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By Hubert Staudigel and David A. Clague

The Geological History , Volume 1, a quarterly 23, Number The O journal of of Deep-Sea Volcanoes Biosphere, Hydrosphere, ceanography S

and Lithosphere ociety. ©

Interactions 2010 by The O ceanography S ceanography S ociety. A ll rights reserved. P reserved. ll rights ociety. S end all correspondence to: [email protected] Th e O or ermission is granted to in teaching copy this and research. for use article R ceanography S ociety, 1931, R Box PO epublication, systemmatic reproduction,

Emergence of a submarine near Nukualofa, Tonga, March 18, 2009. The eruption MD 20849-1931, USA ockville, column displays two very characteristic features for shallow-submarine eruptions, base surges (light colored) that spread out horizontally at the base and “cocks’ tail” jets that emerge from the center of the column. Photo credit: Dana Stephenson/Getty Images .

58 Oceanography Vol.23, No.1 Abstract. The geological evolution of has distinct influence on their to science and society. They have rich interactions with the , their hydrology, geochemical fluxes, biology, resources, fisheries (Pitcher et al., 2010), are marine- and . There are six geological evolutionary stages of seamounts: (1) small biological and microbiological hotspots seamounts (100–1000-m height), (2) mid-sized seamounts (>1000-m height, > 700-m (Danovaro et al., 2009; Emerson and eruption depth), (3) explosive seamounts (< 700-m eruption depth), (4) ocean islands, Moyer, 2010), and have a significant (5) extinct seamounts, and (6) subducting seamounts. Throughout their lifetimes, geochemical impact, with chemical fluxes seamounts offer major passageways for fluid circulation that promotes geochemical that allow mantle-derived materials to exchange between and the volcanic , and seamounts likely interact with the hydrosphere (Fisher host significant microbial communities. Water circulation may be promoted by and Wheat, 2010). Hence, seamounts hydrothermal siphons in conjunction with the underlying oceanic crust, or it may are key points of intersection among the be driven by intrusions inside seamounts from Stage 2 onward. Geochemical fluxes biosphere, hydrosphere, and lithosphere, are likely to be very large, primarily because of the very large number of Stage 1 and they are globally relevant. seamounts. Intrusive growth of seamounts also initiates internal deformation that In this paper, we explore how ultimately may trigger volcano sector collapse that likely peaks at the end of the main seamounts grow, collapse, and age, volcanic activity at large seamounts or islands. Explosive activity at seamounts may and how these processes interact with begin at abyssal depth, but it is most pronounced at eruption depths shallower than the hydrosphere and impact biological 700 m. inhibits the emergence of islands and shortens their lifespans processes. The reader is referred to earlier before they subside due to lithosphere cooling. Once volcanism ends and a reviews by Batiza and White (2000), is submerged, seamounts are largely unaffected by collapse or erosion. Throughout Schmidt and Schmincke (2000), and a their histories, seamounts offer habitats for diverse micro- and macrobiological recent monograph (Pitcher et al., 2007). communities, culminating with the formation of coral reefs in tropical latitudes. Geological hazards associated with seamounts are responsible for some of the largest Defining a Seamount natural recorded in history and include major explosive eruptions and large- There is no uniform definition for the scale that may trigger . term seamount that equally satisfies all scientific disciplines. Definitions reflect Introduction in height and does not offer any topo- diverse interpretative perspectives and Seamounts are the least explored major graphic detail below this scale. This state disciplinary approaches that can be quite morphological features on . of exploration reveals nothing about specific to the needs of a particular scien- Hundreds of thousands of them are a seamount’s . In fact, this lack tific community (see Box 1 on page 20 dispersed through the ocean basins, with of detail would be considered entirely of this issue [Staudigel et al., 2010a]). less than 0.1% known from any direct unacceptable for topographic features We use here a definition that centers observation such as an echosounding, on land, effectively all of which are on seamounts as isolated geological or any kind of sampling. Most of what now sampled and mapped by direct features on the seafloor. Seamounts are we know about seamounts comes from measurements with a resolution better typically volcanoes that form on top of satellite observations that sense them than 30 m in vertical and 100 m in the oceanic crust with their own central indirectly from the centimeter-size horizontal dimensions. Seamounts are supply and hydrothermal system. deflections they impose on sea surface the last major frontier in our exploration This simple distinction can become more topography (e.g., Wessel et al., 2010). of Earth’s surface. complex when isolated volcanoes form This indirect method reliably identi- The minimal level of seamount explo- right on the mid-ocean ridge axis, such fies only features larger than 1500 m ration contrasts with their significance as on the Juan de Fuca

Oceanography March 2010 59 Ridge (see Spotlight 1 on page 38 of this easily from the processes involved in oceanic intraplate volcanoes (OIVs) issue [Chadwick et al., 2010a]). seamount formation (see Koppers and are formed by decompression melting Isolated volcanoes on the seafloor Watts, 2010). It may bulge upward of mantle materials that buoyantly rise come in a range of sizes whereby the from the buoyancy of rising mantle or in upward convective flow and/or in smallest ones are typically referred to due to heating of the lithosphere from response to plate extension and thin- as abyssal hills and the larger ones as below. However, as soon as a seamount ning. zone volcano seamounts. We use 100 m as the tallest has reached a critical size, subsidence are commonly formed by the lowering abyssal hill and smallest seamount, becomes the dominant vertical force, of the melting point of the sub-arc following a definition of Schmidt and caused by the bending of the oceanic mantle due to addition of volatiles from Schmincke (2000). By this definition, crust into the mantle as it yields to the descending slab. These different seamounts may form in any tectonic the seamount load (Watts, 2001). For melting processes have a profound setting; they may have conical, - example, data from drowning coral impact on the chemistry of seamount topped, or complex shapes; and they terraces on Hawai`i suggest a subsid- rocks and the chemical evolution of may or may not have carbonate or ence rate of 2.6 mm yr-1 for the Island the volcanoes formed. sedimented caps. We also include of Hawai`i (Moore and Fornari, 1984; Seamounts close to MORs and in OIV oceanic islands in our definition of a Moore et al., 1996). However, on a much settings erupt mostly basaltic (tholeiitic seamount, considering them as very longer time scale extending millions of and alkalic) magmas, whereas subduc- large seamounts that breached the sea years, seamounts also subside owing to tion zone volcanics are predominantly surface. Viewing islands as a subgroup the cooling of the lithosphere they are composed of calc-alkaline (andesitic or of seamounts is unusual because most built upon. Most seamounts are formed arc-theoleiitic) magmas. All seamount views focus on the distinct nature within a few hundred kilometers of rocks are formed by mantle melting of islands in terms of their volcanic mid-ocean ridges at roughly 2.5-km processes that involve smaller degrees features, subaerial weathering, and water depth. These seamounts will of melting than are characteristic for land-based biology (see Gillepsie and subside with the underlying cooling the generation of the oceanic crust Clague, 2009). However, our grouping lithosphere to about 5-km water depth itself. As a result, their incompatible makes sense from a geological point of within 60 million years and possibly element abundances may be an order of view in which volcanic islands are the down to 10-km depth when they arrive magnitude higher, and seamount fluxes summit regions of very large seamounts at subduction zones. This journey from of these elements may dominate global that are temporarily exposed above sea mid-ocean ridge to subduction zone may mantle fluxes even though seamounts are level. Most oceanic islands start out as involve thousands of kilometers of hori- substantially less voluminous than the seamounts and turn back into seamounts zontal displacement. For example, the elongated volcanoes forming the MORs toward the ends of their life cycles. majority of the seamounts in the central- themselves. Subduction zone magmas western Pacific originally formed in the tend to have overall higher Si, alkali, Geological Setting South Pacific 80–140 million years ago. and volatile abundances, and lower Mg Seamounts form in an oceanic geological contents, than OIVs and MORs. As a setting that is much more dynamic than Geochemistry result, they are more explosive and tend the continents, resulting in much larger Seamount volcanoes are formed by two to have higher . vertical and horizontal displacements. principal mantle melting processes. OIVs display a characteristic Oceanic crust flexes and bends relatively Mid-ocean ridge (MOR) and geochemical evolution over their 5–20-million-year volcanic histories Hubert Staudigel ([email protected]) is Research and Lecturer, Institute (Clague and Dalrymple, 1987). Four of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of geochemical stages are commonly , San Diego, La Jolla, CA, USA. David A. Clague is Senior Scientist, Monterey Bay distinguished, originally defined for Aquarium Research Institute, Moss Landing, CA, USA. the Hawaiian Islands but also found to

60 Oceanography Vol.23, No.1 characterize many other OIVs (Konter volcanoes with pronounced rift zones 2 km (Staudigel and Schmincke, 1984; et al., 2009). OIV formation at Hawai`i’s such as Hawai`i’s Kilauea Volcano Staudigel et al., 1986). Sills may also most recent active volcano, Lō`ihi, starts (Dieterich, 1988; Leslie et al., 2004). play a key role in the growth of some with small erupted volumes of basaltic Kilauea’s East Rift displays volcanic Galápagos volcanoes (Amelung et al., rocks with diverse compositions gener- spreading rates of about 10 cm yr -1, 2000; Chadwick et al., 2006). ated by small degrees of mantle melting. pushing the whole volcano outward The deeper portions of seamounts are The subsequent main, shield-building from along a near-horizontal likely to contain magma reservoirs that stage then produces up to 98% of the slip surface at 8–9-km depth (Hill and crystallize to gabbroic and ultramafic entire volcano typically from rocks Zucca, 1987; Denlinger and Okubo, 1995; intrusions (Upton and Wadsworth, formed by relatively large melt frac- Delaney et al., 1998). Such spreading 1972). Such intrusions may occur at a tions in the mantle, yielding tholeiitic may be entirely intrusive, as indicated by range of depths below the summit and or mildly alkalic . This is closely the common swarms on the are commonly sampled as xenoliths followed by a cap of alkalic rocks that rift zones of Kilauea that are not associ- in later (Clague and Dalrymple, are formed by smaller degrees of partial ated with eruptions (Klein et al., 1987). 1987). At , such intrusive rocks melting than the shield magmas. Some Erosion has also exposed the rift zones have been found at stratigraphic depths OIVs have a last stage of post-erosional on several extinct Hawaiian volcanoes, of almost 5 km below the likely original or rejuvenated volcanism, after a particularly Koolau on Oahu, that also summit of the seamount (Staudigel et al., volcanic hiatus of 1.5–10 million years. show large volumes of dikes (Walker, 1986), whereas active magma chambers Rejuvenated volcanism typically consists 1987; Figure 1). At the uplifted La Palma at Kilauea may be only 800 m below of very small volumes of highly alkalic seamount series (), sills the summit (Almendros et al., 2002). lavas that often include mantle xenoliths. are the dominant form of intrusion. Intrusive activity at seamounts results in These stages of geochemical evolution Their cumulative thickness suggests inflation, over-steepening of flanks, and provide insights into the temporal evolu- uplift of the overlying volcano by about ultimately flank collapse. tion of mantle melting with substan- tial implications for the geodynamic processes involved. An interesting fact is that the majority of seamounts have completed their geochemical evolutions, and hence their surfaces most likely display the latest stages of volcanism. As a consequence, the majority of their earlier shield-building evolution may be largely hidden from direct sampling.

Lithological Rock Types Seamounts, like all volcanoes, are formed by intrusive and eruptive processes, prob- ably in roughly equal proportions, at least in larger volcanoes where these processes have been studied in more detail. Intrusive rocks at seamounts include dikes, sills, and gabbroic to ultramafic Figure 1. Dike intrusions (vertical and slanted features) into the Koolau volcanic shield, Oahu, Hawai`i. intrusions. The intrusion of dikes is likely Photo taken during road construction where the road cut is defined by three, near-horizontal “staircase” to be one of the key means of growth of steps. A person on the second terrace (circled) offers a scale.

Oceanography March 2010 61 Eruptive rocks dominate the surface (Figure 2A,B). Less abundant are sheet tubes that form these flows. Pillow and the upper portions of a seamount, flows and massive flows that tend to lava cross sections commonly display hence they are the major apparent indicate higher mass eruption rates. characteristic “V-shaped” bottoms as component of volcano growth, with two Sheet flows typically are found on the lava tubes mold themselves to the major submarine types: lava flows and shallow slopes and settings where they underlying pillow lavas. Individual volcaniclastics (Staudigel and Schmincke may be ponded (Figure 2C; Staudigel lava tubes may occur in a range of tube 1984; Batiza and White, 2000; Schmidt and Schmincke 1984, Clague et al., 2000, diameters, from a few tens of centimeters and Schmincke, 2000). 2009). All submarine lava flows have to 200 cm in diameter (Clague et al., Pillow lavas are the most common pronounced, quenched, glassy margins. 2009), decreasing in size with distance submarine lava flows and may be found Pillow lavas are named after the curved, from the eruption center and upward in almost any subaqueous setting occasionally round, cross section of the in a section (Figure 7 in Staudigel and Schmincke, 1984). Seamount volcaniclastic rocks ABare sedimentary rocks formed by mechanical breakup of lava flows or by explosive-eruptive fragmentation mech- anisms (Fisher and Schmincke, 1984); they are the most common volcanic rocks in shallow submarine volcanoes (e.g., Staudigel and Schmincke, 1984; 3 m 50 cm Garcia et al., 2007). Mechanical frag- mentation prevails during the collapse CD of unstable formations and spallation of pillow margins. Explosive fragmentation may be caused by the expansion of gas bubbles inside the magma, magma interaction with external water, or grain-to-grain inter- 2 m 1 m action during an eruption. Although mechanical fragmentation processes E F have no (confining) pressure depen- dence, explosive fragmentation does. The water-depth dependence of explosive fragmentation processes is due to pres- sure dependence of magma volatile solu- bility and the pressure dependence of 50 µm 50 µm the extraordinarily large volume change in water when it transforms from liquid Figure 2. Field and microphotographs of submarine volcanic rocks: (A) Pillow lavas from submarine to gas. At one atmosphere pressure, this exposure at Vance Seamounts (1600-m water depth). (B) Dissected pillow formation from Archean pillow lavas in Pechenga, Russia. (C) Surface of a submarine sheet flow resembling a subaerial pahoehoe volume change is by a factor of 1600. It is flow from Vance Seamounts (1712-m water depth). (D) Pillow fragment from Lō`ihi Seamount, important that the pressure dependence 1000-m water depth. (E) Tubular micro tunnels at a crack in a glassy pillow margin from Drilling of gas solubility varies substantially with Project Site 396B, 20R-3, piece 13, 108–112 cm. (F) Segmented tubular alteration and tubules with coiled bore from Troodos ophiolite. Tubular features in (E) and (F) are interpreted as trace fossils formed by the dissolved species. CO2 may outgas at microbial chemical drilling (Staudigel et al., 2008). relatively high pressures, whereas H2O

62 Oceanography Vol.23, No.1 and SO2 tend to outgas at much lower spatter may also be responsible for the . at pressures. Sulfur may separate from formation of well-sorted, fine-grained seamounts is significant, because they magma as an immiscible liquid at a wide hyaloclastite (glass sand; Clague et al., are made of rather permeable volcani- range of pressures, but it contributes to 2009). Most submarine volcaniclastics clastic rocks, and cover does the explosivity of lava only at low pres- are missing most of the fine grain-size not significantly inhibit circulation. sure when SO2 bubbles can form in the fraction, which remains suspended in Seamounts also have a topographic magma. Geological observations and turbulent waters and is deposited slowly advantage in that water can enter on theoretical considerations point to a in more distant areas surrounding the the flanks and then rise within the relatively consistent critical depth for the seamount eruption site. This lack of fine seamount due to its raised geothermal onset of major explosive activity. A more grains gives seamount volcaniclastics a gradients resulting from hydrothermal than 1800-m-thick section of seamount relatively high permeability. activity (Schiffman and Staudigel, volcanic rocks at the La Palma seamount Lithology, particularly the abundance 1994) or simply by their topography. series shows a dramatic increase in of coarser-grained volcaniclastic deposits, Furthermore, seamount volcanic rocks volcaniclastic rocks at about 1000-m has a substantial impact on chemical are rich in volcanic glass, and their water depth (Staudigel and Schmincke, fluxes and on the development of micro- alteration contributes preferentially to 1984). Theoretical considerations place bial habitats. Key controls include the hydrothermal chemical fluxes. Hence, the explosive transition closer to 700 m high permeability of these deposits, seamount hydrothermal fluxes may for explosions caused by the interaction which allows for seawater circulation, be substantially more important than of external water with magma (Peckover and a high abundance of volcanic glass, suggested by their volume relative to et al., 1973) as well as the explosive which is highly reactive with seawater. MORs. Some indications for large fluxes 3 outgassing of magma itself (McBirney, The alteration of volcanic glass is the come from the large fluxes of He or CO2 1963). However, some variance may be primary process that controls low- (Lō`ihi; see Spotlight 3 on page 72 of this expected as a result of magma composi- temperature alteration fluxes between issue [Staudigel et al., 2010c]; Lupton, tion and eruption rates, and minor pyro- seawater and basalt (e.g., Staudigel and 1996; Hilton et al., 1998) or Mn and 3He clastic activity can be widespread even at Hart, 1983), and glass is a particularly anomalies (Vailulu’u; see Spotlight 8 on abyssal depths (Clague et al., 2009). suitable material for colonization by page 164 of this issue [Koppers et al., Pillow fragment are prob- microbial communities (Figure 2E,F), 2010]; Hart et al., 2000; Staudigel et al., ably the most common mechanically up to the moment they are sealed off by 2004). Together, seamount and oceanic fragmented volcaniclastic rocks at authigenic mineral deposition. crust hydrothermal systems play key roles seamounts, often formed during the in buffering the chemical composition collapse of steep flow fronts Hydrothermal and of seawater, but the contribution from (Figure 2D). Pillow breccias may form Biological Activity seamounts remains less well known and when pillow lavas intrude into their own Seafloor volcanism generally produces is likely to vary among elements. fine-grained volcaniclastics, with highly hydrothermal systems that are likely to Microbial communities have been irregular, amoeba-shaped pillow cross be associated with substantial microbial documented from a range of seamounts, sections that are completely enclosed in and possibly macrobiological activity. and there is abundant evidence that they a hyaloclastite matrix. Pillows may also Seamount hydrothermal water flow may might play a role in a potentially very intrude into fine-grained epiclastic sedi- be driven by intrusive activity, or the large deep oceanic crustal biosphere. ments (peperites). Explosive seamount seamount may act as a “hydrothermal Emerson and Moyer (2010) review volcaniclastics from magmatic outgas- siphon” (Fisher and Wheat, 2010), much of the evidence from microbio- sing may form highly vesicular scoria allowing the recharge of seawater to the logical studies at seamounts, showing that may occur as nearly pumiceous oceanic crust and the escape of oceanic that there are very diverse microbial deposits or be broken up into finer crustal fluids that may be otherwise communities, often with novel organisms particles. The fragmentation of dense trapped by a blanket of impermeable with metabolic capabilities (including

Oceanography March 2010 63 heterotrophy) and microbes that oxidize crust largely by eruptive processes. or on thinly sedimented seafloor. Their (and reduce) iron, manganese, and sulfur. (2) Mid-size seamounts are taller than dominant eruptive lithologies include Microbial communities may thrive, in about 1000 m and remain at eruptive mostly pillow lavas and approximately particular, on the reducing potential of depths deeper than 700 m, representing 20% volcaniclastics (as found at Ocean these elements in basalt, which is strongly largely nonexplosive volcanoes with Drilling Program/Deep Sea Drilling enhanced by the high abundance of key magmatic plumbing systems that Project Site 417A and the deep-water nutrients in these rocks that are depleted become an increasingly important part section at La Palma; Staudigel and in seawater (Fe, Mn, Ni, P). Fluids of the seamount itself. (3) Explosive Schmincke, 1984). It is likely that extracted from a sealed drillhole into seamounts form at ocean depths shal- seamounts on sedimented seafloor may Baby Bare Seamount (Cascadia Basin, lower than 700 m and are mostly covered initially form intrusions into the sedi- Northeast Pacific Ocean) yielded a highly by volcaniclastics. (4) Islands form when ments and/or peperites, before they diverse microbial community (Cowen a seamount breaches the sea surface, erupt pillow lavas that quickly cover et al., 2003), suggesting that microbes shifting to subaerial processes, including these rocks. Small seamounts are likely are active inside the crust as well. The subaerial weathering, soil formation, to include almost no intrusive rocks, latter is also suggested by the study of and erosion. (5) Extinct seamounts except for their feeder dikes, which has microbial trace fossils in volcanic glass have completed all volcanic activity and important consequences. The underlying alteration that imply that bioalteration subsided below , sometimes oceanic crust controls these seamounts’ dominates glass alteration in the upper forming . (6) Seamounts meet plumbing systems and tectonics. There oceanic crust, at least to a depth of 500 m the end of their geological life during also is no or very little intrusive inflation (Figure 2E,F; Furnes and Staudigel, 1999; closure of an ocean basin or by subduc- of the freestanding part of the seamount, Staudigel et al., 2008). tion. Although these six stages describe which makes it very unlikely that these The interplay of ocean circulation the complete development of very large volcanoes develop any major collapse and hydrothermal activity at seamounts seamounts, most seamounts never reach features. And, finally, the reaction zone also has a substantial impact on the the island stage. Smaller seamounts may for their hydrothermal fluids is located macrofauna found at their surfaces. be entirely buried by sediments by the inside the oceanic crust, and their hydro- For example, the crater of Vailulu’u time they reach a subduction zone. thermal venting is strongly linked to the Seamount displays substantial metazoan hydrothermal cooling of the underlying mortality in the deepest part of the Stage 1 oceanic crust. Intense low-temperature crater, whereas the summit of a new Small seamounts with total heights of hydrothermal circulation in small volcano growing in the crater displays 100–1000 m are by far the most common seamounts results in extreme enrich- a thriving population of eels (Staudigel type of seamount, even though they ments in seawater-derived chemical et al., 2006). Similar complex benthic are often difficult to distinguish from components, particularly in volcaniclas- zonations likely occur at many other the roughness of the abyssal oceanic tics but also in lava flows (e.g., Site 417A; seamounts, such as the liquid sulfur crust topography that may occur on a Staudigel et al., 1996). It is quite likely lakes at Daikoku and Nikko volcanoes similar scale. The key distinctive feature that the very large number of small (Embley et al., 2007). is their often circular morphology that seamounts and their extreme alteration is superimposed on a seafloor fabric makes them major players in global The Structural Evolution that largely is made up of elongated geochemical cycles and as globally rele- of Seamounts MOR volcanoes or tectonic blocks. vant habitats for microbial communities. Seamounts evolve in six stages that There may be as many as 25 million of are structurally distinct (Figure 3). them worldwide (Wessel et al., 2010). Stage 2 (1) Small seamounts (< 1-km tall) The majority of these seamounts form Mid-size seamounts are defined here and abyssal hills (< 100-m tall) are on relatively young crust, at water as volcanic features that are tall enough seafloor volcanoes built on the oceanic depths between 2–3 km, on bare rock, (at least 1000 m in height) to have a

64 Oceanography Vol.23, No.1 magma plumbing system above the level they increase in size. Based on observa- extent and geometry of volcano collapse. of the oceanic crust, but that have not tions at La Palma, the eruptive rocks on Second, intrusions introduce heat into yet shoaled enough to be dominated by typical mid-size seamounts may consist shallower portions of the volcano, estab- shallow-water explosive activity (which of a majority of pillow lavas with about lishing hydrothermal convective cooling starts at ~ 700-m water depth). There 20% volcaniclastics. systems that are increasingly decoupled may be hundreds of thousands of such As seamounts increase in size, they from the underlying oceanic crust. At seamounts (Wessel et al., 2010), and begin to develop internal magma Vailulu’u Seamount, earthquake loca- some of them may be quite large—all reservoirs above the oceanic crust. This tions cluster below hydrothermal vents Hawaiian seamounts had to grow to attribute has two important conse- along main structural trends defined 4000-m heights above the seafloor quences. First, intrusions begin to by rift zones (Konter et al., 2004), indi- before they became explosive in Stage 3. inflate the volcano, and this inflation cating a close relationship among the Mid-size seamounts are likely to be soon dominates its internal stresses and development of permeability, intrusive mostly made of eruptive rocks, with determines the development of fissures geometry, and hydrothermal cooling. an increasing fraction of intrusives as and rift zones as well as, ultimately, the The depth of the suggests

1 4 Figure 3. Six stages of seamount evolution are defined by seamount size, by the confining pressures at the time of eruption, and by expo- sure to seawater. Stage 1 represents the smallest seamounts (100–1000 m), whose magmatic plumbing systems are located in the oceanic crust, below the volcano. Stage 2 includes 2 5 mid-size volcanoes exceeding 1000 m in height and with erup- tive depths greater than 700 m, the likely critical depth for the beginning of explosive activity. Stage 3 describes the explosive stage of seamounts above 700-m eruption depth, and Stage 4 relates to seamounts with emerged 3 6 summits forming islands and carbonate reefs. Stage 5 includes all seamounts that have reached the end of their magmatic activity. Stage 6 illustrates the breakup of a seamount before it is consumed by subduction.

Oceanography March 2010 65 that the hydrothermal reaction zone is and it is likely that the locations of all oceanic crust upon loading (Koppers and likely centered well above the seafloor, shallow seamounts are known from Watts, 2010). Eruptions from shallow probably guided by intrusive and satellite observations, even though seamounts can also produce rafts recharge systems, including central and gravity anomalies yield only indirect and that float great distances away from the rift-related and recharge from much-less-reliable depth estimates than volcano. Intrusives of shallow seamounts flanks and radial fissures. echosoundings. Shoaling seamounts are likely not to be different from those Mid-size seamounts have a substan- can pose navigational and of the Stage 2 mid-size seamounts, tial impact on ocean circulation and risks. The collision of the submarine although it is likely that the overall frac- on the biosphere. They provide much USS San Francisco with a seamount tion of intrusives increases with the size larger obstacles to currents than small resulted in substantial damage and the of the seamount. seamounts, and they are likely to locally death of one sailor, and a Japanese survey The growth from mid-size to shallow enhance the amplitudes of internal ocean ship, Kaiyo-maru No. 5, was destroyed in and emergent seamounts involves a oscillations. As such, they contribute 1952 by an eruption of Bayonnaise Rocks variety of changes in habitats. The more to ocean mixing (stirring-rod (Myojinshu), killing the crew of 31. increase in volcaniclastics results in effect) and they provide for potential Volcaniclastics are the dominant erup- enhanced production of volcanic development of seamount-encircling tive rock types at shallow seamounts, glass that has a substantial effect on currents that may attract or trap pelagic as directly observed in Mariana geochemical fluxes and provides an life, including larvae. How seamount Arc eruptions at Northwest Rota-1 attractive substrate for microbial growth. morphology and size determines ocean (see Spotlight 10 on page 182 of this It may play an important role within circulation remains a topic of active issue [Chadwick et al., 2010b]; Chadwick the seamount itself, on the summit and research (Lavelle and Mohn, 2010). et al., 2008a; http://www.youtube.com/ flank, and in the sediments surrounding Mid-size seamounts also provide a watch?v=1tPTsZ6PtR8&NR=1) and the seamount. Exploration of the craters settling substrate for sessile benthic at in the Kermadec Arc of mid-size and shallow seamounts filter feeders (such as sponges and deep (Chadwick et al., 2008b), and at an shows that their unusual geochemical sea corals; Young, 2009), defining a emerging seamount volcano near Tonga settings result in ecological conditions biome that may exceed a surface area (see photo in opening spread). There where some specialized biological equivalent to that of Europe (see Box 12 is also abundant evidence from the communities thrive (e.g., polychaetes, on page 206 of this issue [Etnoyer modeling of seamount magnetic anoma- demisponges), although there may be et al., 2010]). These filter feeders record lies, which often require a “nonmagnetic mass mortality for fish (e.g., Vailulu’u) water characteristics that relate to top” that is probably best explained due to extensive hydrothermal plume ocean currents at their particular depth by highly altered volcaniclastics. At activity in an enclosed crater (Staudigel (Robinson et al., 2007). In addition, La Palma, the transition from mid-size et al., 2006). Some unique habitats these seamounts support microbial habi- to shallow seamount is characterized may result from the development of tats that may remain active for a long by an increase from 20% to more than liquid sulfur lakes or the venting of time because of slow, low-temperature 60% volcaniclastics, which are deposited buoyant liquid drops of CO2, as found water circulation. on the summit, flanks, and, in particular, at a range of seamounts in the western the surrounding aprons. It is well known Pacific (Lupton et al., 2006; Staudigel Stage 3 from arc systems that volcaniclastics et al., 2006; Embley et al., 2007). As a Shallow seamounts are geologically make up a very large fraction of the seamount reaches the near surface, it distinct because relatively low hydro- surrounding sediments. A similar may breach the oxygen minimum zone static pressures at depths less than situation may exist for the sediments where microbial communities have to 700 m may allow a substantial increase surrounding very large seamounts where adjust to microaerophilic or anaerobic of explosive volcanism. They are less volcaniclastics are collected in the flexure conditions. Finally, their summits may common than mid-size seamounts, moats caused by a deep flexing of the come close enough to the surface that

66 Oceanography Vol.23, No.1 photosynthesis becomes a viable energy the surrounding seafloor. In particular, are most likely and most prominent in source for carbon fixation. In tropical ocean arcs, but also ocean islands, may the largest seamounts and islands with regions, this can result in the formation produce substantial explosive (Plinian) well-established, long-lived magmatic of massive carbonate reefs. Such reefs are eruptions that can distribute plumbing systems. They are most among the most diverse and productive as discrete layers up to 2000 km down- commonly found near the point of habitats on Earth. wind from the source (Toba/). emergence of major rift zones from These explosive eruptions constitute the main body of a volcano (Figure 4). Stage 4 major volcanic hazards, particularly Sector collapse may mobilize volumes Islands are born when shallow subma- when hydrovolcanic processes resulting up to 3000 km3 (Satake et al., 2002), and rine volcanism allows a seamount to from contact with seawater amplify the the debris from such flows may cover an emerge above sea level and to transi- eruptions. Major catastrophic erup- ocean area of 10,000–15,000 km2 with tion into subaerial volcanic activity. tions include the 1630 BCE > 1-km-size blocks up to 180 km from This transition is a prolonged process. cataclysm that is likely to have termi- their site of origin (Moore and Clague, Historically, most newly born islands nated the Minoan culture, or the 1883 2002). Such events leave a major scar on have disappeared shortly after their eruption that was associated the side of the volcano, have a substantial emergence (e.g., Metis Shoal of Tonga, with more than 30,000 casualties in the effect on benthic life around the volcano, Myojinsho of the Izu Ogosoawara Indian Ocean area. and result in catastrophic tsunamis that group south of Japan, Graham Island Intrusive activity at islands is demon- may have runups reaching inland to in the ). They disap- strably a major factor in their evolu- localities up to 400 m above sea level, pear because the continuous power tion and collapse. Collapse features constituting a major ocean-basin-wide of sea-surface wave erosion tends to out-compete constructive volcanic growth by minimally alteration-resistant DEBRIS volcaniclastic rocks. In fact, persistent emergence may only be possible during periods of very high eruption rates, and the formation of a more resistant 17°20'N lava veneer that protects the underlying clastic rocks from wave erosion. It is also conceivable that emergence is critically aided by intrusive activity that may lift the top of a seamount. 17°00'N HEADWALL Emergence of a volcanic island has SCARP important geological consequences in Figure 4. Diagram of terms of eruptive and intrusive processes flank collapse at Vlinder and volcano collapse, erosion, and 16°40'N Seamount on the northern weathering. Eruptive rocks at ocean edge of its summit platform (Koppers et al., 1998). More islands are dominated by lava flows that than 10 km3 of seamount cool at substantially slower rates than flank were mobilized and their submarine equivalents. However, deposited > 6 km away from 16°20'N the breakout. many of these flows enter the sea, where 154°00'E 154°20'E they produce large quantities of glass- rich volcanic rocks that are deposited on -6000 -5000 -4000 -3000 -2000 -1000 lava benches, submarine island flanks, or Depth (m)

Oceanography March 2010 67 natural hazard (e.g., at Volcano conditions are met, reefs can get estab- oxides and phosphorites, depending in Hawai`i; McMurtry et al., 2004). lished fairly quickly, as is evident at the on their chemical environment. These As soon as islands are formed, they relatively recent Hawaiian volcanoes mineral groups potentially include very are subjected to weathering, soil forma- of Hualalai, Kohala, and Mauna Kea. high concentrations of elements that tion, and erosion, as well as the develop- Overall, coral reefs and volcanic ocean are very scarce in seawater (Mn, P, Co, ment of groundwater, particularly in islands are among the most productive Ni, Ti, Pt, Th, Pb, rare earth elements; environments with significant rainfall. fisheries on Earth. This productivity Hein et al., 1997), and thus seamount Weathering and erosion products intro- may be a result of many processes and surfaces are likely to play a role in the duce a range of new materials into the circumstances, including the constant geochemical budgets of these elements marine environment, including organic- volcanic input, photosynthesis, and in seawater. Such surfaces provide some rich soils and sands with highly reactive complex local ocean circulation, due to unique substrates for microbial growth components (olivine, glass) that can the islands’ roles as ocean “stirring rods.” and the basis for potential seafloor support life in the ocean. In addition, mining, in particular for Co and some the formation of groundwater on islands Stage 5 high-tech metals such as tellurium commonly creates a submarine fresh- Once volcanism ceases, a volcanic island (Hein et al., 2010). water lens that alters the hydrothermal will eventually drown due to erosion Hydrologically, extinct seamounts reactions from a seawater-dominated and subsidence. Coral reefs will meet a may continue to act as long-term hydro- environment to a freshwater system similar fate if coral growth does not keep thermal siphons (Fisher and Wheat, (Clague and Dixon, 2000). Other conse- pace with subsidence. In some cases, 2010), possibly providing the dominant quences of the freshwater lens include volcanically dormant islands may show mechanism for the exchange of fluids freshwater seeps in shallow marine a short period of volcanic rejuvenation between seawater and sediment-covered settings, and the potential “lubrication” that may be reflected in the formation old oceanic crust. This chemical exchange of faults at depth due to increased hydro- of volcanic cones that rise above the remains to be quantified but may be static pressure from a groundwater erosion surface of its otherwise flat profound in terms of global geochemical that commonly is located at elevations top. Following cessation of volcanism, budgets and in providing a habitat for well above sea level. drowning islands and reefs eventually microbial activity. Large submerged Ocean islands and their coastal will become part of a diverse group of seamounts can substantively impact shallow submarine systems are biologi- extinct seamounts that includes many ocean circulation, including vertical cally complex environments that benefit that never reached Stages 2–4. Former mixing and the generation of turbulence in particular from the availability of islands and coral reefs can be recog- that determines the sedimentation photosynthesis as an energy source, as nized as seamounts with flat summits patterns at and down-current from the well as from the fertility of volcanic soils. (“”). After their submergence, seamount (Lavelle and Mohn, 2010). Ocean islands also often provide nesting all the extinct seamounts remain intact Extinct seamounts are considered grounds for seabirds whose presence can and virtually unchanged until they biological hotspots (Danovaro et al., result in deposition of guano that may be are consumed by subduction or ocean 2009), with thriving microbial commu- introduced to the ocean by weathering basin closure. Some may be older than nities and sessile megafauna colonizing or runoff, resulting in increased fertility 140 million years, giving us the oldest, their surfaces (see Box 7 on page 128 of on land and in the surrounding shallow most stable on the planet. this issue [Etnoyer, 2010]). Microbial ocean. Islands and volcanic reefs in Despite the absence of volcanism or communities are known to be associated tropical and subtropical climates provide erosion, extinct seamounts neverthe- with the venting or seepage of fluids substrates for carbonate reefs, that may less serve some significant geological, from the seamount, but it is also likely range from minor shoreline reefs or hydrological, and oceanographic func- that they form communities inside massive coral reefs that eventually may tions. With time, seamount surfaces the seamount, taking advantage of a cover the entire subsiding volcano. If are encrusted with ferromanganese range of chemical nutrient or energy

68 Oceanography Vol.23, No.1 sources provided by hydrothermal a as in the Costa Rica arc and seamounts are facilitated by their circulation. Surface-colonizing fauna at its intersection with the Cocos Ridge. role as hydrothermal siphons and by include, in particular, filter feeders such The tectonism involved in seamount their own hydrothermal convection as deepwater corals, especially along subduction also results in the opening systems. The latter is driven initially ridges and rift zones where they are of new fluid pathways and brings cold by intrusive activity in the underlying exposed to maximum flow of seawater. sediments and seamount materials oceanic crust and then by intrusions Deepwater corals are valuable recorders of paleoclimate for at least the past 200,000 years (Robinson et al., 2007), From over half a century of modest with individual corals recording climate information for their life spans of up to research efforts, it is clear that seamounts are several thousand years. most rewarding and interesting targets for future oceanographic research. Stage 6 “ Seamount life spans end when they reach a subduction zone or when their host ocean basin closes due to the colli- into contact with warmer materials in into seamounts as they develop their sion of two continental plates. Tectonic the subduction system. These effects magma plumbing systems. Fluxes” are processes that affect geohazards, as combine, cause top-down prograde high because of the high permeability well as chemical fluxes and biological metamorphism, and allow fluids to of the volcaniclastic rocks and their processes, mainly control this phase. escape, potentially with consequences for large volume fraction, and the lack of Seamount subduction may follow substantial geochemical fluxes in an arc sedimentary cover that would impede different paths, depending on subduc- system (Staudigel et al., 2010b). fluid flow. Fluxes are relevant to ocean tion geometry. In arc systems with Although there is almost nothing chemistry, arc mass balances, and the pronounced accretionary wedges, known about the biology of this final chemical heterogeneity of the mantle seamount subduction leaves promi- stage of seamount evolution, it is very through seamount subduction. nent scars in the frontal portion of likely that the enhanced fluid flow also Reactive surfaces and the chemical the wedge (e.g., Costa Rica). In the results in substantial microbial activity, fluxes from hydrothermal flow support western Pacific, seamount subduction perhaps providing a link between micro- the development of biological communi- commonly begins with faulting and bial activity and prograde metamor- ties that take advantage of the chemical breakup (e.g., Capricorn and Osborne phism in the subducting slab. energy and nutrients provided by seamounts) before the seamount seamounts. Small seamounts mostly descends into the trench. As seamounts Conclusions act as portals to the underlying oceanic become entirely subducted, they can Seamounts are geological and biological crust, whereas larger seamounts temporarily lift up segments of the over- hotspots that connect lithosphere, are likely to include their own deep lying arc system, potentially creating a hydrosphere, and/or biosphere in ways biosphere. Both are likely to be impor- blockage to continued subduction that that evolve systematically as a seamount tant. Counting the larger seamounts may be released in rupture and associ- grows. These processes are likely to have alone, they represent a biome the size of ated seismic activity (e.g., Scholz and a profound impact on geochemical fluxes Europe or larger (see Box 12 on page 206 Small, 1997; Watts et al., 2010). Descent and the development of seafloor commu- of this issue [Etnoyer et al., 2010]). of a major topographic feature, like a nities and deep biosphere communi- Significant geohazards may be seamount chain, may have a profound ties, and, finally, they may result in associated with very large seamounts, impact on a subduction zone, potentially hazards to humankind. which pose navigational hazards and lifting up the entire arc and/or creating Geochemical fluxes between seawater produce explosive activity and tsunamis

Oceanography March 2010 69 associated with sector collapse. These Chadwick, W.W. Jr., K.V. Cashman, R.W. Embley, Denlinger, R.P., and P.G. Okubo. 1995. Structure H. Matsumoto, R.P. Dziak, C.E.J. de Ronde, of the mobile south flank of Kilauea hazards are responsible for some of T.-K. Lau, N. Deardorff, and S.G. Merle. Volcano, . Journal of Geophysical the largest geologic disasters recorded 2008a. Direct video and Research 100(B12):24,499–24,507. observations of submarine explosive erup- Dieterich, J.H. 1988. Growth and persistence in human history. tions at NW Rota-1 Volcano, Mariana Arc. of Hawaiian volcanic rift zones. Journal of All three processes are likely to be Journal of Geophysical Research 113, B08S10, Geophysical Research 93(B5):4,258–4,270. doi:10.1029/2007JB005215. Embley, R.W., E.T. Baker, D.A. Butterfield, globally relevant, but none are known to Chadwick, W.W., D.J. Geist, S. Jonsson, M. Poland, W.W. Chadwick Jr., J.E. Lupton, J.A. Resing, the extent needed to make meaningful D.J. Johnson, and C.M. Meertens. 2006. A C.E.J. de Ronde, K.-I. Nakamura, V. Tunnicliffe, volcano bursting at the seams: Inflation, J.F. Dower, and S.G. Merle. 2007. Exploring estimate of fluxes, total biomass, extent faulting, and eruption at Sierra Negra Volcano, the submarine ring of . Oceanography of primary carbon fixation, or a realistic Galápagos. Geology 34(12):1,025–1,028. 20(4):68–79. Chadwick, W.W. Jr., I.C. Wright, U. Schwarz- Emerson, D., and C.L. Moyer. 2010. probability of geohazard threats. This Schampera, O. Hyvernaud, D. Reymond, and Microbiology of seamounts: Common lack of detail results from less than C.E.J. de Ronde. 2008b. Cyclic eruptions and patterns observed in community structure. sector collapses at Monowai , Oceanography 23(1):148–163. one percent of all seamounts having Kermadec Arc: 1998–2007. Geochemistry, Etnoyer, P.J. 2010. Box 7: Deep-sea corals on been studied even in a cursory way, Geophysics, Geosystems 9, Q10014, seamounts. Oceanography 23(1):128–129. doi:10010.11029/12008GC002113. Etnoyer, P.J., J. Wood, and T.C. Shirley. 2010. and much of what we know of these Chadwick, W.W., D.A. Butterfield, R.W. Embley, Box 12: How large is a seamount biome? processes is drawn from a few, often V. Tunnicliffe, J.A. Huber, S.L. Nooner, Oceanography 23(1):206–209. and D.A. Clague. 2010a. Spotlight 1: Axial Fisher, R.V., and H.-U. Schmincke. 1984. singular, well-studied examples. From Seamount. Oceanography 23(1):38–39 Pyroclastic Rocks. Springer Verlag, Berlin, over half a century of modest research Chadwick, W.W., R.W. Embley, E.T. Baker, 472 pp. J.A. Resing, J.E. Lupton, K.V. Cashman, Fisher, A.T., and C.G. Wheat. 2010. efforts, it is clear that seamounts are R.P. Dziak, V. Tunnicliffe, D.A. Butterfield, Seamounts as conduits for massive fluid, most rewarding and interesting targets and Y. Tamura. 2010b. 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