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APRIL 2005
492
Garrett Nagle
Island Arcs
Figure 1: Morphology of an island arc system
Sedimentary arc
Bulge
Oceanic
crust
Trench
Outer arc
ridge
Accretionary
prism/wedge
Fore-arc
basin
Volcanic arc
Marginal
back-arc basin
Remnant arc
Island arc
Subduction
complex
Be
100 km
e
on
z
ff
o
ni
0
Introduction
Andes) and inside which basalts
predominated.
Island arc systems are formed when
oceanic lithosphere is subducted
beneath oceanic or continental
lithosphere. They are consequently
typical of the margins of shrinking
oceans such as the Pacific, where the
majority of island arcs are located.
They also occur in the western
Atlantic, where the Lesser Antilles
(Caribbean) and Scotia arcs are
found at the eastern margins of small
oceanic plates isolated by transform
faults against the general westward
trend of movement.
Little could be done to discover the
origin of island arcs until
geophysical data were acquired. It
was not until 1949, when H. Benioff
showed that earthquake epicentres
became progressively deeper as one
went from the ocean side of the
trench to the volcanic arc, that the
idea of a relatively simple, steeply
dipping thrust plane extending from
near the trench to a depth of as
much as 700 km was clearly
established.
Background
Island arcs are recognised as
tectonically active belts of intense
seismic activity containing a chain
or arc of active volcanoes. As early as
the 19th century, W. J. Sollas drew
attention to the correspondence of
the arc-like forms of the
Aleutians/Alaskan Peninsula, the
East Indies (Indonesia), and several
mountain chains to a series of great
circles, and C. Lapworth discussed
the ‘Volcanic Girdle of the Pacific’
(the Pacific ‘Ring of Fire’) as a
continuous ‘septum’ separating
‘plates’ with different histories and
thicknesses. The deepest parts of the
oceans, the deep-sea trenches, were
located on the oceanward side of
these arcs. As the nature of the ‘ring
of fire’ was examined, it was realised
that a line, called the andesite line,
could be drawn around the Pacific
outside which andesites occurred
(named after their type area in the
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By the 1950s, substantial geophysical
data had been acquired around the
Pacific, off Indonesia, and in the
Caribbean suggesting that large slabs
might be dragged down beneath
island arcs along subduction zones
(also known as Benioff zones). It was
not until 1968 that the next
significant advance was made. The
hypothesis of ocean-floor spreading
in the 1960s had postulated that new
lithosphere was being continuously
created. It was recognised that unless
the Earth was expanding, an equal
amount of lithosphere must be being
lost, and this seemed most likely to
happen at the subduction zones. As
the slab of oceanic lithosphere goes
down, it melts partially at about
150–200 km depth, giving birth to
magmas that rise and are extruded in
volcanoes located 150–200 km from
the axis of the trench.
The term ‘island arc’ is commonly
used as a synonym for ‘volcanic arc’,
yet the two terms are not quite the
same. Volcanic arcs include all
volcanically active belts located
above a subduction zone, whether
they are situated as islands in the
middle of oceans or on continents, as
along the west coasts of Central and
South America. True island arcs
include only those separated from
the land by a stretch of water, such
as those in the Caribbean. There is
therefore a continuum of island-arc
types:
1. some are truly intraoceanic,
being situated entirely within the
oceans, for example the
Marianas, New Hebrides,
Solomons, and Tonga in the
Pacific; the Antilles and Scotia
arcs in the Atlantic
2. others are separated from major
continents by small ocean or
marginal basins with a crust that
is intermediate between
continental and oceanic
(Andaman islands, Banda, Japan,
Kuril, and Sulawesi)
3. at the extreme end of the
spectrum are those arcs built
against continental crust, such as
the Burmese and Sumatra/Java
portions of the BurmeseAndaman-Indonesian arc
4. finally the Andean chain, where
the volcanic belt is located
entirely within the continent and
is not therefore an island arc.
The age also varies. Some are very
6
young: less than 10 Ma (10 x 10
years). Others are much older,
dating back at least to the Tertiary
or Cretaceous eras.
April 2005 no.492 Island Arcs
Features of island arcs
Figure 2: The Lesser Antilles volcanic arc: some 500 km3 of volcanic material has
been produced over the last 100,000 years, 20% of which remains in the volcanic arc
The exposed island arc is only one of
a number of features of tectonic
zones that extend from the trench at
the oceanward end to the marginal
or back-arc basin on the continental
side (Figure 1). The generalised
morphology of an island arc system
is shown in Figure 1, although not
all components are present in every
system.
Pyroclastic gravity
flow deposits 1%
Ash fall 0.5%
Dispersed ash 1.5%
Ash fall
60%
Dispersed ash
39%
Pyroclastic gravity
flow deposits 98%
Caribbean
Atlantic
Westerlies
Fore-arc region
Proceeding from the oceanward side
of the system, a bulge about 500 m
high occurs about 120–150 km from
the trench. The fore-arc region
comprises the trench itself, the
subduction complex (the ‘first arc’ or
accretionary wedge or prism) and
the fore-arc basin. The subduction
complex is constructed of thrust
slices of trench fill sediments and
also possibly oceanic crust, which
have been scraped off the downgoing
slab by the leading edge of the
overriding plate. The contact
between the accretionary wedge and
fore-arc basin is often a region of
back-thrusting. The fore-arc basin is
a region of tranquil, flat-bedded
sedimentation between the fore-arc
ridge and island arc. The island arc
(‘second arc’) is made up of an outer
sedimentary arc and an inner
volcanic arc.
Sedimentary arc
The sedimentary arc comprises
coralline and volcaniclastic
sediments underlain by volcanic
rocks older than those found in the
volcanic arc. This volcanic substrate
may represent the initial site of
volcanism as the relatively cool
oceanic plate began its descent. As
the ‘cold’ plate extended further into
the asthenosphere, the position of
extrusive igneous activity moved
backwards to its steady state location
now represented by the volcanic arc.
The island arc and remnant arc
(back-arc ridge or ‘third arc’) enclose
a marginal sea (back-arc basin)
behind the island arc. Such marginal
seas are generally 200–600 km in
width. In some island arc systems
there may be up to three generations
of marginal seas developed on the
landward side of the island arc.
Subduction zone
A subduction zone is identified by
seismic foci, the seismic activity
being concentrated on the upper
surface of the down- going slab of
lithosphere. The seismic activity
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Airfall ash
G
ck ren
-a ad
rc
a
ba
sin
ba
Volcanic
sands
T
re oba
-a
rc go
ba
sin
fo
currents
currents
Arc axis
1.5°Slope
9°Slope
West
defines the ‘seismic plane’ of the
subduction zone, which may be up
to 20–30 km wide. Subduction zones
dip mostly at angles between 30º and
70º, but individual subduction zones
dip more steeply with depth. The
dip of the slab is related inversely to
the velocity of convergence at the
trench, and is a function of the time
since the initiation of subduction.
Because the down-going slab of
lithosphere is heavier than the
plastic asthenosphere below, it tends
to sink passively; and the older the
lithosphere, the steeper the dip.
Trench
Trenches are the deepest features of
ocean basins, with depths ranging
from 7,000 m to almost 11,000 m.
The deepest are the Mariana and
Tonga trenches. Most deep-sea
trenches in the Pacific are formed of
normal basaltic oceanic crust and are
covered with thin layers of pelagic
sediments and ash. This thin
sedimentary layer is easily
subducted under the overriding
plate.
Ocean trenches are the result of
under-thrusting oceanic lithosphere
and are developed on the ocean side
of both island arcs and Andean-type
mountain ranges. They are
remarkable for their depth and
continuity, being the largest
depressed features of the earth’s
surface.
East
The Peru-Chile trench is about 4,500
km long and reaches depths of 2–4
km below the surrounding ocean
floor, so its base is 7–8 km below sea
level. Trenches are generally 50–100
km in width. They have an
asymmetric V-shaped cross-section,
with the steeper side opposite the
under-thrusting ocean crust. The
sediment fill varies from almost
nothing (e.g. Tonga-Kermadec) to
almost complete (e.g. the Lesser
Antilles Trenches).
Volcanic arc
Oceanic volcanic arcs are surrounded
by large volcaniclastic aprons,
kilometres thick. Most of the apron
consists of pyroclastic fragments. As
the submarine slopes of arc-related
volcanoes are steep, there is great
seismic activity and sedimentation is
rapid, caused by slumping, sliding,
and turbidity currents.
Some of the results that have come
from the study of both modern (e.g.
the Lesser Antilles, New Hebrides)
and ancient island arc systems have
shown that:
1. the distribution of sediments
around an arc is usually
asymmetric, owing to the
prevailing wind patterns,
different arc slopes, and ocean
currents (as in the Lesser
Antilles where westerly winds
blow most of the ash into the
Atlantic) (Figure 2);
April 2005 no.492 Island Arcs
Figure 3: Epicentre of Tonga: (a) shallow, (b) intermediate, (c) deep
3
Samoa Is.
Samoa Is.
6 5
15°S
Samoa Is.
3
4
15°S
15°S
2
Fiji Is.
2
Fiji Is.
Fiji Is.
6
2
2
1
23 4
8
Tonga Is.
Tonga Is.
20°
6
8
Tonga Is.
20°
20°
South Fiji Ridge
South
Fiji
Basin
4
3
2
2
2
1
6
8
Tonga
Trench
8
6 Tonga
6
Trench
888
2
Tonga-Fiji
earthquakes
Focal depth
25°
12 4
6 6
1
8
Kermadec
Trench
180°
25°
25°
0–100 km
100–200 km
Water depth
contours in km
0–6
2. the position of the island or
individual volcanoes can migrate,
often oceanward, towards the
trench or along the arc;
3. as the volcano grows into shallow
water and emerges, eruptions
become more explosive, with ash
dispersed over greater distances
from the volcano;
4. sedimentary processes
continually sort volcaniclastic
fragments by grain size and
density into a proximal coarsegrained facies of pillow breccias,
debris-avalanche, and lahar
(mudflow) deposits; a medial
debris-flow facies; and a distal
facies consisting of thin distal
turbidites and fallout ashes.
As island arcs develop, enlarge, and
become more mature, as in Japan
and the North Island of New
Zealand, terrestrial sediments and
plants abound, and lagoons and
lakes develop, especially within the
calderas of the volcanoes.
Back-arc basin
Marginal seas (back-arc basins) are
small ocean basins lying on the
inner, concave sides of island arcs,
bounded on the side opposite the arc
by a back-arc ridge (remnant arc).
They are most common in the
Western Pacific but are also found in
the Atlantic behind the Caribbean
and Scotia arcs.
Marginal basins may develop in
response to tensional tectonics
whereby an existing island arc is
rifted along its length, and the two
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6
Kermadec
Trench
Tonga-Fiji
earthquakes
200–300 km
300–400 km
400–500 km
8
>6
175°W
Tonga
Trench
180°
Tonga-Fiji earthquakes
500–600 km
> 600 km
Volcanoes
175°W
halves separate to give rise to the
marginal basin. A striking feature of
the western Pacific Ocean is the
enormous area covered by a large
and complex pattern of basins that
lie behind the volcanic arcs and are
marginal to the continent. These
marginal basins have been a source
of controversy ever since it was
realised that their crusts, while
usually having a thickness close to
that of continental crust, have
seismic velocities closer to those of
oceanic crust. Most marginal basins
are now known to be old ocean floor
trapped behind an island arc and are
recognised not only in the western
Pacific but also in the Andaman Sea
behind the Burmese- Indonesian
volcanic arc, and behind the
Antillean and Scotia arcs. They
range in age from very young backarc basins that have developed
within oceanic crust relatively
recently (intraoceanic back-arc
basins) to those mature basins
adjacent to continents, such as the
Japan Sea, which is inactive at
present (continental back-arc
basins).
Benioff zone
Island arc systems exhibit intense
volcanic activity. A large number of
events take place on a plane which
dips on average at an angle of about
45° away from the under-thrusting
oceanic plate. The plane is known as
the Benioff (or Benioff-Wadati)
zone, after its discoverer(s), and
earthquakes on it extend from the
surface, at the trench, down to a
maximum depth of about 680 km.
6
Kermadec
Trench
8
180°
175°W
Figure 4: Plate model of subduction
zones
a
b
Lithosphere
Asthenosphere
c
Figure 3 illustrates how shallow,
intermediate and deep focus
earthquakes in the south-western
Pacific occur at progressively greater
distances away from the site of
under-thrusting at the Tonga
Trench.
The earthquake activity of the downgoing slab occurs as a result of three
distinct processes (Figure 4). In
region ‘a’, earthquakes are generated
in response to the bending of the
lithosphere as it begins its descent.
Region ‘b’ is characterised by
earthquakes generated from thrust
faulting along the contact between
the overriding and under-thrusting
plates. Indeed, the overriding plate
suffers compressional deformation
for several tens of kilometres to the
landward side of the trench. At these
depths earthquakes occur as a result
of the internal deformation of the
strong descending slab of lithosphere,
so that the majority of events lie
about 30–40 km beneath the top of
the slab. The presence of earthquakes
at depths in excess of 70 km
April 2005 no.492 Island Arcs
Figure 5: Section through the Eastern Caribbean
Tobago Trough
(fore-arc basin)
Depth (km)
Aves Ridge
W (remnant arc)
0
Grenada Trough
(marginal sea)
Lesser Antilles
(island arc)
Atlantic Ocean
(oceanic crust)
Barbados Ridge
(subduction complex)
E
20
Oceanic
crust
40
Moho
Moho
60
0
200km
80
(Figure 4c) is paradoxical in that
below this level, the high pressure
causes materials to flow rather than
fracture.
Ridges
Ridges similar to mid-ocean ridges
occur at the margins of oceans; the
East Pacific Rise is an example.
There are other spreading ridges
behind the volcanic arcs of
subduction zones. These are usually
termed back-arc spreading centres.
The reason why the ridges are
elevated above the ocean floor is that
they consist of rock that is hotter
and less dense than the older, colder
plate. Hot mantle material wells up
beneath the ridges to fill the gap
created by the separating plates; as
this material rises it is decompressed
and undergoes partial melting.
Figure 6: Lesser Antilles island arc,
Eastern Caribbean
Key
Volcanic
islands
Sedimentary
islands
Sombrero
0
100km
18°N
St Kitts
Guadeloupe
16°N
h
A xis o f t r e n c
Martinique
Aves
Ridge
14°N
St Lucia
Grenada
Trough
St Vincent
Relatively young island arcs are
structurally simple e.g. TongaKermadec, the New Hebrides,
Aleutians, and Lesser Antilles.
Older, mature island arcs are more
complex as they were formed on
earlier subducting plates. They are
generally underlain by thicker crust
20–35 km thick, e.g. the Japanese
and Indonesian arcs.
Further reading
Busby, C. J. and Ingersoll, C. V.
(1995) Tectonics of sedimentary basins.
Blackwell Science, Cambridge,
Massachusetts.
Nicholas, A. (1995) The mid-oceanic
ridge: mountains below sea level,
Springer-Verlag, Berlin
Kearey, P., and Vine, F.J., (1996)
Global tectonics, Blackwell Scientific
Publications
The Lesser Antilles
Subduction Zone
The Eastern Caribbean shows all the
main features of an island arc
(Figures 5 and 6). The Atlantic
Oceanic crust is subducting at a rate
Focus Questions
a d os
B arb
Grenada
1000 fathom isobath
62°W
When subducting oceanic
lithosphere reaches a depth of over
80 km, an island arc is created at the
surface by volcanic and plutonic
activity some 150–200 km from the
trench axis. Most of the world’s
island arcs are situated along the
western and northern margins of the
Pacific Ocean, although there are
others, such as those in the
Caribbean.
Island arcs are associated with the
subduction of oceanic crust. They
are areas of intense volcanic and
earthquake activity. Research
suggests that there is considerable
variety in island arc systems and that
they are complex features. The
Caribbean (Lesser Antilles) island
arc system is an excellent example of
an island arc system, and there are
many ‘classic’ examples in the
Pacific, too.
Barbados
Tobago
Trough
64°W
Volcanic and plutonic
activity
Conclusion
Rid g e
Venezuela Basin
Dominica
At the ridge crest the lithosphere
may consist only of oceanic crust,
which is basaltic in composition. As
it moves away from the spreading
centre, it cools and contracts and the
peridotitic (olivine-rich) mantle
component grows rapidly. The
contraction leads to an increase in
water depth.
of c. 20 mm/year. The ocean trench
is largely filled by sediment from the
Orinoco River in Venezuela. These
sediments have been deformed into a
large accretionary wedge, over 20 km
thick, known as the Barbados Ridge.
Between the ridge and the island arc
is the Tobago Trough, a fore-arc
basin. The island arc stretches from
Sombrero to Grenada, and comprises
an outer sedimentary arc and an
inner volcanic arc. These merge at
Guadeloupe. The Grenada Trough –
a back-arc basin – flanks the inner
side of the island arc, and is bounded
to the west by the Aves Ridge possibly a remnant island arc.
60°W
Geofile Online © Nelson Thornes 2005
12°N
1. Describe, and account for, the main features associated with island
arc systems.
2. Explain why island arcs are associated with volcanic activity.