismic ridges seem to emerge from mid-ocean ridges, such as the Sojourn Ridge and the Pukapuka Ridges in the southern Pacific. Seamounts also appear in clusters. Classic examples of the cluster pattern include the Mid-Pacific Mountains and the Hawaiian North Arch volcanic field. Finally, isolated seamounts are fairly common around fast-spreading mid-ocean ridges, where large numbers of very small seamounts (less than 1 km high) occur randomly on the seafloor. Tectonic Setting: Intraplate Seamounts
The largest seamounts and the longest seamount chains known on Earth are intraplate seamounts. In the Atlantic, several intraplate seamount chains can be traced back to the edge of the continent, indicating 80–120 Ma of continuous activity. Probably the most famous example of an intraplate seamount chain, the Hawaiian–Emperor chain, stretches for 5000 km from its source to 80 Ma seamounts at its end (Figure 2). The oldest parts of this chain may have been subducted, so
its true longevity is unknown. The Hawaiian chain also boasts the largest volcano on Earth: Mauna Loa (with an estimated volume of 42 500 km3). What causes this great volume of volcanic activity is still a subject of debate. The most widely accepted hypothesis for the origin of intraplate seamounts is that they are formed above hotspots supplied by mantle plumes (see Mantle Plumes and Hot Spots). The widely accepted model for hotspots states that they are fixed with respect to the Earth’s centre of mass and thus leave a trail of consecutively older seamounts as the plate moves over the stationary hotspot. The trend and age progression of the hotspot seamount chain should record the absolute (relative to the Earth’s centre of mass) plate-motion vector. The strongest evidence supporting the hotspot model is the linear arrangement and age progression along seamount chains. When a hotspot occurs underneath a mid-ocean ridge it can send out seamount chains or ridges on both sides of the plate boundary. The Carnegie and Cocos ridges from
478 SEAMOUNTS
SEAMOUNTS 479
the Galapagos hotspot, and the Walvis Ridge and Rio Grande Rise from the St Helena hotspot are two examples of this dual-ridge phenomenon. However, many intraplate seamount chains fail to show the simple age progression predicted by the hotspot model. Cocos Island on the Cocos Ridge has lavas that are much younger than predicted by a simple age-progression model consistent with a hotspot origin. Likewise, the Line Islands, the Cameroon Line, and the Pratt–Welker chain show no regular age progression. The lack of age progression in many, even most, seamount chains has resulted in the advancement of several hypotheses to the explain them. Recent palaeomagnetic studies suggest that the hotspot source of the Emperor seamount drifted south at a rate of approximately 30 mm a1 before becoming stationary at 43 Ma. Several other seamount chains in the Pacific exhibit changes in direction near 43 Ma (the Gambier–Tuamotus, Gilbert–Marshall, and Austral chains) but the significance of these kinks in the seamount chains is not yet clear. Lack of age progression and hotspot fixity make it unlikely that all intraplate seamounts result from mantle plumes rising from the deep mantle. Other hypotheses for the origin of seamount chains are extensional stress across zones of pre-existing weakness in the lithosphere, feedback between plate flexure and tectonic stress, and small-scale convective rolls in the upper mantle. Tectonic Setting: Mid-Ocean Ridges
Seamounts are most abundant near mid-ocean ridges (see Tectonics: Mid-Ocean Ridges). These seamounts are predominately small; thus, high-resolution bathymetric mapping is necessary to detect them. Most of the studies mapping seamounts near mid-ocean ridges rely on morphometric assumptions that they have an aspect ratio of 2 : 1 or less and that their heights are several times the resolution of the mapping instrument to distinguish them from abyssal hills or other tectonic seafloor topography. The near-ridge population consists mostly of seamounts less than 1 km high, with the overwhelming majority being small isolated seamounts less than 100 m high (Table 1). On slowspreading ridges, such as the Mid-Atlantic Ridge, seamounts occur directly over the spreading axis within the rift valley of the ridge, while on fast-spreading ridges, such as the East Pacific Rise, there is no rift valley and no seamounts along the spreading axis. The
largest on-axis seamount yet discovered (more than 5 km high) is on the ultra-slow spreading Gakkel Ridge, while recent surveys have found tiny (less than 20 m high) pillow-lava mounds as the largest edifices on the axis of the fast-spreading East Pacific Rise. Thus, seamounts over the spreading axis grow larger but become less abundant as the spreading rate decreases. However, small (less than 200 m high) seamounts are ubiquitous away from the axis of fast-spreading ridges and increasingly rare on slowerspreading ridges. Small but ubiquitous seamounts form on lithosphere younger than 0.2 Ma, regardless of spreading rate. This suggests that the thickness of the brittle lithosphere controls the near-ridge seamount population. Many seamount chains originate at mid-ocean ridges. Near-ridge chains are typically shorter and composed of smaller volcanoes than the intraplate seamount chains. Near-ridge chains become more common as the spreading rate increases, with the largest seamount chains being associated with locations where the ridge axis exhibits signs of a high magma budget. For example, the Axial seamount on the Juan de Fuca ridge is a place where excess magma has produced the Cobb–Eikelberg seamount chain. The highest concentration of seamount chains is adjacent to the most inflated and shallowest part of the superfast-spreading southern East Pacific Rise (Figure 3). Rarely seamount chains are found near fracture zones, but these are smaller lower-volume seamounts than those found near the middle of ridge segments. Unlike the hotspot chains discussed above, most nearridge seamount chains are orientated between the direction of absolute plate motion and that of relative plate motion at the ridge. Most of the chains are aligned with the direction of shallow asthenospheric flow beneath the ridge flanks. Tectonic Setting: Island Arcs
Seamounts occur in island arcs as a product of subduction-zone volcanism. Seamounts are distributed in a linear or curved pattern following the curvature of the trench. Tectonic segmentation of the arc by cross-faults creates discontinuities in the seamount distribution, illustrating the strong tectonic control on the distribution of volcanoes in island-arc settings. Seamounts in island arcs are more diverse in both morphology and composition than seamounts in the other two tectonic settings. The Izu–Ogasawara Arc
Figure 2 Bathymetry surrounding the Hawaiian Islands, a prominent intraplate seamount chain stretching across half of the Pacific plate (see inset), reveals several long rift zones and smaller seamounts. Volcanically and hydrothermically active Loihi, to the southeast of Hawaii, is the youngest volcano in the chain. Reproduced with permission from Eakins BW, Robinson JE, Kanamatsu T, et al. (2003) Hawaii’s Volcanoes Revealed. Geological Investigations Series I-2809. US Geological Survey.
480 SEAMOUNTS
of Cretaceous seamounts is almost double that of Cenozoic seamounts.
Morphology
Figure 3 Seamount chains in the Rano Rahi Field adjacent to the southern East Pacific Rise. The shaded-relief bathymetry reveals a remarkable abundance of seamounts near the part of the spreading axis with the greatest inferred magma budget in the region.
contains eight submarine calderas that have erupted large (more than 100 km3) volumes of rhyolite pumice. In contrast, Kick-Em-Jenny, the southernmost island in the Lesser Antilles, is an island-arc seamount that has erupted 10 times since 1939, forming lava domes of basaltic magma within the summit caldera of its very steep-sided edifice. Almost every island arc has a number of seamounts that are smaller versions of the emergent volcanic islands making up the arc. Mud volcanoes are a unique amagmatic type of seamount found in the fore-arc regions of several subduction zones. The widely accepted explanation for mud volcanism is the upwelling of overpressured serpentinite diapirs from the subduction de´ collement through normal faults in the fore arc. One of the largest and best-studied mud volcanoes is the Conical Seamount (25 km in diameter and 2 km high), which is located in the fore arc of the Marianas Arc. As more seafloor mapping is done along outer fore arcs, more examples of mud volcanoes are discovered. Distribution through Time: Cretaceous Seamounts
During the Cretaceous (85–120 Ma) incredibly intense seafloor volcanism occurred throughout what is now the western Pacific (Figure 1). Not only were many of the large oceanic plateaus emplaced in the Pacific during the Cretaceous (Ontong–Java, Manihiki, Shatsky), but also the topography of the seafloor reveals a striking abundance of large seamounts (Mid-Pacific Mountains, Magellan Seamounts, Wake Guyots, etc.). Both the highest density of seamounts per unit area and the highest density of large (more than 3.5 km high) seamounts occur in the Pacific. Satellite altimetry data reveal that the concentration
The vast majority of seamounts are predominately constructed of effusive lava flows. Thus, the overall shapes of seamounts are primarily controlled by the geometry of the magma plumbing system, the eruption rate, the lava type, and the local stresses on the volcano edifice. The cooling effect of water leads to the construction of much steeper edifices than those on land. Most seamounts have steep (10 – 30 ) sides but nearly flat summit areas, in contrast to the low slopes of shield volcanoes (5 – 10 ). As a seamount grows, it is subject to more complex stresses and tends to develop a more complex shape (Figure 4). In map-view, seamount shape evolves from nearly circular small seamounts to the more complex stellate forms of large seamounts and ocean islands. As a seamount grows larger, mass wasting from gravitational instability or wave erosion becomes more important in modifying its morphology. Seamount Growth and Development
Deep-water stage Basaltic magma erupted under high hydrostatic pressure produces non-explosive eruptions forming edifices built from lava flows (Figure 4A). Repeated eruptions build extrusive edifices composed of pillow lavas with minor sheet lava flows. Intrusion of magma into the pelagic sediments on the seafloor produces a mixture of lava and wet sediment called peperite. Small seamounts (less than 1 km high and less than 10 km3 in volume) usually form edifices that can best be described as low lava domes. Many seamounts even of this small size have relatively steep slopes and flat summits, unlike subaerial lava shields. Studies of small isolated seamounts near mid-ocean ridges show that a few seamounts of this size develop complex shapes, including relatively large summit craters. The seamount shape becomes generally more complex as the seamount grows. When seamounts reach a somewhat larger size (ca. 1 km high and 10–100 km3 in volume) they more often have an irregular outline and an ‘overturned soup bowl’ or truncated-cone shape. Shoaling stage Large seamounts (more than 2 km high and with volumes of over 200 km3) may develop a stellate shape as a result of long rift zones concentrating lateral eruptions of lava from a central feeding system (Figure 4B). Not all large seamounts develop rift zones (e.g. Galapagos), but the rift zones on some seamounts become very pronounced ridges extending for more than 100 km (e.g. Hawaii). The presence of a magma chamber above the level of the seafloor and
SEAMOUNTS 481
Figure 4 The growth stages of seamounts from (A) early deep-water stage, through (B) shoaling stage, showing the development of rift zones, and (C) ocean island stage, including the development of a subaerial shield volcano and mass wasting, to (D) an atoll in the guyot stage.
within the edifice itself aids the development of rift zones on large seamounts by enabling lateral dikes and flank eruptions in focused zones of weakness. Within about 1 km of sea-level, reduced hydrostatic pressure allows exsolution of gas, and explosive eruptions become frequent. Lava flows of scoriaceous pillows and lapilli breccia become important lava types. Emergent stage At sea-level, eruptions become surtseyan in type, producing fine-grained base surge and air-fall tephra deposits. Besides the famous and welldescribed eruption of Surtsey off Iceland (1963– 1967), the only recorded observations during the emergent stage are from Graham Island (1831), Metis Shoal (1968, 1979), Myujin-sho (1952–1953), and Shin-Iwo-Jima (1986). Phreatomagmatic eruptions form tuff rings and cones consisting of easily eroded deposits of loose lapilli and ash. If eruptions stop at this stage, wave action will quickly erode the seamount. The small islands of Myujin-sho disappeared within months of formation. Ocean island stage Once the volcanic vent rises above sea-level, phreatomagmatic eruptions become rare and effusive eruptions resume (Figure 4C). Pahoehoe and aa are the important lava types, and in most cases a lava shield is created from numerous low-relief lava flows. Giant landslides may cover the flanks of the island with debris from slumping or debris avalanches. Some of the largest landslides on Earth (involving volumes of 5000 km3) have occurred around the Hawaiian Islands (Figure 2), and some
studies speculate that up to 50% of the original volume of the older Hawaiian Islands has been lost owing to mass wasting. Guyot stage Thermal subsidence and erosion combine to drown all islands after volcanic activity ceases. Wave erosion planes off a flat summit plateau, but the sides of the volcano remain steep, creating the distinctive guyot cross-section. Atolls (including the Marshall, Tuamotu, Kiribati, and Maldive Islands) are a type of guyot found in tropical oceans, where coral forms a fringing reef around an ocean island. If the upward growth of the coral reef keeps pace with the subsidence of the seamount, a ring of coral islands form, encircling a lagoon, and a thick layer of carbonate sediment develops, which caps the old seamount basement rocks (Figure 4D). Why Seamounts Have Flat Tops
The characteristic steep-sided flat-topped shape of seamounts is a morphology rarely seen in subaerial volcanoes. The flat tops of guyots can be explained by wave erosion, but many seamounts with flat tops have never been near the wave base. Several models have been advanced to explain why so many seamounts have a summit plateau or flat-topped shape. It is still unclear whether one model can explain all the occurrences of flat-topped volcanoes, or whether each of various models is appropriate under certain conditions. In the ring-dike model proposed by Batiza and Vanko, eruptions occur from a series of circumferential dikes encircling the summit area rather than from
482 SEAMOUNTS
a central feeder vent. The steep slopes and flat summits of the Galapagos volcanoes, which essentially have the same form as flat-topped seamounts, are attributed to lateral extension by ring-dike intrusion. Circumferential fractures observed in the summit areas of some near-ridge seamounts support the ring-dike model. Ring dikes are thought to form when magma stalls within the crust, creating a small laccolith. Ring dikes shoot upwards from the edge of the laccolith as it inflates. A variation on this model proposes dipping cone sheets instead of more vertical ring dikes as the intrusive body. In the caldera-fill model proposed by Clague and others, large calderas form early in seamount growth and are filled by later eruptions. A repeated sequence of caldera collapse and re-filling allows the seamount to grow upwards and outwards while maintaining a flat top. This model is based on new high-resolution mapping that suggests that lava shields (erupted from central vents) formed on the summits of flat-topped volcanoes in the north-eastern Pacific. In the neutral-buoyancy model proposed by Barone and Ryan, low-density basaltic crust counterbalances the buoyancy of the magma so that eruptions will rise only to a limited height. Seamounts will grow only to the height that balances the magma pressure in their magma chambers and will then grow laterally instead of upwards, forming a flat top. Other studies, however, have questioned why this model would lead to circular volcano outlines and what continues to drive eruptions once the critical height is reached.
Broader Effects of Seamounts
seamount chains and aseismic ridges coincides with a reduced dip angle in the subducting plate, for example in the case of the Juan Fernandez chain and the flat slab in northern Chile. Hydrothermal Circulation
Active seamounts may host high-temperature focused hydrothermal flow. The Loihi seamount in the Hawaiian chain hosts high-temperature hydrothermal vents within its summit caldera, which are very similar to vents found at mid-ocean ridges (Figure 2). Most seamounts have neither the magma supply volume nor the longevity of Loihi, but sulphide hydrothermal deposits have been found on the summits of several other seamounts during submersible dives. These hydrothermal deposits indicate that seamount and mid-ocean ridge hydrothermal systems are broadly similar and contribute the same ions to seawater. Possibly more significant is the role that inactive seamounts play as basement outcrops that provide easy paths for the escape of fluid and heat. Oceanographic Circulation
The topography created by seamounts has a large effect on the local water masses, primarily generating upwelling and enhancing currents. Anticyclonic circulation over the tops of seamounts creates a socalled ‘cold dome’ as a result of enhanced upwelling along the sides of the edifice. Also, long aseismic ridges and seamount chains are barriers to deep- and midwater circulation. Aseismic ridges standing 2–4 km above the seafloor and stretching for hundreds of kilometres across the Pacific deflect north–south currents at depth.
Subduction Asperities
Seamounts being subducted erode the fore arc and may also cause erosion at the base of the overriding plate. Huge furrows, mass wasting, and structural disruption of the fore arc are the effects attributed to seamount subduction (Figure 5). There are many areas that exhibit the morphological signs of seamount subduction, but proving the existence of buried seamounts depends on interpreting seismic and magnetic data. However, seamounts being subducted are sometimes directly visible, for example Daiichi-Kashima Guyot in the Japan Trench and the Bannock (Seamount) Structure in the Mediterranean. Seamount subduction may also be responsible for seismic asperities on the subduction thrust-fault interface. Seamount tunnelling, the process by which a seamount going down on the subducting plate scrapes material off the overriding plate, has been inferred to have an important role in crustal development and fluid flow in the margin. The subduction of large
Critical Habitat
Seamounts play a key role in ocean environments by providing habitats for fishes and suspension feeders. The enhanced upwelling combined with the potential for a reef environment in very shallow seamounts make large seamounts sites of high primary productivity. Seamounts have been known as productive fishing grounds for centuries, but their role in oceanic biodiversity has been appreciated only in the last 50 years. Seamounts host a relatively large percentage (estimated at 15–35%) of endemic species and may be important sites of speciation for deep-sea fauna. Seamounts in the south-west Pacific show highly localized species distributions and fairly limited recruitment among seamounts, even between closely spaced seamounts. The formation of atolls by coral growth on subsiding seamounts creates an important habitat niche for filter feeders and reef fishes. Finally, many humans find ocean islands and
SEAMOUNTS 483
Figure 5 Seamount subduction has produced huge furrows and re-entrants in the fore-arc margin of Costa Rica. Shaded-relief bathymetry reproduced from Dominguez S, Lallemand SE, Malavieille J, and von Huene R (1998) Upper plate deformation associated with seamount subduction. Tectonophysics 293: 207–224, copyright 1998, with permission from Elsevier.
484 SEAMOUNTS
atolls highly desirable locations for settlement and vacation.
Glossary Abyssal hill A low-relief elongate hill formed by horst-and-graben style faulting around a mid-ocean ridge. Alkali basalt A variety of basalt that has relatively high sodium and potassium contents. Atoll A ring of carbonate reef islands enclosing a lagoon built atop a subsiding seamount in tropical latitudes. Pillow lava A type of subaqueous lava flow consisting of spherical or elongate tubes formed by the rapid quenching of lava. Tubes seen in cross-section often resemble pillows, with a flat base and a convex upper surface. Scoria Glassy porous pyroclastic lava. Surtseyan A type of volcanic eruption, based on observations at Surtsey in Iceland, which is the phreatomagmatic equivalent of a Hawaiian or quiescent lava flow eruption. Tholeiitic basalt A variety of basalt characterized by the presence of orthopyroxene in addition to clinopyroxene and calcium-plagioclase.
See Also History of Geology Since 1962. Large Igneous Provinces. Lava. Mantle Plumes and Hot Spots. Sedimentary Environments: Reefs (‘Build-Ups’). Sedimentary Processes: Particle-Driven Subaqueous Gravity Processes. Tectonics: Hydrothermal Activity; Mid-Ocean Ridges. Volcanoes.
Further Reading Barone AM and Ryan WBF (1990) Single plume model for asynchronous formation of Lamont Seamounts and adjacent Pacific Rise terrains. Journal of Geophysical Research 95 N. B7: 10801–10827. Batiza R (1989) Seamounts and seamount chains of the eastern Pacific. In: Winterer EL, Hussong DM, and
Decker RW (eds.) The Geology of North America, Volume N, The Eastern Pacific Ocean and Hawaii, pp. 289–306. Boulder: Geological Society of America. Batiza R and Vanko D (1984) Volcanic development of small oceanic central volcanoes on the flanks of the East Pacific Rise inferred from narrow beam echo-sounder surveys. Marine Geology 54: 53–90. Clague DA, Reynolds JR, and Davis AS (2000) Nearridge seamount chains in the north-eastern Pacific Ocean. Journal of Geophysical Research 105: 16 541–16 561. Davis EE (2000) Earth science: volcanic action at Axial Seamount. Nature 403: 379–480. Johnson HP and Embley RW (eds.) (1990) Axial Seamount: an active ridge axis volcano on the central Juan de Fuca Ridge. Journal of Geophysical Research 95: 12 689–12 966. Keating BH and McGuire WJ (2000) Island edifice failures and associated tsunami hazards. Pure and Applied Geophysics 157: 899–955. Keating BH, Fryer P, Batiza R, and Boehlert W (eds.) (1987) Seamounts, Islands, and Atolls. Geophyiscal Monograph Series Volume 43. Washington DC: American Geophysical Union. Mitchell NC (2001) Transition from circular to stellate forms of submarine volcanoes. Journal of Geophysical Research 106: 1987–2004. Moore JG, Normark WR, and Holcomb RT (1994) Giant Hawaiian landslides. Annual Review of Earth and Planetary Sciences 22: 119–144. Ranero CR and Von Huene R (2000) Subduction erosion along the Middle America convergent margin. Nature 404: 748–752. Schmidt R and Schminke H-U (2000) Seamounts and island building. In: Sigurdsson H, Houghton B, McNutt SR, Rymer H, and Stix J (eds.) Encyclopedia of Volcanoes, pp. 383–402. San Diego: Academic Press. Takahashi E, Lipman PW, Garcia MO, Naka J, and Aramaki S (eds.) (2002) Hawaiian Volcanoes: Deep Underwater Perspectives. Geophysical Monograph Series Volume 128. Washington DC: American Geophysical Union. Thorarinsson S (1967) Surtsey: The New Island in the North Atlantic. New York: Viking. Wessel P (2001) Global distribution of seamounts inferred from gridded Geosat/ERS-1 altimetry. Journal of Geophysical Research 106: 19 431–19 441.
SATURN See SOLAR SYSTEM: Jupiter, Saturn and Their Moons
SEAMOUNTS S M White, University of South Carolina, Columbia, SC, USA ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction Maps of the seafloor reveal a surface that is dotted with a vast number of cones and truncated cones, arranged in lines, grouped in clusters, or isolated. These are known as seamounts, the name given to any steep-sloped more-or-less conical feature on the seafloor. The overwhelming majority of seamounts are volcanoes. In fact, seamounts are the most common volcanic landform on Earth, but one of the least studied, owing to their wide dispersal and relative inaccessibility. As a consequence, much of the information presented here is derived from a small number of well-studied seamounts and applied to the general population of seamounts. Seamounts are officially defined as volcanic cones on the seafloor with at least 1 km of relief. This distinction arose during the days of seafloor mapping with wide-beam echosounders when only these relatively large features could be unambiguously identified. Technological advances in remote sensing have improved our ability to locate and image seamounts of increasingly smaller size. Modern shipboard multibeam echosounders provide high-resolution maps showing the morphology and distribution of small (>50 m high) seamounts. Deep-towed sonars reveal tiny (<20 m high) seamounts along the axes of midocean ridges. Satellite altimetry has enabled complete mapping of the ocean basins, but is capable of imaging only relatively large (>2 km high) seamounts. There is little scientific justification for a 1 km height cut-off for seamounts. All volcanoes start growing from the seafloor, regardless of their current size. For the purpose of this work, seamounts are defined broadly to include submarine volcanic edifices of any size. Seamounts, ocean islands, and guyots form a natural continuum in the process of submarine volcanic construction from submerged, through emergent, to
erosional. This article focuses on the processes that build and erode seamounts and their distribution in the oceans. The quasi-conical shape and volcanic origin of seamounts distinguish them from other relief-forming features on the seafloor, such as abyssal hills and carbonate reefs, which are not dealt with here. One exception is the process of serpentine mud volcanism, which builds seamounts in convergentmargin settings.
Geochemical and Geophysical Characteristics Nearly all seamounts, aside from some in island arcs, are composed of basalt. Geophysical studies also suggest that seamounts have densities, magnetizations, and seismic velocities that are consistent with porous basalt. Most seamounts consist of a base layer of alkali basalt that grades into voluminous edifice-building tholeiitic basalt, which is capped by alkali basalt. Occasionally differentiation products of alkali basalt (hawaiite, mugearite, benmoreite, and trachyte) are observed in the latter stages of growth. However, not all seamounts exhibit all these growth stages, and small seamounts in particular may cease activity before reaching the main tholeiite stage. Smaller seamounts located near mid-ocean ridges have a composition that strongly resembles that of the lava erupted at the mid-ocean ridge but have a tendency to be both more primitive and more depleted than ridge-axis basalt. Seismic, geodetic, and gravity studies have identified magma chambers beneath several ocean islands. Geophysical studies of the Kilauea volcano on Hawaii provide perhaps the clearest picture of a magma system anywhere. These studies define a partly molten conduit that rises to about 5 km beneath the summit caldera, with large blade-like dikes running laterally down the rift zones. Large positive gravity anomalies centred beneath many of the islands of French Polynesia suggest large (100 km3) frozen magma chambers at shallow levels (2–4 km). At Krafla on Iceland, attenuation of seismic shear waves suggests a large
476 SEAMOUNTS
magma body (10–100 km3) at a depth of 3 km below the surface. Small melt bodies that have been detected seismically in the uppermost mantle near the EPR (East Pacific Rise) may be evidence that even very small near-ridge seamounts have ephemeral magma chambers.
Distribution Global Distribution and Spatial Arrangement
The earliest studies of seamount distribution and abundance in the ocean basins relied on estimates of height and volume obtained from single-beam echosounder crossings of seamounts. Since single-beam data provide only a cross-sectional view of the topography, with no guarantee of crossing the summit, approximations were required to estimate true seamount height and shape. Nevertheless, these early studies provided valuable indications of the distribution of seamounts in the ocean basins. Modern multi-beam echosounders have revealed the complexity and variety of seamount shapes and distributions, but still only cover regional areas (a few hundreds of square kilometres). The development of satellite altimetry has allowed the complete mapping of the larger seamounts in the ocean basins
(Figure 1). Studies of seamounts suggest that satellite altimetry reliably detects seamounts that are at least 2 km high. The global distribution of seamounts is best approximated by power-law models. Power-law models predict that there are in the order of 105 seamounts over 1 km high in the ocean basins and in the order of 107 seamounts including seamounts that are much less than 1 km high. Over 50% of all seamounts are found in the Pacific Ocean. Statistical studies of seamount distribution find very clear differences in the distributions of seamounts in different oceans and in different tectonic settings (Table 1). Seamounts are distributed in several spatial patterns. The most widely known pattern is the seamount chain (Figure 2). In seamount chains, several very closely spaced individual volcanoes occur in a line. The Hawaiian–Emperor Seamounts are a classic example of a seamount chain. In this chain, as in many others, the bases of individual seamounts nearly touch or may completely overlap, but each volcano creates an individual peak. When seamounts are even more closely spaced, so that individual seamounts along the chain are difficult to recognize, the arrangement is called a volcanic or aseismic ridge. Aseismic ridges often occur along the track of a hotspot, for example the Ninety
Figure 1 Global distribution of seamounts (indicated by plus symbols scaled according to the size of the seamount edifice) detected by Geosat/ERS-1 altimetry. Approximately 14 675 seamounts ranging in size from approximately 1 km to 7 km were identified. Reproduced by permission of American Geophysical Union from Wessel P (2001) Global distribution of seamounts inferred from gridded Geosat/ERS-1 altimetry. Journal of Geophysical Research 106: 19 431–19 441. Copyright 2001 American Geophysical Union.
SEAMOUNTS 477
Table 1 Seamount abundances in intraplate and mid-ocean ridge settings
Region
Mid-Atlantic Ridge, 24 – 30 N Reykjanes Ridge, 57 – 62 N Mid-Atlantic Ridge, 25 – 27 N Galapagos Ridge, 91 – 97.6 W East Pacific Rise, 8 – 17 N East Pacific Rise, 15.5 – 20 S Easter Seamount Chain Pacific plate Global
Inclusive area (km2)
Lithospheric age (Ma)
Observed seamount abundance (103 km2 )
Minimum height cutoff (m)
6027
0–0.5
79.8
50
5075
0–0.6
106
50
73 000
0–28
18.35
70
Smith and Cann (1992) Magde and Smith (1995) Jaroslow et al. (2000)
14 100
0–0.5
18.4
50
Behn et al. (2003)
200 400
0–2.9
0.9
200
113 000
0–1.9
9.5
50
243 000 Pacific plate
0–9.8 0–169
1.6 Total ¼ 8882
200 1500
All ocean basins
0–180
Total ¼ 14 675
1500
Reference
Schierer and Macdonald (1995) White et al. (1998) Rappaport et al. (1997) Wessel and Lyons (1997) Wessel (2001)
Data based on raw seamount counts and seafloor survey areas selected on the basis of complete seafloor coverage with modern mapping technology in the following references: Behn MD, Sinton JM, and Detrick RS (2003) Effect of the Galapagos Hotspot on seafloor volcanism along the Galapagos Spreading Center (90.9 – 97.6 W). Earth and Planetary Science Letters, vol 217, pp. 331–347 (2004); Jaroslow GE, Smith DK, and Tucholke BE (2000) Record of seamount production and off-axis evolution in the western North Atlantic Ocean, 25 25 – 27 10 N. Journal of Geophysical Research 105: 2737–2760; Magde L and Smith DK (1995) Seamount volcanism at the Reykjanes Ridge: relationship to the Iceland Hotspot. Journal of Geophysical Research 100: 8449–8468; Scheirer DS and Macdonald KC (1995) Near-axis seamounts on the flanks of the East Pacific Rise, 8 – 17 N. Journal of Geophysical Research 100: 7871–7885; Smith DK and Cann JR (1992) The role of seamount volcanism in crustal construction at the Mid-Atlantic Ridge (24 30 N) Journal of Geophysical Research 97: 1645–1658; Rappaport Y, Naar DF, Barton CC, Liu ZJ, and Hey RN (1997) Morphology and distribution of seamounts surrounding Easter Island. Journal of Geophysical Research 102: 24 719–24 728; Wessel P (2001) Global distribution of seamounts inferred from gridded Geosat/ERS-1 altimetry. Journal of Geophysical Research 106: 19 431–19 441; Wessel P and Lyons S (1997) Distribution of large Pacific seamounts from Geosat/ERS-1: implications for the history of intraplate volcanism. Journal of Geophysical Research 102: 22 459–22 475; White SM, Macdonald KC, Scheirer DS, and Cormier M-H (1998) Distribution of isolated volcanoes on the flanks of the East Pacific Rise, 15.3 – 20 S. Journal of Geophysical Research 103: 30 371–30 384.
East Ridge in the Indian Ocean. However, some aseismic ridges seem to emerge from mid-ocean ridges, such as the Sojourn Ridge and the Pukapuka Ridges in the southern Pacific. Seamounts also appear in clusters. Classic examples of the cluster pattern include the Mid-Pacific Mountains and the Hawaiian North Arch volcanic field. Finally, isolated seamounts are fairly common around fast-spreading mid-ocean ridges, where large numbers of very small seamounts (less than 1 km high) occur randomly on the seafloor. Tectonic Setting: Intraplate Seamounts
The largest seamounts and the longest seamount chains known on Earth are intraplate seamounts. In the Atlantic, several intraplate seamount chains can be traced back to the edge of the continent, indicating 80–120 Ma of continuous activity. Probably the most famous example of an intraplate seamount chain, the Hawaiian–Emperor chain, stretches for 5000 km from its source to 80 Ma seamounts at its end (Figure 2). The oldest parts of this chain may have been subducted, so
its true longevity is unknown. The Hawaiian chain also boasts the largest volcano on Earth: Mauna Loa (with an estimated volume of 42 500 km3). What causes this great volume of volcanic activity is still a subject of debate. The most widely accepted hypothesis for the origin of intraplate seamounts is that they are formed above hotspots supplied by mantle plumes (see Mantle Plumes and Hot Spots). The widely accepted model for hotspots states that they are fixed with respect to the Earth’s centre of mass and thus leave a trail of consecutively older seamounts as the plate moves over the stationary hotspot. The trend and age progression of the hotspot seamount chain should record the absolute (relative to the Earth’s centre of mass) plate-motion vector. The strongest evidence supporting the hotspot model is the linear arrangement and age progression along seamount chains. When a hotspot occurs underneath a mid-ocean ridge it can send out seamount chains or ridges on both sides of the plate boundary. The Carnegie and Cocos ridges from
478 SEAMOUNTS
SEAMOUNTS 479
the Galapagos hotspot, and the Walvis Ridge and Rio Grande Rise from the St Helena hotspot are two examples of this dual-ridge phenomenon. However, many intraplate seamount chains fail to show the simple age progression predicted by the hotspot model. Cocos Island on the Cocos Ridge has lavas that are much younger than predicted by a simple age-progression model consistent with a hotspot origin. Likewise, the Line Islands, the Cameroon Line, and the Pratt–Welker chain show no regular age progression. The lack of age progression in many, even most, seamount chains has resulted in the advancement of several hypotheses to the explain them. Recent palaeomagnetic studies suggest that the hotspot source of the Emperor seamount drifted south at a rate of approximately 30 mm a1 before becoming stationary at 43 Ma. Several other seamount chains in the Pacific exhibit changes in direction near 43 Ma (the Gambier–Tuamotus, Gilbert–Marshall, and Austral chains) but the significance of these kinks in the seamount chains is not yet clear. Lack of age progression and hotspot fixity make it unlikely that all intraplate seamounts result from mantle plumes rising from the deep mantle. Other hypotheses for the origin of seamount chains are extensional stress across zones of pre-existing weakness in the lithosphere, feedback between plate flexure and tectonic stress, and small-scale convective rolls in the upper mantle. Tectonic Setting: Mid-Ocean Ridges
Seamounts are most abundant near mid-ocean ridges (see Tectonics: Mid-Ocean Ridges). These seamounts are predominately small; thus, high-resolution bathymetric mapping is necessary to detect them. Most of the studies mapping seamounts near mid-ocean ridges rely on morphometric assumptions that they have an aspect ratio of 2 : 1 or less and that their heights are several times the resolution of the mapping instrument to distinguish them from abyssal hills or other tectonic seafloor topography. The near-ridge population consists mostly of seamounts less than 1 km high, with the overwhelming majority being small isolated seamounts less than 100 m high (Table 1). On slowspreading ridges, such as the Mid-Atlantic Ridge, seamounts occur directly over the spreading axis within the rift valley of the ridge, while on fast-spreading ridges, such as the East Pacific Rise, there is no rift valley and no seamounts along the spreading axis. The
largest on-axis seamount yet discovered (more than 5 km high) is on the ultra-slow spreading Gakkel Ridge, while recent surveys have found tiny (less than 20 m high) pillow-lava mounds as the largest edifices on the axis of the fast-spreading East Pacific Rise. Thus, seamounts over the spreading axis grow larger but become less abundant as the spreading rate decreases. However, small (less than 200 m high) seamounts are ubiquitous away from the axis of fast-spreading ridges and increasingly rare on slowerspreading ridges. Small but ubiquitous seamounts form on lithosphere younger than 0.2 Ma, regardless of spreading rate. This suggests that the thickness of the brittle lithosphere controls the near-ridge seamount population. Many seamount chains originate at mid-ocean ridges. Near-ridge chains are typically shorter and composed of smaller volcanoes than the intraplate seamount chains. Near-ridge chains become more common as the spreading rate increases, with the largest seamount chains being associated with locations where the ridge axis exhibits signs of a high magma budget. For example, the Axial seamount on the Juan de Fuca ridge is a place where excess magma has produced the Cobb–Eikelberg seamount chain. The highest concentration of seamount chains is adjacent to the most inflated and shallowest part of the superfast-spreading southern East Pacific Rise (Figure 3). Rarely seamount chains are found near fracture zones, but these are smaller lower-volume seamounts than those found near the middle of ridge segments. Unlike the hotspot chains discussed above, most nearridge seamount chains are orientated between the direction of absolute plate motion and that of relative plate motion at the ridge. Most of the chains are aligned with the direction of shallow asthenospheric flow beneath the ridge flanks. Tectonic Setting: Island Arcs
Seamounts occur in island arcs as a product of subduction-zone volcanism. Seamounts are distributed in a linear or curved pattern following the curvature of the trench. Tectonic segmentation of the arc by cross-faults creates discontinuities in the seamount distribution, illustrating the strong tectonic control on the distribution of volcanoes in island-arc settings. Seamounts in island arcs are more diverse in both morphology and composition than seamounts in the other two tectonic settings. The Izu–Ogasawara Arc
Figure 2 Bathymetry surrounding the Hawaiian Islands, a prominent intraplate seamount chain stretching across half of the Pacific plate (see inset), reveals several long rift zones and smaller seamounts. Volcanically and hydrothermically active Loihi, to the southeast of Hawaii, is the youngest volcano in the chain. Reproduced with permission from Eakins BW, Robinson JE, Kanamatsu T, et al. (2003) Hawaii’s Volcanoes Revealed. Geological Investigations Series I-2809. US Geological Survey.
480 SEAMOUNTS
of Cretaceous seamounts is almost double that of Cenozoic seamounts.
Morphology
Figure 3 Seamount chains in the Rano Rahi Field adjacent to the southern East Pacific Rise. The shaded-relief bathymetry reveals a remarkable abundance of seamounts near the part of the spreading axis with the greatest inferred magma budget in the region.
contains eight submarine calderas that have erupted large (more than 100 km3) volumes of rhyolite pumice. In contrast, Kick-Em-Jenny, the southernmost island in the Lesser Antilles, is an island-arc seamount that has erupted 10 times since 1939, forming lava domes of basaltic magma within the summit caldera of its very steep-sided edifice. Almost every island arc has a number of seamounts that are smaller versions of the emergent volcanic islands making up the arc. Mud volcanoes are a unique amagmatic type of seamount found in the fore-arc regions of several subduction zones. The widely accepted explanation for mud volcanism is the upwelling of overpressured serpentinite diapirs from the subduction de´ collement through normal faults in the fore arc. One of the largest and best-studied mud volcanoes is the Conical Seamount (25 km in diameter and 2 km high), which is located in the fore arc of the Marianas Arc. As more seafloor mapping is done along outer fore arcs, more examples of mud volcanoes are discovered. Distribution through Time: Cretaceous Seamounts
During the Cretaceous (85–120 Ma) incredibly intense seafloor volcanism occurred throughout what is now the western Pacific (Figure 1). Not only were many of the large oceanic plateaus emplaced in the Pacific during the Cretaceous (Ontong–Java, Manihiki, Shatsky), but also the topography of the seafloor reveals a striking abundance of large seamounts (Mid-Pacific Mountains, Magellan Seamounts, Wake Guyots, etc.). Both the highest density of seamounts per unit area and the highest density of large (more than 3.5 km high) seamounts occur in the Pacific. Satellite altimetry data reveal that the concentration
The vast majority of seamounts are predominately constructed of effusive lava flows. Thus, the overall shapes of seamounts are primarily controlled by the geometry of the magma plumbing system, the eruption rate, the lava type, and the local stresses on the volcano edifice. The cooling effect of water leads to the construction of much steeper edifices than those on land. Most seamounts have steep (10 – 30 ) sides but nearly flat summit areas, in contrast to the low slopes of shield volcanoes (5 – 10 ). As a seamount grows, it is subject to more complex stresses and tends to develop a more complex shape (Figure 4). In map-view, seamount shape evolves from nearly circular small seamounts to the more complex stellate forms of large seamounts and ocean islands. As a seamount grows larger, mass wasting from gravitational instability or wave erosion becomes more important in modifying its morphology. Seamount Growth and Development
Deep-water stage Basaltic magma erupted under high hydrostatic pressure produces non-explosive eruptions forming edifices built from lava flows (Figure 4A). Repeated eruptions build extrusive edifices composed of pillow lavas with minor sheet lava flows. Intrusion of magma into the pelagic sediments on the seafloor produces a mixture of lava and wet sediment called peperite. Small seamounts (less than 1 km high and less than 10 km3 in volume) usually form edifices that can best be described as low lava domes. Many seamounts even of this small size have relatively steep slopes and flat summits, unlike subaerial lava shields. Studies of small isolated seamounts near mid-ocean ridges show that a few seamounts of this size develop complex shapes, including relatively large summit craters. The seamount shape becomes generally more complex as the seamount grows. When seamounts reach a somewhat larger size (ca. 1 km high and 10–100 km3 in volume) they more often have an irregular outline and an ‘overturned soup bowl’ or truncated-cone shape. Shoaling stage Large seamounts (more than 2 km high and with volumes of over 200 km3) may develop a stellate shape as a result of long rift zones concentrating lateral eruptions of lava from a central feeding system (Figure 4B). Not all large seamounts develop rift zones (e.g. Galapagos), but the rift zones on some seamounts become very pronounced ridges extending for more than 100 km (e.g. Hawaii). The presence of a magma chamber above the level of the seafloor and
SEAMOUNTS 481
Figure 4 The growth stages of seamounts from (A) early deep-water stage, through (B) shoaling stage, showing the development of rift zones, and (C) ocean island stage, including the development of a subaerial shield volcano and mass wasting, to (D) an atoll in the guyot stage.
within the edifice itself aids the development of rift zones on large seamounts by enabling lateral dikes and flank eruptions in focused zones of weakness. Within about 1 km of sea-level, reduced hydrostatic pressure allows exsolution of gas, and explosive eruptions become frequent. Lava flows of scoriaceous pillows and lapilli breccia become important lava types. Emergent stage At sea-level, eruptions become surtseyan in type, producing fine-grained base surge and air-fall tephra deposits. Besides the famous and welldescribed eruption of Surtsey off Iceland (1963– 1967), the only recorded observations during the emergent stage are from Graham Island (1831), Metis Shoal (1968, 1979), Myujin-sho (1952–1953), and Shin-Iwo-Jima (1986). Phreatomagmatic eruptions form tuff rings and cones consisting of easily eroded deposits of loose lapilli and ash. If eruptions stop at this stage, wave action will quickly erode the seamount. The small islands of Myujin-sho disappeared within months of formation. Ocean island stage Once the volcanic vent rises above sea-level, phreatomagmatic eruptions become rare and effusive eruptions resume (Figure 4C). Pahoehoe and aa are the important lava types, and in most cases a lava shield is created from numerous low-relief lava flows. Giant landslides may cover the flanks of the island with debris from slumping or debris avalanches. Some of the largest landslides on Earth (involving volumes of 5000 km3) have occurred around the Hawaiian Islands (Figure 2), and some
studies speculate that up to 50% of the original volume of the older Hawaiian Islands has been lost owing to mass wasting. Guyot stage Thermal subsidence and erosion combine to drown all islands after volcanic activity ceases. Wave erosion planes off a flat summit plateau, but the sides of the volcano remain steep, creating the distinctive guyot cross-section. Atolls (including the Marshall, Tuamotu, Kiribati, and Maldive Islands) are a type of guyot found in tropical oceans, where coral forms a fringing reef around an ocean island. If the upward growth of the coral reef keeps pace with the subsidence of the seamount, a ring of coral islands form, encircling a lagoon, and a thick layer of carbonate sediment develops, which caps the old seamount basement rocks (Figure 4D). Why Seamounts Have Flat Tops
The characteristic steep-sided flat-topped shape of seamounts is a morphology rarely seen in subaerial volcanoes. The flat tops of guyots can be explained by wave erosion, but many seamounts with flat tops have never been near the wave base. Several models have been advanced to explain why so many seamounts have a summit plateau or flat-topped shape. It is still unclear whether one model can explain all the occurrences of flat-topped volcanoes, or whether each of various models is appropriate under certain conditions. In the ring-dike model proposed by Batiza and Vanko, eruptions occur from a series of circumferential dikes encircling the summit area rather than from
482 SEAMOUNTS
a central feeder vent. The steep slopes and flat summits of the Galapagos volcanoes, which essentially have the same form as flat-topped seamounts, are attributed to lateral extension by ring-dike intrusion. Circumferential fractures observed in the summit areas of some near-ridge seamounts support the ring-dike model. Ring dikes are thought to form when magma stalls within the crust, creating a small laccolith. Ring dikes shoot upwards from the edge of the laccolith as it inflates. A variation on this model proposes dipping cone sheets instead of more vertical ring dikes as the intrusive body. In the caldera-fill model proposed by Clague and others, large calderas form early in seamount growth and are filled by later eruptions. A repeated sequence of caldera collapse and re-filling allows the seamount to grow upwards and outwards while maintaining a flat top. This model is based on new high-resolution mapping that suggests that lava shields (erupted from central vents) formed on the summits of flat-topped volcanoes in the north-eastern Pacific. In the neutral-buoyancy model proposed by Barone and Ryan, low-density basaltic crust counterbalances the buoyancy of the magma so that eruptions will rise only to a limited height. Seamounts will grow only to the height that balances the magma pressure in their magma chambers and will then grow laterally instead of upwards, forming a flat top. Other studies, however, have questioned why this model would lead to circular volcano outlines and what continues to drive eruptions once the critical height is reached.
Broader Effects of Seamounts
seamount chains and aseismic ridges coincides with a reduced dip angle in the subducting plate, for example in the case of the Juan Fernandez chain and the flat slab in northern Chile. Hydrothermal Circulation
Active seamounts may host high-temperature focused hydrothermal flow. The Loihi seamount in the Hawaiian chain hosts high-temperature hydrothermal vents within its summit caldera, which are very similar to vents found at mid-ocean ridges (Figure 2). Most seamounts have neither the magma supply volume nor the longevity of Loihi, but sulphide hydrothermal deposits have been found on the summits of several other seamounts during submersible dives. These hydrothermal deposits indicate that seamount and mid-ocean ridge hydrothermal systems are broadly similar and contribute the same ions to seawater. Possibly more significant is the role that inactive seamounts play as basement outcrops that provide easy paths for the escape of fluid and heat. Oceanographic Circulation
The topography created by seamounts has a large effect on the local water masses, primarily generating upwelling and enhancing currents. Anticyclonic circulation over the tops of seamounts creates a socalled ‘cold dome’ as a result of enhanced upwelling along the sides of the edifice. Also, long aseismic ridges and seamount chains are barriers to deep- and midwater circulation. Aseismic ridges standing 2–4 km above the seafloor and stretching for hundreds of kilometres across the Pacific deflect north–south currents at depth.
Subduction Asperities
Seamounts being subducted erode the fore arc and may also cause erosion at the base of the overriding plate. Huge furrows, mass wasting, and structural disruption of the fore arc are the effects attributed to seamount subduction (Figure 5). There are many areas that exhibit the morphological signs of seamount subduction, but proving the existence of buried seamounts depends on interpreting seismic and magnetic data. However, seamounts being subducted are sometimes directly visible, for example Daiichi-Kashima Guyot in the Japan Trench and the Bannock (Seamount) Structure in the Mediterranean. Seamount subduction may also be responsible for seismic asperities on the subduction thrust-fault interface. Seamount tunnelling, the process by which a seamount going down on the subducting plate scrapes material off the overriding plate, has been inferred to have an important role in crustal development and fluid flow in the margin. The subduction of large
Critical Habitat
Seamounts play a key role in ocean environments by providing habitats for fishes and suspension feeders. The enhanced upwelling combined with the potential for a reef environment in very shallow seamounts make large seamounts sites of high primary productivity. Seamounts have been known as productive fishing grounds for centuries, but their role in oceanic biodiversity has been appreciated only in the last 50 years. Seamounts host a relatively large percentage (estimated at 15–35%) of endemic species and may be important sites of speciation for deep-sea fauna. Seamounts in the south-west Pacific show highly localized species distributions and fairly limited recruitment among seamounts, even between closely spaced seamounts. The formation of atolls by coral growth on subsiding seamounts creates an important habitat niche for filter feeders and reef fishes. Finally, many humans find ocean islands and
SEAMOUNTS 483
Figure 5 Seamount subduction has produced huge furrows and re-entrants in the fore-arc margin of Costa Rica. Shaded-relief bathymetry reproduced from Dominguez S, Lallemand SE, Malavieille J, and von Huene R (1998) Upper plate deformation associated with seamount subduction. Tectonophysics 293: 207–224, copyright 1998, with permission from Elsevier.
484 SEAMOUNTS
atolls highly desirable locations for settlement and vacation.
Glossary Abyssal hill A low-relief elongate hill formed by horst-and-graben style faulting around a mid-ocean ridge. Alkali basalt A variety of basalt that has relatively high sodium and potassium contents. Atoll A ring of carbonate reef islands enclosing a lagoon built atop a subsiding seamount in tropical latitudes. Pillow lava A type of subaqueous lava flow consisting of spherical or elongate tubes formed by the rapid quenching of lava. Tubes seen in cross-section often resemble pillows, with a flat base and a convex upper surface. Scoria Glassy porous pyroclastic lava. Surtseyan A type of volcanic eruption, based on observations at Surtsey in Iceland, which is the phreatomagmatic equivalent of a Hawaiian or quiescent lava flow eruption. Tholeiitic basalt A variety of basalt characterized by the presence of orthopyroxene in addition to clinopyroxene and calcium-plagioclase.
See Also History of Geology Since 1962. Large Igneous Provinces. Lava. Mantle Plumes and Hot Spots. Sedimentary Environments: Reefs (‘Build-Ups’). Sedimentary Processes: Particle-Driven Subaqueous Gravity Processes. Tectonics: Hydrothermal Activity; Mid-Ocean Ridges. Volcanoes.
Further Reading Barone AM and Ryan WBF (1990) Single plume model for asynchronous formation of Lamont Seamounts and adjacent Pacific Rise terrains. Journal of Geophysical Research 95 N. B7: 10801–10827. Batiza R (1989) Seamounts and seamount chains of the eastern Pacific. In: Winterer EL, Hussong DM, and
Decker RW (eds.) The Geology of North America, Volume N, The Eastern Pacific Ocean and Hawaii, pp. 289–306. Boulder: Geological Society of America. Batiza R and Vanko D (1984) Volcanic development of small oceanic central volcanoes on the flanks of the East Pacific Rise inferred from narrow beam echo-sounder surveys. Marine Geology 54: 53–90. Clague DA, Reynolds JR, and Davis AS (2000) Nearridge seamount chains in the north-eastern Pacific Ocean. Journal of Geophysical Research 105: 16 541–16 561. Davis EE (2000) Earth science: volcanic action at Axial Seamount. Nature 403: 379–480. Johnson HP and Embley RW (eds.) (1990) Axial Seamount: an active ridge axis volcano on the central Juan de Fuca Ridge. Journal of Geophysical Research 95: 12 689–12 966. Keating BH and McGuire WJ (2000) Island edifice failures and associated tsunami hazards. Pure and Applied Geophysics 157: 899–955. Keating BH, Fryer P, Batiza R, and Boehlert W (eds.) (1987) Seamounts, Islands, and Atolls. Geophyiscal Monograph Series Volume 43. Washington DC: American Geophysical Union. Mitchell NC (2001) Transition from circular to stellate forms of submarine volcanoes. Journal of Geophysical Research 106: 1987–2004. Moore JG, Normark WR, and Holcomb RT (1994) Giant Hawaiian landslides. Annual Review of Earth and Planetary Sciences 22: 119–144. Ranero CR and Von Huene R (2000) Subduction erosion along the Middle America convergent margin. Nature 404: 748–752. Schmidt R and Schminke H-U (2000) Seamounts and island building. In: Sigurdsson H, Houghton B, McNutt SR, Rymer H, and Stix J (eds.) Encyclopedia of Volcanoes, pp. 383–402. San Diego: Academic Press. Takahashi E, Lipman PW, Garcia MO, Naka J, and Aramaki S (eds.) (2002) Hawaiian Volcanoes: Deep Underwater Perspectives. Geophysical Monograph Series Volume 128. Washington DC: American Geophysical Union. Thorarinsson S (1967) Surtsey: The New Island in the North Atlantic. New York: Viking. Wessel P (2001) Global distribution of seamounts inferred from gridded Geosat/ERS-1 altimetry. Journal of Geophysical Research 106: 19 431–19 441.