The Geological History of Deep-sea Volcanoes

8/3/2019 The Geological History of Deep-sea Volcanoes

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Oceanography V.23, N.158

M o u N ta i N s i N t h e s e a

the GeoloGical historyo Deep-sea VolcaNoesBiosphere,

hyDrosphere,aND lithosphere

iNteractioNs


B y h u B e r t s t a u D i G e l a N D D a V i D a . c l a G u e

emgn bmn vn n N, tng, M 18, 2009. T n

mn d v bmn n, b g

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m n mn. Photo credit: Dana Stephenson/Getty Images


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Oceanography M 2010 59

aBstract. Te geological evolution o seamounts has distinct inuence on their

interactions with the ocean, their hydrology, geochemical uxes, biology, resources,and geohazards. Tere are six geological evolutionary stages o seamounts: (1) small

seamounts (1001000-m height), (2) mid-sized seamounts (>1000-m height, > 700-m

eruption depth), (3) explosive seamounts (< 700-m eruption depth), (4) ocean islands,

(5) extinct seamounts, and (6) subducting seamounts. Troughout their lietimes,

seamounts oer major passageways or uid circulation that promotes geochemical

exchange between seawater and the volcanic oceanic crust, and seamounts likely

host signicant microbial communities. Water circulation may be promoted by

hydrothermal siphons in conjunction with the underlying oceanic crust, or it may

be driven by intrusions inside seamounts rom Stage 2 onward. Geochemical uxes

are likely to be very large, primarily because o the very large number o Stage 1seamounts. Intrusive growth o seamounts also initiates internal deormation that

ultimately may trigger volcano sector collapse that likely peaks at the end o the main

volcanic activity at large seamounts or islands. Explosive activity at seamounts may

begin at abyssal depth, but it is most pronounced at eruption depths shallower than

700 m. Wave erosion inhibits the emergence o islands and shortens their liespans

beore they subside due to lithosphere cooling. Once volcanism ends and a seamount

is submerged, seamounts are largely unaected by collapse or erosion. Troughout

their histories, seamounts oer habitats or diverse micro- and macrobiological

communities, culminating with the ormation o coral rees in tropical latitudes.

Geological hazards associated with seamounts are responsible or some o the largest

natural disasters recorded in history and include major explosive eruptions and large-

scale landslides that may trigger tsunamis.

iNtroDuctioN

Seamounts are the least explored major

morphological eatures on Earth.

Hundreds o thousands o them are

dispersed through the ocean basins, with

less than 0.1% known rom any direct

observation such as an echosounding,

or any kind o sampling. Most o what

we know about seamounts comes rom

satellite observations that sense them

indirectly rom the centimeter-size

deections they impose on sea surace

topography (e.g., Wessel et al., 2010).

Tis indirect method reliably identi-

es only eatures larger than 1500 m

in height and does not oer any topo-

graphic detail below this scale. Tis state

o exploration reveals nothing about

a seamounts geology. In act, this lack

o detail would be considered entirely

unacceptable or topographic eatures

on land, eectively all o which are

now sampled and mapped by direct

measurements with a resolution better

than 30 m in vertical and 100 m in

horizontal dimensions. Seamounts are

the last major rontier in our exploration

o Earths surace.

Te minimal level o seamount explo-

ration contrasts with their signicance

to science and society. Tey have rich

sheries (Pitcher et al., 2010), are marine-biological and microbiological hotspots

(Danovaro et al., 2009; Emerson and

Moyer, 2010), and have a signicant

geochemical impact, with chemical uxes

that allow mantle-derived materials to

interact with the hydrosphere (Fisher

and Wheat, 2010). Hence, seamounts

are key points o intersection among the

biosphere, hydrosphere, and lithosphere,

and they are globally relevant.In this paper, we explore how

seamounts grow, collapse, and age,

and how these processes interact with

the hydrosphere and impact biological

processes. Te reader is reerred to earlier

reviews by Batiza and White (2000),

Schmidt and Schmincke (2000), and a

recent monograph (Pitcher et al., 2007).

DeiNiNG a seaMouNt

Tere is no uniorm denition or the

term seamount that equally satises all

scientic disciplines. Denitions reect

diverse interpretative perspectives and

disciplinary approaches that can be quite

specic to the needs o a particular scien-

tic community (see Box 1 on page 20

o this issue [Staudigel et al., 2010a]).

We use here a denition that centers

on seamounts as isolated geological

eatures on the seaoor. Seamounts are

typically volcanoes that orm on top o

the oceanic crust with their own central

magma supply and hydrothermal system

Tis simple distinction can become more

complex when isolated volcanoes orm

right on the mid-ocean ridge axis, such

as Axial Seamount on the Juan de Fuca


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Ridge (see Spotlight 1 on page 38 o this

issue [Chadwick et al., 2010a]).

Isolated volcanoes on the seaoor

come in a range o sizes whereby the

smallest ones are typically reerred to

as abyssal hills and the larger ones asseamounts. We use 100 m as the tallest

abyssal hill and smallest seamount,

ollowing a denition o Schmidt and

Schmincke (2000). By this denition,

seamounts may orm in any tectonic

setting; they may have conical, at-

topped, or complex shapes; and they

may or may not have carbonate or

sedimented caps. We also include

oceanic islands in our denition o aseamount, considering them as very

large seamounts that breached the sea

surace. Viewing islands as a subgroup

o seamounts is unusual because most

views ocus on the distinct nature

o islands in terms o their volcanic

eatures, subaerial weathering, and

land-based biology (see Gillepsie and

Clague, 2009). However, our grouping

makes sense rom a geological point o

view in which volcanic islands are the

summit regions o very large seamounts

that are temporarily exposed above sea

level. Most oceanic islands start out as

seamounts and turn back into seamounts

toward the ends o their lie cycles.

GeoloGical settiNG

Seamounts orm in an oceanic geological

setting that is much more dynamic than

the continents, resulting in much larger

vertical and horizontal displacements.

Oceanic crust exes and bends relatively

easily rom the processes involved in

seamount ormation (see Koppers and

Watts, 2010). It may bulge upward

rom the buoyancy o rising mantle or

due to heating o the lithosphere rom

below. However, as soon as a seamounthas reached a critical size, subsidence

becomes the dominant vertical orce,

caused by the bending o the oceanic

crust into the mantle as it yields to

the seamount load (Watts, 2001). For

example, data rom drowning coral

terraces on Hawai`i suggest a subsid-

ence rate o 2.6 mm yr -1 or the Island

o Hawai`i (Moore and Fornari, 1984;

Moore et al., 1996). However, on a muchlonger time scale extending millions o

years, seamounts also subside owing to

the cooling o the lithosphere they are

built upon. Most seamounts are ormed

within a ew hundred kilometers o

mid-ocean ridges at roughly 2.5-km

water depth. Tese seamounts will

subside with the underlying cooling

lithosphere to about 5-km water depth

within 60 million years and possibly

down to 10-km depth when they arrive

at subduction zones. Tis journey rom

mid-ocean ridge to subduction zone may

involve thousands o kilometers o hori-

zontal displacement. For example, the

majority o the seamounts in the central-

western Pacic originally ormed in the

South Pacic 80140 million years ago.

GeocheMistry

Seamount volcanoes are ormed by two

principal mantle melting processes.

Mid-ocean ridge (MOR) basalt and

oceanic intraplate volcanoes (OIVs)

are ormed by decompression melting

o mantle materials that buoyantly rise

in upward convective ow and/or in

response to plate extension and thin-

ning. Subduction zone volcano magmasare commonly ormed by the lowering

o the melting point o the sub-arc

mantle due to addition o volatiles rom

the descending slab. Tese dierent

melting processes have a proound

impact on the chemistry o seamount

rocks and the chemical evolution o

the volcanoes ormed.

Seamounts close to MORs and in OIV

settings erupt mostly basaltic (tholeiiticand alkalic) magmas, whereas subduc-

tion zone volcanics are predominantly

composed o calc-alkaline (andesitic or

arc-theoleiitic) magmas. All seamount

rocks are ormed by mantle melting

processes that involve smaller degrees

o melting than are characteristic or

the generation o the oceanic crust

itsel. As a result, their incompatible

element abundances may be an order o

magnitude higher, and seamount uxes

o these elements may dominate global

mantle uxes even though seamounts are

substantially less voluminous than the

elongated volcanoes orming the MORs

themselves. Subduction zone magmas

tend to have overall higher Si, alkali,

and volatile abundances, and lower Mg

contents, than OIVs and MORs. As a

result, they are more explosive and tend

to have higher viscosities.

OIVs display a characteristic

geochemical evolution over their

520-million-year volcanic histories

(Clague and Dalrymple, 1987). Four

geochemical stages are commonly

distinguished, originally dened or

the Hawaiian Islands but also ound to

Hubert Staudigel ([email protected]) is Research Geologist and Lecturer, Institute

of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of

California, San Diego, La Jolla, CA, USA. David A. Clague is Senior Scientist, Monterey Bay

Aquarium Research Institute, Moss Landing, CA, USA.


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characterize many other OIVs (Konter

et al., 2009). OIV ormation at Hawaiìs

most recent active volcano, Lìhi, starts

with small erupted volumes o basaltic

rocks with diverse compositions gener-

ated by small degrees o mantle melting.Te subsequent main, shield-building

stage then produces up to 98% o the

entire volcano typically rom rocks

ormed by relatively large melt rac-

tions in the mantle, yielding tholeiitic

or mildly alkalic basalts. Tis is closely

ollowed by a cap o alkalic rocks that

are ormed by smaller degrees o partial

melting than the shield magmas. Some

OIVs have a last stage o post-erosionalor rejuvenated volcanism, aer a

volcanic hiatus o 1.510 million years.

Rejuvenated volcanism typically consists

o very small volumes o highly alkalic

lavas that oen include mantle xenoliths.

Tese stages o geochemical evolution

provide insights into the temporal evolu-

tion o mantle melting with substan-

tial implications or the geodynamic

processes involved. An interesting act

is that the majority o seamounts have

completed their geochemical evolutions,

and hence their suraces most likely

display the latest stages o volcanism.

As a consequence, the majority o their

earlier shield-building evolution may be

largely hidden rom direct sampling.

litholoGical rock types

Seamounts, like all volcanoes, are ormed

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 ultramac

intrusions. Te intrusion o dikes is likely

to be one o the key means o growth o

volcanoes with pronounced ri zones

such as Hawaiìs Kilauea Volcano

(Dieterich, 1988; Leslie et al., 2004).

Kilaueas East Ri displays volcanic

spreading rates o about 10 cm yr -1,

pushing the whole volcano outwardrom Mauna Loa along a near-horizontal

slip surace at 89-km depth (Hill and

Zucca, 1987; Denlinger and Okubo, 1995;

Delaney et al., 1998). Such spreading

may be entirely intrusive, as indicated by

the common earthquake swarms on the

ri zones o Kilauea that are not associ-

ated with eruptions (Klein et al., 1987).

Erosion has also exposed the ri zones

on several extinct Hawaiian volcanoes,particularly Koolau on Oahu, that also

show large volumes o dikes (Walker,

1987; Figure 1). At the uplied La Palma

seamount series (Canary Islands), sills

are the dominant orm o intrusion.

Teir cumulative thickness suggests

upli o the overlying volcano by about

2 km (Staudigel and Schmincke, 1984;

Staudigel et al., 1986). Sills may also

play a key role in the growth o some

Galpagos volcanoes (Amelung et al.,

2000; Chadwick et al., 2006).

Te deeper portions o seamounts arelikely to contain magma reservoirs that

crystallize to gabbroic and ultramac

intrusions (Upton and Wadsworth,

1972). Such intrusions may occur at a

range o depths below the summit and

are commonly sampled as xenoliths

in later lavas (Clague and Dalrymple,

1987). At La Palma, such intrusive rocks

have been ound at stratigraphic depths

o almost 5 km below the likely originalsummit o the seamount (Staudigel et al.,

1986), whereas active magma chambers

at Kilauea may be only 800 m below

the summit (Almendros et al., 2002).

Intrusive activity at seamounts results in

ination, over-steepening o anks, and

ultimately ank collapse.

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Eruptive rocks dominate the surace

and the upper portions o a seamount,

hence they are the major apparent

component o volcano growth, with two

major submarine types: lava ows and

volcaniclastics (Staudigel and Schmincke1984; Batiza and White, 2000; Schmidt

and Schmincke, 2000).

Pillow lavas are the most common

submarine lava ows and may be ound

in almost any subaqueous setting

(Figure 2A,B). Less abundant are sheet

ows and massive ows that tend to

indicate higher mass eruption rates.

Sheet ows typically are ound on

shallow slopes and settings where they

may be ponded (Figure 2C; Staudigeland Schmincke 1984, Clague et al., 2000,

2009). All submarine lava ows have

pronounced, quenched, glassy margins.

Pillow lavas are named aer the curved,

occasionally round, cross section o the

lava tubes that orm these ows. Pillow

lava cross sections commonly display

characteristic V-shaped bottoms as

the lava tubes mold themselves to the

underlying pillow lavas. Individual

lava tubes may occur in a range o tubediameters, rom a ew tens o centimeters

to 200 cm in diameter (Clague et al.,

2009), decreasing in size with distance

rom the eruption center and upward

in a section (Figure 7 in Staudigel

and Schmincke, 1984).

Seamount volcaniclastic rocks

are sedimentary rocks ormed by

mechanical breakup o lava ows or by

explosive-eruptive ragmentation mech-anisms (Fisher and Schmincke, 1984);

they are the most common volcanic

rocks in shallow submarine volcanoes

(e.g., Staudigel and Schmincke, 1984;

Garcia et al., 2007). Mechanical rag-

mentation prevails during the collapse

o unstable volcanic rock ormations

and spallation o pillow margins.

Explosive ragmentation may be caused

by the expansion o gas bubbles inside

the magma, magma interaction with

external water, or grain-to-grain inter-

action during an eruption. Although

mechanical ragmentation processes

have no (conning) pressure depen-

dence, explosive ragmentation does. Te

water-depth dependence o explosive

ragmentation processes is due to pres-

sure dependence o magma volatile solu-

bility and the pressure dependence o

the extraordinarily large volume change

in water when it transorms rom liquid

to gas. At one atmosphere pressure, this

volume change is by a actor o 1600. It is

important that the pressure dependence

o gas solubility varies substantially with

the dissolved species. CO2 may outgas at

relatively high pressures, whereas H2O

3 m

2 m

50 m

50 cm

1 m

50 m

A B

C D

E F

g 2. d nd mg bmn vn : (a) p v m bmn

x Vn smn (1600m d). (B) Dd mn m an

v n png, r. (c) s bmn f mbng b

f m Vn smn (1712m d). (D) p gmn b m l` smn,

1000m d. (e) tb m nn n g mgn m D s Dng

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and SO2 tend to outgas at much lower

pressures. Sulur may separate rom

magma as an immiscible liquid at a wide

range o pressures, but it contributes to

the explosivity o lava only at low pres-

sure when SO2 bubbles can orm in themagma. Geological observations and

theoretical considerations point to a

relatively consistent critical depth or the

onset o major explosive activity. A more

than 1800-m-thick section o seamount

volcanic rocks at the La Palma seamount

series shows a dramatic increase in

volcaniclastic rocks at about 1000-m

water depth (Staudigel and Schmincke,

1984). Teoretical considerations placethe explosive transition closer to 700 m

or explosions caused by the interaction

o external water with magma (Peckover

et al., 1973) as well as the explosive

outgassing o magma itsel (McBirney,

1963). However, some variance may be

expected as a result o magma composi-

tion and eruption rates, and minor pyro-

clastic activity can be widespread even at

abyssal depths (Clague et al., 2009).

Pillow ragment breccias are prob-

ably the most common mechanically

ragmented volcaniclastic rocks at

seamounts, oen ormed during the

collapse o steep pillow lava ow ronts

(Figure 2D). Pillow breccias may orm

when pillow lavas intrude into their own

ne-grained volcaniclastics, with highly

irregular, amoeba-shaped pillow cross

sections that are completely enclosed in

a hyaloclastite matrix. Pillows may also

intrude into ne-grained epiclastic sedi-

ments (peperites). Explosive seamount

volcaniclastics rom magmatic outgas-

sing may orm highly vesicular scoria

that may occur as nearly pumiceous

deposits or be broken up into ner

particles. Te ragmentation o dense

spatter may also be responsible or the

ormation o well-sorted, ne-grained

hyaloclastite (glass sand; Clague et al.,

2009). Most submarine volcaniclastics

are missing most o the ne grain-size

raction, which remains suspended inturbulent waters and is deposited slowly

in more distant areas surrounding the

seamount eruption site. Tis lack o ne

grains gives seamount volcaniclastics a

relatively high permeability.

Lithology, particularly the abundance

o coarser-grained volcaniclastic deposits,

has a substantial impact on chemical

uxes and on the development o micro-

bial habitats. Key controls include thehigh permeability o these deposits,

which allows or seawater circulation,

and a high abundance o volcanic glass,

which is highly reactive with seawater.

Te alteration o volcanic glass is the

primary process that controls low-

temperature alteration uxes between

seawater and basalt (e.g., Staudigel and

Hart, 1983), and glass is a particularly

suitable material or colonization by

microbial communities (Figure 2E,F),

up to the moment they are sealed o by

authigenic mineral deposition.

hyDrotherMal aND

BioloGical actiVity

Seaoor volcanism generally produces

hydrothermal systems that are likely to

be associated with substantial microbial

and possibly macrobiological activity.

Seamount hydrothermal water ow may

be driven by intrusive activity, or the

seamount may act as a hydrothermal

siphon (Fisher and Wheat, 2010),

allowing the recharge o seawater to the

oceanic crust and the escape o oceanic

crustal uids that may be otherwise

trapped by a blanket o impermeable

sediments. Hydrothermal circulation at

seamounts is signicant, because they

are made o rather permeable volcani-

clastic rocks, and sediment cover does

not signicantly inhibit circulation.

Seamounts also have a topographicadvantage in that water can enter on

the anks and then rise within the

seamount due to its raised geothermal

gradients resulting rom hydrothermal

activity (Schiman and Staudigel,

1994) or simply by their topography.

Furthermore, seamount volcanic rocks

are rich in volcanic glass, and their

alteration contributes preerentially to

hydrothermal chemical uxes. Hence,seamount hydrothermal uxes may

be substantially more important than

suggested by their volume relative to

MORs. Some indications or large uxes

come rom the large uxes o3He or CO2

(L`ihi; see Spotlight 3 on page 72 o this

issue [Staudigel et al., 2010c]; Lupton,

1996; Hilton et al., 1998) or Mn and 3He

anomalies (Vailuluu; see Spotlight 8 on

page 164 o this issue [Koppers et al.,

2010]; Hart et al., 2000; Staudigel et al.,

2004). ogether, seamount and oceanic

crust hydrothermal systems play key roles

in buering the chemical composition

o seawater, but the contribution rom

seamounts remains less well known and

is likely to vary among elements.

Microbial communities have been

documented rom a range o seamounts,

and there is abundant evidence that they

might play a role in a potentially very

large deep oceanic crustal biosphere.

Emerson and Moyer (2010) review

much o the evidence rom microbio-

logical studies at seamounts, showing

that there are very diverse microbial

communities, oen with novel organisms

with metabolic capabilities (including


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heterotrophy) and microbes that oxidize

(and reduce) iron, manganese, and sulur.

Microbial communities may thrive, in

particular, on the reducing potential o

these elements in basalt, which is strongly

enhanced by the high abundance o keynutrients in these rocks that are depleted

in seawater (Fe, Mn, Ni, P). Fluids

extracted rom a sealed drillhole into

Baby Bare Seamount (Cascadia Basin,

Northeast Pacic Ocean) yielded a highly

diverse microbial community (Cowen

et al., 2003), suggesting that microbes

are active inside the crust as well. Te

latter is also suggested by the study o

microbial trace ossils in volcanic glassalteration that imply that bioalteration

dominates glass alteration in the upper

oceanic crust, at least to a depth o 500 m

(Figure 2E,F; Furnes and Staudigel, 1999;

Staudigel et al., 2008).

Te interplay o ocean circulation

and hydrothermal activity at seamounts

also has a substantial impact on the

macroauna ound at their suraces.

For example, the crater o Vailuluu

Seamount displays substantial metazoan

mortality in the deepest part o the

crater, whereas the summit o a new

volcano growing in the crater displays

a thriving population o eels (Staudigel

et al., 2006). Similar complex benthic

zonations likely occur at many other

seamounts, such as the liquid sulur

lakes at Daikoku and Nikko volcanoes

(Embley et al., 2007).

the structural eVolutioN

o seaMouNts

Seamounts evolve in six stages that

are structurally distinct (Figure 3).

(1) Small seamounts (< 1-km tall)

and abyssal hills (< 100-m tall) are

seaoor volcanoes built on the oceanic

crust largely by eruptive processes.

(2) Mid-size seamounts are taller than

about 1000 m and remain at eruptive

depths deeper than 700 m, representing

largely nonexplosive volcanoes with

magmatic plumbing systems thatbecome an increasingly important part

o the seamount itsel. (3) Explosive

seamounts orm at ocean depths shal-

lower than 700 m and are mostly covered

by volcaniclastics. (4) Islands orm when

a seamount breaches the sea surace,

shiing to subaerial processes, including

subaerial weathering, soil ormation,

and erosion. (5) Extinct seamounts

have completed all volcanic activity andsubsided below sea level, sometimes

orming atolls. (6) Seamounts meet

the end o their geological lie during

closure o an ocean basin or by subduc-

tion. Although these six stages describe

the complete development o very large

seamounts, most seamounts never reach

the island stage. Smaller seamounts may

be entirely buried by sediments by the

time they reach a subduction zone.

sg 1

Small seamountswith total heights o1001000 m are by ar the most common

type o seamount, even though they

are oen difcult to distinguish rom

the roughness o the abyssal oceanic

crust topography that may occur on a

similar scale. Te key distinctive eature

is their oen circular morphology that

is superimposed on a seaoor abric

that largely is made up o elongated

MOR volcanoes or tectonic blocks.

Tere may be as many as 25 million o

them worldwide (Wessel et al., 2010).

Te majority o these seamounts orm

on relatively young crust, at water

depths between 23 km, on bare rock,

or on thinly sedimented seaoor. Teir

dominant eruptive lithologies include

mostly pillow lavas and approximately

20% volcaniclastics (as ound at Ocean

Drilling Program/Deep Sea Drilling

Project Site 417A and the deep-watersection at La Palma; Staudigel and

Schmincke, 1984). It is likely that

seamounts on sedimented seaoor may

initially orm intrusions into the sedi-

ments and/or peperites, beore they

erupt pillow lavas that quickly cover

these rocks. Small seamounts are likely

to include almost no intrusive rocks,

except or their eeder dikes, which has

important consequences. Te underlyingoceanic crust controls these seamounts

plumbing systems and tectonics. Tere

also is no or very little intrusive ination

o the reestanding part o the seamount,

which makes it very unlikely that these

volcanoes develop any major collapse

eatures. And, nally, the reaction zone

or their hydrothermal uids is located

inside the oceanic crust, and their hydro-

thermal venting is strongly linked to the

hydrothermal cooling o the underlying

oceanic crust. Intense low-temperature

hydrothermal circulation in small

seamounts results in extreme enrich-

ments in seawater-derived chemical

components, particularly in volcaniclas-

tics but also in lava ows (e.g., Site 417A;

Staudigel et al., 1996). It is quite likely

that the very large number o small

seamounts and their extreme alteration

makes them major players in global

geochemical cycles and as globally rele-

vant habitats or microbial communities.

sg 2

Mid-size seamountsare dened hereas volcanic eatures that are tall enough

(at least 1000 m in height) to have a


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magma plumbing system above the level

o the oceanic crust, but that have not

yet shoaled enough to be dominated by

shallow-water explosive activity (which

starts at ~ 700-m water depth). Tere

may be hundreds o thousands o suchseamounts (Wessel et al., 2010), and

some o them may be quite largeall

Hawaiian seamounts had to grow to

4000-m heights above the seaoor

beore they became explosive in Stage 3.

Mid-size seamounts are likely to be

mostly made o eruptive rocks, with

an increasing raction o intrusives as

they increase in size. Based on observa-

tions at La Palma, the eruptive rocks on

typical mid-size seamounts may consist

o a majority o pillow lavas with about

20% volcaniclastics.

As seamounts increase in size, theybegin to develop internal magma

reservoirs above the oceanic crust. Tis

attribute has two important conse-

quences. First, intrusions begin to

inate the volcano, and this ination

soon dominates its internal stresses and

determines the development o ssures

and ri zones as well as, ultimately, the

extent and geometry o volcano collapse.

Second, intrusions introduce heat into

shallower portions o the volcano, estab-

lishing hydrothermal convective cooling

systems that are increasingly decoupled

rom the underlying oceanic crust. AtVailuluu Seamount, earthquake loca-

tions cluster below hydrothermal vents

along main structural trends dened

by ri zones (Konter et al., 2004), indi-

cating a close relationship among the

development o permeability, intrusive

geometry, and hydrothermal cooling.

Te depth o the earthquakes suggests

1

2

3

4

5

6

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mn vn

dnd b mn

z, b nnng

m

n, nd b x

. sg 1

n m

mn (1001000 m),

mgm mbng

m d n

n , b vn. sg 2 nd

mdz vn

xdng 1000 m n

g nd

v d g n

700 m,

d bgnnng

xv v. sg 3

db xv

g mn bv

700m n d,

nd sg 4

mn mgd

mm mng ndnd bn .

sg 5 nd

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nmd b bdn.


9/14


10/14


photosynthesis becomes a viable energy

source or carbon xation. In tropical

regions, this can result in the ormation

o massive carbonate rees. Such rees are

among the most diverse and productive

habitats on Earth.

sg 4

Islands are born when shallow subma-

rine volcanism allows a seamount to

emerge above sea level and to transi-

tion into subaerial volcanic activity.

Tis transition is a prolonged process.

Historically, most newly born islands

have disappeared shortly aer their

emergence (e.g., Metis Shoal o onga,Myojinsho o the Izu Ogosoawara

group south o Japan, Graham Island

in the Mediterranean Sea). Tey disap-

pear because the continuous power

o sea-surace wave erosion tends to

out-compete constructive volcanic

growth by minimally alteration-resistant

volcaniclastic rocks. In act, persistent

emergence may only be possible during

periods o very high eruption rates,

and the ormation o a more resistant

lava veneer that protects the underlying

clastic rocks rom wave erosion. It is also

conceivable that emergence is critically

aided by intrusive activity that may li

the top o a seamount.

Emergence o a volcanic island has

important geological consequences in

terms o eruptive and intrusive processes

and volcano collapse, erosion, and

weathering. Eruptive rocks at ocean

islands are dominated by lava ows that

cool at substantially slower rates than

their submarine equivalents. However,

many o these ows enter the sea, where

they produce large quantities o glass-

rich volcanic rocks that are deposited on

lava benches, submarine island anks, or

the surrounding seaoor. In particular,

ocean arcs, but also ocean islands, may

produce substantial explosive (Plinian)

eruptions that can distribute volcanic ash

as discrete layers up to 2000 km down-

wind rom the source (oba/Sumatra).Tese explosive eruptions constitute

major volcanic hazards, particularly

when hydrovolcanic processes resulting

rom contact with seawater ampliy the

eruptions. Major catastrophic erup-

tions include the 1630 BCE Santorini

cataclysm that is likely to have termi-

nated the Minoan culture, or the 1883

Krakatoa eruption that was associated

with more than 30,000 casualties in theIndian Ocean area.

Intrusive activity at islands is demon-

strably a major actor in their evolu-

tion and collapse. Collapse eatures

are most likely and most prominent in

the largest seamounts and islands with

well-established, long-lived magmatic

plumbing systems. Tey are most

commonly ound near the point o

emergence o major ri zones romthe main body o a volcano (Figure 4).

Sector collapse may mobilize volumes

up to 3000 km3 (Satake et al., 2002), and

the debris rom such ows may cover an

ocean area o 10,00015,000 km2 with

> 1-km-size blocks up to 180 km rom

their site o origin (Moore and Clague,

2002). Such events leave a major scar on

the side o the volcano, have a substantia

eect on benthic lie around the volcano,and result in catastrophic tsunamis that

may have runups reaching inland to

localities up to 400 m above sea level,

constituting a major ocean-basin-wide

-6000 -5000 -4000 -3000 -2000 -1000

Depth (m)

15400'E 15420'E

1700'N

1640'N

1620'N

1720'N

DEBRIS

HEADWALL

SCARP

g 4. Dgm

fn Vnd

smn n nn

dg mm m

(k ., 1998). M

n 10 m3 mn

fn mbzd nd

dd > 6 m m

b.


11/14


natural hazard (e.g., at Kohala Volcano

in Hawai`i; McMurtry et al., 2004).

As soon as islands are ormed, they

are subjected to weathering, soil orma-

tion, and erosion, as well as the develop-

ment o groundwater, particularly inenvironments with signicant rainall.

Weathering and erosion products intro-

duce a range o new materials into the

marine environment, including organic-

rich soils and sands with highly reactive

components (olivine, glass) that can

support lie in the ocean. In addition,

the ormation o groundwater on islands

commonly creates a submarine resh-

water lens that alters the hydrothermalreactions rom a seawater-dominated

environment to a reshwater system

(Clague and Dixon, 2000). Other conse-

quences o the reshwater lens include

reshwater seeps in shallow marine

settings, and the potential lubrication

o aults at depth due to increased hydro-

static pressure rom a groundwater table

that commonly is located at elevations

well above sea level.

Ocean islands and their coastal

shallow submarine systems are biologi-

cally complex environments that benet

in particular rom the availability o

photosynthesis as an energy source, as

well as rom the ertility o volcanic soils.

Ocean islands also oen provide nesting

grounds or seabirds whose presence can

result in deposition o guano that may be

introduced to the ocean by weathering

or runo, resulting in increased ertility

on land and in the surrounding shallow

ocean. Islands and volcanic rees in

tropical and subtropical climates provide

substrates or carbonate rees, that may

range rom minor shoreline rees or

massive coral rees that eventually may

cover the entire subsiding volcano. I

conditions are met, rees can get estab-

lished airly quickly, as is evident at the

relatively recent Hawaiian volcanoes

o Hualalai, Kohala, and Mauna Kea.

Overall, coral rees and volcanic ocean

islands are among the most productivesheries on Earth. Tis productivity

may be a result o many processes and

circumstances, including the constant

volcanic input, photosynthesis, and

complex local ocean circulation, due to

the islands roles as ocean stirring rods.

staGe 5

Once volcanism ceases, a volcanic island

will eventually drown due to erosionand subsidence. Coral rees will meet a

similar ate i coral growth does not keep

pace with subsidence. In some cases,

volcanically dormant islands may show

a short period o volcanic rejuvenation

that may be reected in the ormation

o volcanic cones that rise above the

erosion surace o its otherwise at

top. Following cessation o volcanism,

drowning islands and rees eventually

will become part o a diverse group o

extinct seamounts that includes many

that never reached Stages 24. Former

islands and coral rees can be recog-

nized as seamounts with at summits

(guyots). Aer their submergence,

all the extinct seamounts remain intact

and virtually unchanged until they

are consumed by subduction or ocean

basin closure. Some may be older than

140 million years, giving us the oldest,

most stable landorms on the planet.

Despite the absence o volcanism or

erosion, extinct seamounts neverthe-

less serve some signicant geological,

hydrological, and oceanographic unc-

tions. With time, seamount suraces

are encrusted with erromanganese

oxides and phosphorites, depending

on their chemical environment. Tese

mineral groups potentially include very

high concentrations o elements that

are very scarce in seawater (Mn, P, Co,

Ni, i, Pt, T, Pb, rare earth elements;Hein et al., 1997), and thus seamount

suraces are likely to play a role in the

geochemical budgets o these elements

in seawater. Such suraces provide some

unique substrates or microbial growth

and the basis or potential seaoor

mining, in particular or Co and some

high-tech metals such as tellurium

(Hein et al., 2010).

Hydrologically, extinct seamountsmay continue to act as long-term hydro-

thermal siphons (Fisher and Wheat,

2010), possibly providing the dominant

mechanism or the exchange o uids

between seawater and sediment-covered

old oceanic crust. Tis chemical exchange

remains to be quantied but may be

proound in terms o global geochemical

budgets and in providing a habitat or

microbial activity. Large submerged

seamounts can substantively impact

ocean circulation, including vertical

mixing and the generation o turbulence

that determines the sedimentation

patterns at and down-current rom the

seamount (Lavelle and Mohn, 2010).

Extinct seamounts are considered

biological hotspots (Danovaro et al.,

2009), with thriving microbial commu-

nities and sessile megaauna colonizing

their suraces (see Box 7 on page 128 o

this issue [Etnoyer, 2010]). Microbial

communities are known to be associated

with the venting or seepage o uids

rom the seamount, but it is also likely

that they orm communities inside

the seamount, taking advantage o a

range o chemical nutrient or energy


12/14


sources provided by hydrothermal

circulation. Surace-colonizing auna

include, in particular, lter eeders such

as deepwater corals, especially along

ridges and ri zones where they are

exposed to maximum ow o seawater.Deepwater corals are valuable recorders

o paleoclimate or at least the past

200,000 years (Robinson et al., 2007),

with individual corals recording climate

inormation or their lie spans o up to

several thousand years.

sg 6

Seamount lie spans end when they

reach a subduction zone or when theirhost ocean basin closes due to the colli-

sion o two continental plates. ectonic

processes that aect geohazards, as

well as chemical uxes and biological

processes, mainly control this phase.

Seamount subduction may ollow

dierent paths, depending on subduc-

tion geometry. In arc systems with

pronounced accretionary wedges,

seamount subduction leaves promi-

nent scars in the rontal portion o

the wedge (e.g., Costa Rica). In the

western Pacic, seamount subduction

commonly begins with aulting and

breakup (e.g., Capricorn and Osborne

seamounts) beore the seamount

descends into the trench. As seamounts

become entirely subducted, they can

temporarily li up segments o the over-

lying arc system, potentially creating a

blockage to continued subduction that

may be released in rupture and associ-

ated seismic activity (e.g., Scholz and

Small, 1997; Watts et al., 2010). Descent

o a major topographic eature, like a

seamount chain, may have a proound

impact on a subduction zone, potentially

liing up the entire arc and/or creating

a slab window as in the Costa Rica arc

at its intersection with the Cocos Ridge.

Te tectonism involved in seamount

subduction also results in the opening

o new uid pathways and brings cold

sediments and seamount materials

into contact with warmer materials in

the subduction system. Tese eects

combine, cause top-down prograde

metamorphism, and allow uids to

escape, potentially with consequences or

substantial geochemical uxes in an arc

system (Staudigel et al., 2010b).

Although there is almost nothing

known about the biology o this nal

stage o seamount evolution, it is very

likely that the enhanced uid ow also

results in substantial microbial activity,

perhaps providing a link between micro-

bial activity and prograde metamor-

phism in the subducting slab.

coNclusioNs

Seamounts are geological and biological

hotspots that connect lithosphere,

hydrosphere, and/or biosphere in ways

that evolve systematically as a seamount

grows. Tese processes are likely to have

a proound impact on geochemical uxes

and the development o seaoor commu-

nities and deep biosphere communi-

ties, and, nally, they may result in

hazards to humankind.

Geochemical uxes between seawater

and seamounts are acilitated by their

role as hydrothermal siphons and by

their own hydrothermal convection

systems. Te latter is driven initially

by intrusive activity in the underlying

oceanic crust and then by intrusions

into seamounts as they develop their

magma plumbing systems. Fluxes are

high because o the high permeability

o the volcaniclastic rocks and their

large volume raction, and the lack o

sedimentary cover that would impede

uid ow. Fluxes are relevant to ocean

chemistry, arc mass balances, and the

chemical heterogeneity o the mantle

through seamount subduction.

Reactive suraces and the chemical

uxes rom hydrothermal ow support

the development o biological communi-

ties that take advantage o the chemical

energy and nutrients provided by

seamounts. Small seamounts mostly

act as portals to the underlying oceanic

crust, whereas larger seamounts

are likely to include their own deep

biosphere. Both are likely to be impor-

tant. Counting the larger seamounts

alone, they represent a biome the size o

Europe or larger (see Box 12 on page 206

o this issue [Etnoyer et al., 2010]).

Signicant geohazards may be

associated with very large seamounts,

which pose navigational hazards and

produce explosive activity and tsunamis

roM oVer hal a ceNtury o MoDestresearch eorts, it is clear that seaMouNts areMost rewarDiNG aND iNterestiNG tarGets oruture oceaNoGraphic research.


13/14


associated with sector collapse. Tese

hazards are responsible or some o

the largest geologic disasters recorded

in human history.

All three processes are likely to be

globally relevant, but none are known tothe extent needed to make meaningul

estimate o uxes, total biomass, extent

o primary carbon xation, or a realistic

probability o geohazard threats. Tis

lack o detail results rom less than

one percent o all seamounts having

been studied even in a cursory way,

and much o what we know o these

processes is drawn rom a ew, oen

singular, well-studied examples. Fromover hal a century o modest research

eorts, it is clear that seamounts are

most rewarding and interesting targets

or uture oceanographic research. Tis

body o work points the way to a series

o targeted investigations that have

much potential or better understanding

how Earth works.

ackNowleDGeMeNts

We acknowledge Anthony Koppers,

John Mahoney, Bob Duncan, and Rodey

Batiza or helpul and insightul critical

reviews and assistance with illustrations

by Patty Keizer.

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Documents

The Geological History of Deep-sea Volcanoes