Island arc

• 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.
6

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.
7

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 Andes) and inside which basalts
predominated.
8

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.
9

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
10

•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’, however
•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.
11

• An island arc is a type of archipelago, often composed of a
chain of volcanoes, with arc-shaped alignment, situated
parallel and close to a boundary between two
converging tectonic plates.
• Most of these island arcs are formed as one oceanic tectonic
plate subducts another one and, in most cases,
produces magma at depth below the over-riding plate.
However, this is only true for those island arcs that are part of
the group of mountain belts which are called volcanic arcs, a
term which is used when all the elements of the arc-shaped
mountain belt are composed of volcanoes.
• For example, large parts of the Andes/Central
American/Canadian mountain chain may be known as a
volcanic arc, but they are not islands (being situated upon and
along a continental area) and are thus not classified as an
island arc.
• On the other hand, the Aegean or Hellenic arc in
the Mediterranean area, composed of numerous islands such
as Crete, is an island arc, but is not volcanic. Parallel to it is
the South Aegean Volcanic Arc, which is the volcanic island
arc of the same tectonic 13

• In the subduction zone, loss of volatiles from the
subducted slab induces partial melting of the
overriding mantle. This process, called flux melting,
generates low-density calc-alkaline magma that
buoyantly rises to intrude and be extruded through
the lithosphere of the overriding plate.
• The resulting volcano chain has the shape of an arc
parallel to the convergent plate boundary and
convex toward the subducting plate.
• One of the theories to explain the arc shape views
this as a consequence of the geometry of the
spherical plate crumpling along a line on a spherical
surface, but only the more broadly shaped arcs can
be explained in this way
14

• On the subducting side of the island arc is a deep
and narrow oceanic trench, which is the trace at
the Earth’s surface of the boundary between the
downgoing and overriding plates.
• This trench is created by the gravitational pull of
the relatively dense subducting plate pulling the
leading edge of the plate downward.
Multiple earthquakes occur along this
subduction boundary with
the seismic hypocenters located at increasing
depth under the island arc: these quakes define
the Wadati–Benioff zones
15

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 Burmese Andaman-
Indonesian arc
4. finally the Andean chain, where the volcanic belt is
located entirely within the continent and is not
therefore an island arc.
16

• The age also varies. Some are very young: less
than 10 Ma Others are much older, dating
back at least to the Tertiary or Cretaceous
eras.
17

• A Wadati–Benioff zone (also Benioff–Wadati zone or Benioff
zone or Benioff seismic zone) is a planar zone of seismicity
corresponding with the down-going slab in
a subduction zone. Differential motion along the zone produces
numerous earthquakes, the foci of which may be as deep as about
670 kilometres. The term was named for the
two seismologists, Hugo Benioff of the California Institute of
Technology and Kiyoo Wadati of the Japan Meteorological Agency,
who independently discovered the zones.
• Wadati–Benioff zone earthquakes develop beneath volcanic island
arcs and continental margins above active subduction
zones.[3] They can be produced by slip along the subduction thrust
fault or slip on faults within the downgoing plate, as a result of
bending and extension as the plate is pulled into the
mantle.[4] The deep-focus earthquakes along the zone allow
seismologists to map the three-dimensional surface of a subducting
slab of oceanic crust and mantle.
18

20
(1) New Zealand to Tonga; (2) Melanesia; (3) Indonesia; (4) Philippines; (5) Formosa (Taiwan) and west
Japan; (6) Marianas and east Japan; (7) Kurile and Kamchatka; (8) Aleutian and Alaska; (9) Central
America; (10) West Indies; (11) South America; and (12) western Antarctica (Sugimura, 1967b).

The "active" island arcs are decidedly anomalous areas of the
earth, having the following major characteristics (Fig. 2 - 7 ) :
(1) Arcuate continuation of islands.
(2) Prominent volcanic activity at present (Fig.3).
(3) Deep trench on the oceanic side (Fig.7) and shallow tray-
shaped seas on the continental side.
(4) Marked gravity anomaly belt that indicates large departures
from isostasy (Fig.2).
(5) Active seismicity, including deep and intermediate
earthquakes (Fig.4, 5).
(6) Earth movement in progress.
(7) Coincidence of arcs with recent orogenic belts.
In recent years, some further characteristic features such as the
distribution of heat flow (Fig.6), the composition of volcanic
rocks and so forth, which also show remarkable zonalities, have
become known to us
21

Not all of the island arcs have been investigated with respect to each of
the above characteristics, but upon the three criteria:
(a) recent volcanic activity;
(b) Oceanic trenches deeper than 6,000 m (Fisher and Hess, 1963); and
(c) earthquake foci deeper than 70 km (Gutenberg and Richter, 1954),
the following may be identified as island arcs
(1) New Zealand to Tonga; (2) Melanesia; (3) Indonesia; (4) Philippines; (5)
Formosa (Taiwan) and west Japan; (6) Marianas and east Japan; (7) Kurile
and Kamchatka; (8) Aleutian and Alaska; (9) Central America; (10) West
Indies; (11) South America; and (12) western Antarctica (Sugimura,
1967b).
Among these arcs, Central and South America are not islands, but they are
included in the list because they appear to have most of the other
characteristic features. Each island arc has a length of the order of several
thousands of kilometres with a narrow width ( 2 0 0 - 3 0 0 km including
the oceanic trench).
22

Banda Arc
• The Banda Arc (main arc, Inner, and Outer) is a set
of island arcs that exist in eastern Indonesia. It
manifests the collision of a continent and an intra-
oceanic island arc. The presently active arc is located
on what appears to be oceanic crust whereas the
associated subduction trench is underlain
by continental crust.[1] The convergence of the Indo-
Australian plates and Eurasiaand resulted in the
formation of the Sunda and Banda island arcs. The
transitional zone between the arcs is located south
of Flores Island and is characterized by the change in
the tectonic regime along the boundary
37

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.
56

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 down going 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.
57

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.
58

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.
59

Subduction zone: A subduction zone is identified by
seismic foci, the seismic activity being concentrated on
the upper surface of the downgoing slab of lithosphere.
The seismic activity 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.
60

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.
61

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.
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).
62

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
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.
63

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 halves
separate to give rise to the marginal basin.
64

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.
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)
65

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
66

The earthquake activity of the down going slab
occurs as a result of three distinct processes:
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
The presence of earthquakes at depths in excess of
70 km region ‘c’ is paradoxical in that below this
level, the high pressure causes materials to flow
rather than fracture
67

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.
68

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 of 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.
72

Structure of an Island Arc
Schematic cross section through a typical island arc after Gill (1981), Orogenic Andesites and Plate
Tectonics. Springer-Verlag. HFU= heat flow unit (4.2 x 10-6 joules/cm2/sec)
73

ISLAND ARCS IN THE WORLD
The Pacific Ocean is surrounded, at least around
its western and south-eastern margins, by belts
of seismicity and volcanism: the circum-Pacific
island a r c - t r e n c h belts.
It has long been noted that in the island arcs
there is a distinct regularity in the arrangement
of crust—mantle features, suggesting that all
the island arc—trench systems have been
brought about by a common mechanism.
74

Many theories or hypotheses have been postulated on
this mechanism. Among the older theories are those of
Solías (1903), Molengraaff (1914), Argand (1916), Hobbs
(1925), Lake (1931) and Lawson (1932).
Typical examples of the more modern theories are the
earth's contraction hypotheses (Jeffreys, 1952; Wilson,
1959), down-buckling and convection current hypotheses
(Vening Meinesz, 1930, 1964; Kuenen, 1936; Umbgrove,
1938, 1947; Griggs, 1939), serpentinization hypothesis
(Hess, 1937), mantle fault hypotheses (Ewing and Heezen,
1955).
In the last decade, the mantle convection hypothesis
seems to have acquired favour among geo-scientists.
75

Island Arc Magmatism
 Arcuate volcanic island chains along subduction
zones
 Distinctly different from mainly basaltic provinces
thus far
 Composition more diverse and silicic
 Basalt generally subordinate
 More explosive
 Strato-volcanoes most common volcanic
landform
76

Igneous activity related to convergent plate
situations- subduction of one plate beneath
another
The initial petrologic model:
Subducted oceanic crust is partially melted
Partial melts- more silicic than source
Melts rise through the overriding plate
volcanoes just behind leading plate edge
Unlimited supply of oceanic crust to melt
77

Ocean-ocean  Island Arc (IA)
Ocean-continent  Continental Arc or Active Continental Margin (ACM)
Figure : Principal subduction zones associated with orogenic volcanism and plutonism. Triangles are on the overriding plate.
PBS = Papuan-Bismarck-Solomon-New Hebrides arc. After Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer.

Subduction Products
 Characteristic igneous associations
 Distinctive patterns of metamorphism
 Orogeny and mountain belts
Complexly
Interrelated

Structure of an Island Arc
Figure 16.2. Schematic cross section through a typical island arc after Gill (1981), Orogenic Andesites
and Plate Tectonics. Springer-Verlag. HFU= heat flow unit (4.2 x 10-6 joules/cm2/sec)

Table 16-1. Relative Proportions of Analyzed
Locality B B-A A D R
Mt. Misery, Antilles (lavas)2
17 22 49 12 0
Ave. Antilles2
17 ( 42 ) 39 2
Lesser Antilles1
71 22 5
Nicaragua/NW Costa Rica1
64 33 3 1 0
W Panama/SE Costa Rica1
34 49 16 0 0
Aleutians E of Adak1
55 36 9 0 0
Aleutians, Adak & W1
18 27 41 14 0
Little Sitkin Island, Aleutians2
0 78 4 18 0
Ave. Japan (lava, ash falls)2
14 ( 85 ) 2 0
Isu-Bonin/Mariana1
47 36 15 1 < 1
Kuriles1
34 38 25 3 < 1
Talasea, Papua2
9 23 55 9 4
Scotia1
65 33 3 0 0
1
from Kelemen (2003a and personal comunication).
2
after Gill (1981, Table 4.4) B = basalt B-A = basaltic andesite
A = andesite, D = dacite, R = rhyolite
Island Arc Volcanic Rock Types
( 3 )
Volcanic Rocks of Island Arcs
 Complex tectonic situation and broad spectrum of
volcanic products
 High proportion of basaltic andesite and andesite
 Most andesites occur in subduction zone settings
Basalts are still very common
and important!

Major Elements and Magma Series
Tholeiitic (MORB, OIT)
Alkaline (OIA)
Calc-Alkaline (~ restricted to SZ)
Characteristic
Series Convergent Divergent Oceanic Continental
Alkaline yes yes yes
Tholeiitic yes yes yes yes
Calc-alkaline yes
Plate Margin Within Plate

Major Elements and
Magma Series
Figure 16.3. Data compiled by Terry
Plank (Plank and Langmuir, 1988)
Earth Planet. Sci. Lett., 90, 349-370.
a. Alkali vs. silica
b. AFM
c. FeO*/MgO vs. silica
diagrams for 1946 analyses from
~ 30 island and continental arcs
with emphasis on the more
primitive volcanics

Figure 16.4 The three andesite series
of Gill (1981). A fourth very high K
shoshonite series is rare. Contours
represent the concentration of 2500
analyses of andesites stored in the
large data file RKOC76 (Carnegie
Institute of Washington).

Figure 16.6. a. K2O-SiO2 diagram distinguishing high-K, medium-K and low-K series. Large squares = high-K, stars = med.-K,
diamonds = low-K series from Table 16-2. Smaller symbols are identified in the caption. Differentiation within a series (presumably
dominated by fractional crystallization) is indicated by the arrow. Different primary magmas (to the left) are distinguished by
vertical variations in K2O at low SiO2. After Gill, 1981, Orogenic Andesites and Plate Tectonics. Springer-Verlag.

Figure 16.6. b. AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows
represent differentiation trends within a series.

Figure 16.6. c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series. The gray arrow
near the bottom is the progressive fractional melting trend under hydrous conditions of Grove et al. (2003).

Calc-alkaline differentiation
 Early crystallization of Fe-Ti oxide
Probably related to the high water content of calc-
alkaline magmas in arcs, dissolves  high fO2
 High PH2O also depresses plagioclase liquidus  more
An-rich
 As hydrous magma rises, DP  plagioclase liquidus
moves to higher T  crystallization of considerable An-
rich-SiO2-poor plagioclase
 The crystallization of anorthitic plagioclase and low-
silica, high-Fe hornblende may be an alternative
mechanism for the observed calc-alkaline differentiation
trend

Figure 17-23. Schematic cross section of an active continental margin subduction zone, showing the dehydration of the subducting slab,
hydration and melting of a heterogeneous mantle wedge (including enriched sub-continental lithospheric mantle), crustal underplating of
mantle-derived melts where MASH processes may occur, as well as crystallization of the underplates. Remelting of the underplate to
produce tonalitic magmas and a possible zone of crustal anatexis is also shown. As magmas pass through the continental crust they may
differentiate further and/or assimilate continental crust. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice
Hall.

References
 (Developments in Geotectonics 3) A. SUGIMURA and S. UYEDA
(Eds.)-Island ARCSJapan and Its Environs-Academic Press,
Elsevier (1973)
 Island Arcs, Deep Sea Trenches and Back-Arc Basins-by the
American Geophysical Union (1977)
 (Developments in Geotectonics 7) R.W. GIRDLER (Eds.)-East
African Rifts-Academic Press, Elsevier (1972) .
 Evgenii V. Sharkov (Editor)-New Frontiers in Tectonic Research -
General Problems, Sedimentary Basins and Island Arcs -InTech
(2011)
 Andrew Goudie (editor)-Encyclopedia of Geomorphology ( 2
Volume Set). vol 1-Routledge (2003)
 Peter Atkinson, Giles M. Foody, Steven E. Darby, Fulong Wu-
GeoDynamics-CRC Press (2004)

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