South Pacific Studies Vol.33, No.2, 2013

101

Geological Origin of the Volcanic Islands of the Caroline
Group in the Federated States of Micronesia, Western Pacific
REHMAN Hafiz Ur1*, NAKAYA Hideo1 and KAWAI Kei2
1: Department of Earth and Environmental Sciences, Graduate School of Science andEngineering, Kagoshima University, Korimoto 1-21-35, Kagoshima, 890-0065 Japan
2: Research Center for the Pacific Islands, Kagoshima University, Korimoto 1-21-24,
Kagoshima, 890-8580 Japan*Corresponding author
E-mail: hafiz@sci.kagoshima-u.ac.jp

Abstract
We present a geological review of the volcanic islands of the Caroline Islands group (Yap, Chuuk,
Pohnpei and Kosrae) of the Federated States of Micronesia, to understand the origin and mechanism of
formation of these volcanic islands. Previous geochronological data from the three volcanic islands (Chuuk:
14.8 to 4.3 Ma; Pohnpei: 8.7 to 0.92; and Kosrae: 2 to 1 Ma), indicate a progressive increase in age towards
west, based on which a hot-spot origin was assigned. However, overlap in age of the volcanic islands andthe absence of a hot spot east of Kosrae suggest involvement of other tectonic activity for their formation
besides the simple hot-spot model. By observing geological features in the field (e.g., occurrence ofstructurally younger dikes which cross-cut the older volcanic sequences, presence of extensive dikes withcolumnar joints; indicative of extensional environment) and literature survey, we consider that the Caroline
volcanic islands were not only remnants of a hot spot trace; as generally agreed by many authors, but werealso produced in a fracture-induced subduction-related tectonic environment.
Key words: age, the Caroline Islands, hot-spot, plate motion volcanics

Introduction
The Federated States of Micronesia (FSM) is a group of 607 small islands, known
as the Caroline Islands, in the western Pacific. It comprises four main states: (1) Yap, (2)Chuuk (Truk), (3) Pohnpei (Ponape), and (4) Kosrae (Kusaie), from west to east in the
Pacific low latitudes in the northern equatorial region (Fig. 1).
The islands of Yap form an island arc system on the eastern convergent margin of
the Philippine Sea Plate (PSP) and are connected to the Palau island arc in the south
and the Izu-Mariana arc system in the north (Fig. 2). Tectonically the Yap island arc
Received: 4 January, 2013
Accepted: 13 February, 2013

102

South Pacific Studies Vol.33, No.2, 2013

o

o

140 E

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Guam (US)Philippine Sea
MarshalIslands

CAROLINE ISLANDS

Ulithi
Fais
Colonia

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Ngulu

Sorol

Fayu

YAP State

Minto Reef

NomwinOroluk

Moen
Nama
Lasap

Woleai
Eauripik

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Ant

Pingelap

400

Kosraeo

POHNPEI StateCHUUK State
NukuroNational CapitalState Capital

200

Palikir
Pohnpei
Mokil

Etal
Satawan

FEDERATED STATES OFMICRONESIA

0

o

10 N

Murilo5N
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600 kmKopingamarangi

Equator0oFig. 1. Geographic location of the Caroline Islands in the Federated States of Micronesia, western Pacific(map modified from the FSM government official site: http://www.fsmgov.org/info/map.html).

o

20 N

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rough

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Fig. 1.
o

140 E

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Fig. 2. Simplified sketch map of the Pacific Plate, the Philippine Sea Plate and the Caroline Plate (modified
after KOBAYASHI 2004). The map shows approximate position of the Yap Trench, the Caroline Island ridge,
and trenches and troughs in the surrounding areas.

REHMAN et al.: Geological Origin of the Volcanic Islands of the Caroline Group103

is situated on margin of the Caroline Plate which has almost been subducted beneaththe PSP. Other three states of the FSM (Chuuk, Pohnpei, and Kosrae) lie on the PacificPlate, east of the Mariana-Yap-Palau trench system along the Caroline ridge (Fig. 2).
A number of publications describe the volcanic activity in the Caroline Islands
and their petrological characteristics (e.g., IWASAKI 1915, KINOSHITA 1926, TAYAMA
1936, 1952, YOSII 1936, IWAO 1941, YAGI 1960, STARK and HAY 1963, ISHIKAWA andYAGI 1969). However, the origin of these islands and the source of magma remained
controversial. In the late twentieth century several publications focused on the volcanicislands in which geochemical and age-data were reported (e.g., MATTEY 1982, DIXON
et al. 1984, KEATING et al. 1984a, 1984b, SPENGLER 1990, LEE et al. 2001). Most of theabove authors suggested a hot-spot origin for the Caroline Islands, however, differencesin geochemical nature of the volcanic rocks and presence of silica undersaturated andoversaturated lavas on same islands pose a number of questions on the hot-spot only
origin for these islands. For example (1) if the Caroline Islands were formed only froma hot spot, where did the hot spot go after the last event of volcanism in Kosrae Islandca. 1 Ma? (2) The Caroline Islands do not lie along a line, unlike the Hawaiian chain
which lies along a narrow line and the volcanic islands increase in age away from thecurrent hot spot location; rather they mark a broad zone within the Pacific Plate along
the Caroline ridge. (3) The overlap in age of the volcanic islands suggests that volcanic
activity occurred simultaneously on these different islands, which is not likely if they
were formed from a hot spot. (4) Moreover, the geochemical and isotope compositionof volcanic rocks on the Caroline Islands are somehow different from the rocks of
Hawai`i suggesting a different magma source than the Hawaiian lavas.
This paper aims at presenting a review of the geological set up of the Caroline
Islands and discusses the possible source of magma which formed these islands. Our
observations in the field and literature study reveal that assigning only a hot-spot
origin for the Caroline Islands is not likely. There might have been some other tectonic
activities along the western margin of the Pacific Plate besides an unknown hot spot
(which does not exist anymore), which possibly produced volcanic islands along theCaroline ridge.

Previous Studies
First geological report on the Caroline Islands was presented by IWASAKI (1915).
Based on his preliminary investigation, he used the terms “Takashima: high islands
and Hikushima: low islands”. The Chuuk, Pohnpei, and Kosrae islands were partsof Takashima which are formed by volcanic activity. The Yap Islands were part ofHikushima, which are composed mainly of amphibolite facies metamorphic rocks withminor volcanics. Based on petrographic and geochemical study on the rock samples
collected by IWASAKI (1915), KINOSHITA (1926) suggested that nepheline basalts on
Chuuk are similar to the rocks of Pohnpei and concluded that their magma source was

104South Pacific Studies Vol.33, No.2, 2013

same. Several other Japanese researchers described geology of the Caroline Islands (e.g.,
TAYAMA 1936, 1952, YOSII 1936, IWAO 1941, YAGI 1960 etc.). STARK and HAY (1963)
conducted detailed field survey in the Chuuk Islands for military geology and presented
detailed petrography of volcanic rocks in Chuuk Atoll. BRACEY and ANDREWS (1974)
suggested that the Caroline Islands could have been part of a relict island arc becauseof the complex bathymetry of the islands. JARRARD and CLAGUE (1977) proposed a hotspot origin to the Caroline Islands. Their results were based on a comparative studyof the bathymetry of the Caroline Islands with other hot-spot origin islands. Theysuggested that the Caroline and Kodiak-Bowie chains, presently being subducted into
the Pacific trenches, were once as long as the Hawaiian chain (the longest of the WNW
trending chain in Pacific). First geochronological work on the volcanics of Pohnpei
was done by DIXON et al. (1984). They reported major and trace elements compositions
and K-Ar ages from five volcanic rocks samples from Pohnpei and proposed a multistage (polybaric) fractional crystallization which formed these islands. KEATING et al.
(1984a, 1984b) reported K-Ar age, paleomagnetic and geochemical data for the Chuuk,
Pohnpei and Kosrae Islands and suggested a hot-spot origin. Several other publications
also supported the idea of a hot spot (e.g., SPENGLER 1990, LEE et al. 2001).

Geologic Setting
Geology of Yap
Yap State is located in the western Pacific from 7° and 10° N latitude and 137°
and 148° E longitude (Fig. 1). It is comprised of four large islands, seven small islands,
and 134 atolls (Fig. 3A; modified from Yap Visitor’s Bureau homepage: http://www.
fsminvest.fm/yap/index.html). The large islands, namely Yap, Map (Maap), Tomil, andRumong; are volcanically raised islands and are mainly composed of metamorphosed
ultramafic and volcanic rocks with some coral sand and mangrove mud (NEDACHI et al.
2001).
NEDACHI et al. (2001) described geological environment of the Yap Island which
was previously subdivided into the following geological units as Yap formation,
Map formation, and the Tomil volcanics. The Yap formation (mainly composed ofgreenschist and amphibolites) constitutes the basement rock whose metamorphic grade
increases eastward from greenschist facies to amphibolite facies (NEDACHI et al. 2001).
It is derived from a melange trapped in the Yap Trench. Sporadic unmetamorphosed
ultramafic units (clinopyroxinite to olivine websterite) intrude into amphibolites.
The Map formation is composed mainly of tectonic breccia, sandstone and siltstone.
The breccia contains metamorphosed fragments of the Yap formation and some otherigneous rocks; therefore it is younger than the Yap formation. Based on foraminiferafrom the sandstones and siltstones of the Map formation, a Miocene age was assigned
to it (COLE et al. 1960). SHIRAKI et al. (1978) suggested mid-ocean-ridge basalt origin
to the mafic and ultramafic rocks on Yap. The Tomil volcanics (andesitic; exposed

REHMAN et al.: Geological Origin of the Volcanic Islands of the Caroline Group

(a)
（A）

105

(b)
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Moen
(Weno)
Ulalu
Udot

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Fala-Beguets

Dublon
Fefen
Uman

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12.5

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10 km

(c)
（Ｃ）

(d)
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Kolonia

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Madolenihm
Kitti

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7.5

WalungVillage

Nan Madol

15 km

0

5

Utwa

Lelu

Lelu Village

Male

10 km

Fig. 3. Four states of the FSM; (A) Yap, (B) Chuuk (Truk), (C) Pohnpei (Ponape), and (D) Kosrae (Kusaie).
Maps are modified from the FSM visitors’ center.

over a wide area of the Tomil and Map Islands) are composed mainly of agglomerate,
breccia, tuff and lava. SHIRAKI et al. (1978) considered that the Tomil volcanics aregeochemically and mineralogically similar to andesite on the Mariana and Palau arcs.
Geology of Chuuk
Chuuk State lies at about 7° 08' and 7° 41' N latitude and 151° 26' and 152° 2'
E longitude in the western Pacific (Fig. 1). It includes 12 volcanic high islands and24 low coral-reef islands in a lagoon enclosed by a barrier reef forming a near-atoll(Fig. 3B). Forty one low coral islands exist along the atoll. STARK and HAY (1963)
subdivided the volcanic islands of Chuuk into three groups as (1) eastern group (Moen106South Pacific Studies Vol.33, No.2, 2013

(Weno), Uman, Dublon, and Fefen), (2) central group (Udot, Eot, Eiol), and (3) westerngroup (Tol, Fala-Beguets, and Ulalu). Based on their field observation, S TARK and
HAY (1963) interpreted that the volcanic islands are remnants of a large shield volcano
in which lava flows predominate with minor pyroclastic deposits. They also reported
that most of the lava flows were ejected from fissure vents as dikes and dike swarms.
Petrographically, lava flows and dikes on the Chuuk Islands represent the alkaliolivine-basalt-trachyte association, similar to other volcanic islands of the Pacific. The
undersaturated lavas (nepheline basalts) on the Chuuk Islands are similar to that of
Hawai`i, therefore, they suggested same magma source for the rocks on the Caroline
Islands to that of Hawai`i.
Islands of the eastern group were made of olivine-basalt with minor andesite.
The lava flows on central islands are mainly andesite and basalt which intermingle
with breccias. Islands of the western group show different rock types than those of
the central and eastern groups. Tol, the biggest island in the western group, is entirelycomposed of olivine-basalts and andesite flows that are cut by dikes. Based on presenceof olivine-basalt, nepheline-basalt and mililite-nepheline-basalt, STARK and HAY (1963)
suggested that these rocks originated from a different magma source. Most of the
Chuuk lavas were formed by fractional crystallization of undersaturated olivine-basalt
parent magma; however the source of such lava is unclear.
Based on chemical composition, MATTEY (1982) recognized three magma types
in Chuuk i.e. (1) the Chuuk Main Lava Series (CMLS), (2) the Chuuk Transitional
Lava Series (CTLS), and (3) the Chuuk Nephelinite Series (CNS). Rocks of the CMLS
are aphyric to highly porphyritic basalts, hawaiites, mugearites and trachytes. Theirmajor and trace element composition are similar to those of Hawaiian alkali basalts.
Rocks of the CTLS, exposed in Tol, are mainly basalts which were characterized byslightly lower ratios of more incompatible to less-incompatible trace elements (suchas Nb/Zr, Ba/Zr, Ce/Zr etc.). These volcanics are less alkalic than the CMLS and wereinterpreted by MATTEY (1982) as a different magma source. Rocks of the CNS arenepheline basalts which are exposed on Tol and unconformably overlie the older CTLS
and CMLS lavas. These rocks are highly enriched in Rb, Th, Ba, Sr, La and Zr relative
to the CMLS.
Geology of Pohnpei
The island of Pohnpei is situated at 6°54' N longitude and 158°14' E latitude in
the western Pacific (Fig. 1). Pohnpei has a land area of 338 km2 and is surrounded bya barrier reef (Fig. 3C). The island is roughly circular in shape and is characterized bysteep mountains in the deeply dissected interior.
Rocks exposed on Pohnpei are alkali olivine basalts, trachytes, hawaiites,
ankramites, and nephlinites to basanites. Numerous dykes of several cm to several ten
cm intrude the volcanics. MATTEY (1982) recognized three magma types from Pohnpei
and termed them as (1) the Pohnpei Main Lava Series (PMLS), (2) the Pohnpei
Transitional Lava Series (PTLS), and (3) the Pohnpei basanite Series (PBS). SPENGLER

REHMAN et al.: Geological Origin of the Volcanic Islands of the Caroline Group107(1990) reclassified the volcanic rocks on Pohnpei into two major units i.e. the shieldbuilding lavas and the overlying post-shield deposits.
Rocks of the PMLS are comagmatic and consist of basalts, hawaiite, mugearite
and a trachyte. Unlike the Chuuk volcanics, the PMLS are characterized by phenocrysts
of olivine and relative Ti- and Na-rich clinopyroxene. Hawaiites are the most abundantin this series and consists of phenocrysts of olivine, clinopyroxene and rare magnetite.
Major element compositions are similar to those of the Hawaiian alkali basalts. Minorand trace elements show some differences from the CMLS. Rocks of the PTLS containslightly lower incompatible ratios as compared with the PMLS. In them olivine,clinopyroxene and plagioclase phenocrysts are common. The magma type for theserocks was considered as different from the PMLS i.e. less alkalic. Rocks of the PBS
are distinguished from the PMLS by a small range of higher more-incompatible to lessincompatible element ratios. Silica is undersaturated. These lavas gave K-Ar age of lessthan 1 million years and are significantly younger than the PMLS which has an averageage of 6 Ma (KEATING et al. 1984a). This seems to be post-erosional magma similar to
the CNS on Chuuk.
SPENGLER (1990) subdivided the volcanic rocks on Pohnpei into two major unitsas (1) the shield-building lavas and (2) the overlying post-shield deposits. K-Ar age
data suggest that the shield building stage initiated at around 8.5 Ma, and the volcanic
activity remained until 1 Ma (DIXON et al. 1984, KEATING et al. 1984a). The postshield deposits were further subdivided into two groups, the Awak Volcanics and the
Kupwuriso Volcanics, based on differences in age, general flow morphology, andchemical composition, although, the contact between them was not distinguished on
geological map (SPENGLER 1990). SPENGLER (1990) reported that the Awak stage lasted
in between 7 to 3 Ma, the Kupwuriso stage extended from 2.1 to less than 1.0 Ma, andboth stages have similar origin like the rocks of Hawai`i. On the basis of geochemical
and isotope data, SPENGLER (1990) interpreted that the data for Pohnpei fall within themodern-day oceanic island or hot spot field. However, based on their Pb isotope data
(SPENGLER 1990), the three volcanic series do not show isotopically homogeneous
source. SPENGLER (1990) also compared isotopic data for basalt of the Pohnpei withbasalt from the Ontong Java Plateau (OJP) but observed very little or no mixing
between magmas of Pohnpei and the OJP. He reported that Pohnpei lavas are moreradiogenic in 208Pb/204Pb than the overall Hawaiian lavas. The general trend toward
less radiogenic 87Sr/86Sr values going from shield-building to alkalic capping thepost-erosional stages observed in many individual Hawaiian volcanoes was not seen in
the Pohnpei data set.
Geology of Kosrae
Kosrae is located at approximately 5° N latitude and 163° E longitude and covers
an area of 110 km2 (Fig. 1). It is the second largest island and the most easterly stateof the FSM. Kosrae is also a volcanic island and is surrounded by mangrove swamp(Fig. 3D). MATTEY (1982) reported only two magma types from Kosrae i.e. (1) the108South Pacific Studies Vol.33, No.2, 2013

Kosrae Main Lava Series (KMLS) and (2) the Kosrae Nepehlinite Series (KNS). Rocksof the KMLS are mainly basalts, ankaramites, and hawaiites containing phenocrysts
of olivine, Ti-clinopyroxene, kaersutite and phlogopite. These rocks are similar to the
PMLS basalts. Rocks of the KNS are a group of highly to moderately undersaturated
lavas and dikes and are distinguished from the KMLS.

Formation of the Caroline Volcanic Islands
The volcanic activity on Yap was associated with the convergence of the Caroline
and Pacific Plates. The volcanic activity in the remaining three volcanic islands (Chuuk,
Pohnpei and Kosrae) of the Caroline Islands started at 14 Ma and remained until 0.9
Ma. Age of these islands increases progressively towards west. These three islands
lie in a broad zone between 4° and 8° latitudes (Fig. 1), forming a sub-linear chain.
In contrast, the hot-spot originated islands such as the Hawaiian chain form along anarrow line and the age of the sea mounts increases away from the hot spot due totraveling of the lithosphere over a mantle plume. In case of the Caroline Islands, it istrue that the age of the volcanic islands decreases towards east; however, there is abroad range in the age of volcanism even within the same island (e.g., in Pohnpei therocks range in age from 8.7 to 0.92 Ma; DIXON et al. 1984).
Age of rocks on the Yap Island
WEISSEL and ANDERSON (1978) reported that the island arc volcanism on Yap
probably terminated in Late Oligocene or Miocene. Based on sedimentation rates
from the rocks dredged from the rift axis in Ayu Trough, they proposed that marginsof the basin were formed in mid-Miocene (ca. 10 to 12 Ma). BECCALUVA et al. (1980)
reported K-Ar age (ca. 11 to 7 Ma) for the Yap Trench volcanics. There is no evidenceof volcanism after that time in the Yap arc.
Plate convergence at the Yap Trench ranges from 0.6 to 0.3 cm yr-1 whereas, theYap island is moving together with the rigid PSP at 9.2 cm yr-1 (N73°W) with neither
backarc opening nor intra-plate deformation (KOBAYASHI 2004 and references therein).
Along the Chuuk Islands, the motion of the Pacific Plate is 8.3 cm yr-1 (N60°W) and thewestern end of the Caroline ridge is moving together with the Pacific Plate (KOBAYASHI
2004). The movement of the PSP with respect to its present pole of rotation at 3 to 4
Ma caused to reduce the convergence rate significantly at Yap and Palau due to which
the old volcanic islands of Yap and Palau uplifted by the upward pressures (KOBAYASHI
2004). Deep seated rocks beneath the remnant rift have been thrust up to form
metamorphosed ophiolites in the Yap Island.
Age of rocks in the Chuuk Islands
STARK and HAY (1963) proposed that age of the Chuuk volcanoes could be no later
than Miocene because the limestone xenoliths in Udot and Eot contained Cycloclypeus

REHMAN et al.: Geological Origin of the Volcanic Islands of the Caroline Group109

and Miogypsina fossils (TAYAMA 1952). On the basis of geochemical, K-Ar age,and paleomagnetic data, KEATING et al. (1984b) interpreted that the volcanics in the
Chuuk lagoon were formed from a fractional crystallization of heterogeneous magma
source. K-Ar dates for the shield-building lavas range in age from 14.8 to 8.4 Ma
for the CMLS, 10.6 to 7.2 Ma for the CTLS and 5.4 to 4.3 Ma for the post-erosional
CNS lavas. Paleomagnetic results suggest that the Chuuk Islands was formed by a hot
spot during 1 to 14 million years at 4.4° north of the equator. They considered these
volcanics as the equivalent of the Hawaiian lavas but the abundance of alkali lavas in
Chuuk and their rare occurrence in the Hawaiian lavas raise questions on their origin.
In the Hawaiian Islands, most of the early and shield-building lavas are tholeiitic but inChuuk the early shield-building lavas are alkali and the post-erosional volcanics grade
into silica-undersaturated nephelinites. LEE et al. (2001) also reported K-Ar age data
for lava samples collected from Weno which ranged from 11.3 to 6.7 Ma.
Age of rocks on the Pohnpei Island
DIXON et al. (1984) subdivided the volcanic rocks on Pohnpei into two major
units i.e. the shield-building lavas and the overlying post-shield deposits and reported
their K-Ar age data. They reported 8.7 Ma for the early shield-building stage and< 1 Ma for the post-erosional event. They argued that the age span for the Pohnpei
volcanics is significantly large relative to other Pacific islands which span mostly < 3
Ma. Another issue was that the volcanics rocks which yielded 8.7 Ma and < 1 Ma were
petrographically or by appearance same.
Age of rocks on the Kosrae Island
The shield-building stage of rocks on the Kosrae Island (i.e., the KMLS andthe KNS) was reported to be between 2 to 1 Ma (KEATING et al. 1984a). There is noevidence of volcanic activity east of Kosrae. Based on this feature, most of the authors
concluded that hot spot died after forming the seamounts of Kosrae. However, why the
hot spot died was explained by none.

Constraints on the Magma Source and Age of the Volcanic Islands
Two different models can be proposed for the origin of volcanic islands of the
Caroline Islands: (1) formation of the volcanic chain (Chuuk, Pohnpei and Kosrae) bya hot spot and (2) formation of seamounts (Caroline ridge) along a fractures zone in
the western Pacific due to subduction-related tectonic setting. STARK and HAY (1963)
correlated the undersaturated lavas of Chuuk to that of Hawai`i and based on theirsimilarity they suggested similar magma source. They pointed out that the presenceof nepheline-basalt and melilite-nepheline basalt on Tol Island suggests that the rocks
originated from a different source. Below we explain the mechanism of formation of
the Caroline Islands by presenting geophysical and geochemical evidence both fromfield and through the literature survey.

110South Pacific Studies Vol.33, No.2, 2013

Hot-spot model
Generally, hot spots are considered stationary or fixed with respect to mantle
and plates move over them. When plates move the upwelling mantle materials fromthese hot spots form linear chains of seamounts (MORGAN 1971). The distribution of
seamounts, thus, marks the history of the motion if their age has been determined.
Based on the progressive change in the age of the Caroline seamounts (Chuuk, Pohnpei
and Kosrae) which increases towards NNW, JARRARD and CLAGUE (1977) proposed a
hot-spot model hypothesis. They suggested that Caroline and Kodiak-Bowie chains,presently being subducted into the Pacific trenches, may once have been as long as
the Hawaiian chain (the Hawaiian chain is the longest of the WNW trending chain
in Pacific). KEATING et al. (1984a) interpreted that Chuuk, Pohnpei and Kosrae wereformed at the same paleolatitudes based on their paleomagnetic data. Moreover, theprogressive increase in age towards west led the authors to consider the three islands
as a trace of a young melting anomaly or hot spot across part of the oldest portion of
the Pacific Ocean crust. If their idea of a fixed hot spot is correct, the hot spot would
have been located somewhere in the east of Kosrae (note that the age of Kosrae is <1 Ma). The absence of volcanoes east of Kosrae indicates either the hot spot has diedout or there was a drastic change in the Pacific Plate’s motion which misplaced the
hot spot somewhere in the Pacific. However, why such a young hot spot dies requires
geophysical evidence.
Hot-spot origin for the Caroline volcanic islands was also supported by several
other authors (e.g., KEATING et al. 1984a, 1984b, POLLITZ 1986, SPENGLER 1990, LEE et
al. 2001) on the basis of pieces of evidence such as (1) the islands of Chuuk, Pohnpei
and Kosrae form a sub-linear chain in the western Pacific and show a progressive
younging direction towards east, (2) similarities in the field occurrence, textural andmineralogical features, and chemical compositions with the Hawaiian volcanics, and (3)the convex-northward configuration of the island trend from Pohnpei to Kosrae, and(4) minor change in the absolute motion of the Pacific Plate between 5.0 and 3.2 Ma.
The above features support the formation of these islands by a hot spot which was once
located in the low northern equatorial region, however where did it go is unclear. In
addition, SPENGLER (1990), on the basis of Pb isotope data, interpreted that generationof Pohnpei lavas require at least three different source components with discrete Pb
contents. If they had same source as those of the Hawaiian volcanoes, their isotopic
compositions should have been similar which are not. This infers that the volcanism on
Caroline seamounts was not produced by a simple hot spot process with should havemore or less homogenous magma source.
Moreover, based on geochemical data and K-Ar dating, DIXON et al. (1984)
argued that simply a hot-spot model hypothesis for the Caroline ridge is not likely
because the volcanics on Pohnpei have younger age (ca. < 1 Ma) which overlap withthe age data of the Kosrae volcanics (ca. 1 to 2 Ma). Besides, the volcanics on Chuuk(14.8 to 4.3 Ma) and on Pohnpei (8.7 to 0.9 Ma) show an age overlap (Fig. 4). Theseage values are too long for a hot spot activity on a single island based on the motion

REHMAN et al.: Geological Origin of the Volcanic Islands of the Caroline Group

111

Kosrae Main Lava Series
Pohnpei Post-erosional Lava
Pohnpei Shield-building Lava Series
Chuuk Nephelinite Series
Chuuk Transitional Lava Series
Chuuk Main Lava Series
Yap Trench Volcanics

0

2

4

6

8
Age (Ma)

10

12

14

Fig. 4. Diagram indicating age of the volcanism in the Caroline Islands. The volcanic series with their age
values for Yap, Chuuk, Pohnpei and Kosrae are shown from bottom to top and are separated by brokenlines. Shaded portion shows age overlap in the volcanism.

of Pacific Plate which was to be > 8 cm yr-1 (KOBAYASHI 2004). The shield-building
event for most of the volcanoes is usually < 1 Ma and in well-documented linear chainsof the hot-spot type in the Pacific, the maximum age range is < 3 Ma. This means
volcanism on Pohnpei does not seem to be related only to the hot spot. Moreover, theCaroline Islands and the Caroline Ridge itself are not a liner structure but form a broadfeature. Therefore, a simple hot-spot model (volcanism on a plate due to passage over a
fixed mantle) cannot fit here.
WEISSEL and ANDERSON (1978) suggested that relative motion of the Caroline
Plate (south of the Caroline Ridge) may have undergone some modification beginningin the Late Miocene (12 to13 Ma). This is the time when volcanism on the Caroline
Ridge initiated (Chuuk lavas of 12 Ma). This volcanism can be due to interaction of the
Caroline Plate with the Pacific Plate beginning in the Early Miocene and continuing
to the present. The occurrence of shallow seismicity in the eastern Caroline Ridge(WEISSEL and ANDERSON 1978) supports this hypothesis.
Based on sedimentation rates from the rocks dredged from the rift axis in Ayu
Trough, WEISSEL and ANDERSON (1978) proposed that margins of the basin near Yap
and Ayu trenches were formed in mid-Miocene (ca. 10 to 12 Ma). The spreading
activity along the Ayu Trough ceased 5 to 6 million years ago. Based on seismic dataand geophysical and physiographic features, WEISSEL and ANDERSON (1978) proposed
presence of extensional environment along the Pacific-Caroline boundary. The presenceof numerous normal faults with throws of up to 3 km along the plate boundary supports
their idea.

112South Pacific Studies Vol.33, No.2, 2013

（Ａ）

（B）

Fig. 5. Photographs of the dike complex (A) which shows vertical to subvertical columnar joints and (B) aview from the top of dikes in which hexagonal shape of dikes is seen clearly (hammer in the middle is forscale). Photographs were taken in a stone quarry north of the Weno (Moen) in the Chuuk Islands (looking
west in A, and from top towards south in B).

REHMAN et al.: Geological Origin of the Volcanic Islands of the Caroline Group

(a)
（A）

113

Yap Trench volcanics(11~7 Ma)
Chuuk
(12~8 Ma)

Philippine Sea Plate

Pacific Plate

(b)
（B）

Pohnpei(8~0.9 Ma)

Caro

Kosrae(2~1 Ma)line R

idge

(c)
（C）
Pacific Plate

OldestYoungest

Fixed

ot

hot spFig. 6. Schematic diagram comparing (A and B) the formation of the Caroline Islands in a subductionrelated tectonic setting versus (C) the formation of volcanic island chain by a simple hot-spot model (exampleof the Hawaiian chain; figure modified and redrawn from the URL: http://pubs.usgs.gov/gip/dynamic/
hotspots.html#anchor19596916). Curved black lines on the figures “A and B” are the fractures forming theCaroline Islands (highlighted band), whereas the dashed-line in figure “C” shows the linear trend of a hot
spot-produced volcanic islands. Black arrows indicate the plate motion. Grey arrows (double-sided) indicate
thickness of the crust (figures not to scale).

114

South Pacific Studies Vol.33, No.2, 2013

Fracture-induced volcanism
Formation of the seamounts on the converging plate by the subduction-related
deformation is likely. If two oceanic plates are colliding, the older plate will possibly
subduct because of its high density and coldness (as is the case with the PSP and thePacific Plate; the Pacific Plate is subducting beneath the PSP). WESSEL and KROENKE(1997, 1998) reported that some seamounts appear to form in the proximity ofdivergent plate boundaries and adjacent fracture zone systems and are not related to
hot spot activity. Because seamounts produced by hot spot volcanism, must be traced
back to their known hot spot location. WESSEL and KROENKE (1997) discovered ageometric relationship between hot-spot-produced seamounts and the hot spot locations
which allowed them to determine hot spot locations from seamount locations without
requiring seamount age information. They applied the hot-spotting method to all
seamounts in the Pacific Plate and obtained the cumulative volcano amplitude image
in which the location of well developed hot spots were marked with “X” sign. In theirfigure 5 (WESSEL and KROENKE 1997: 368) they got a very prominent “X” mark forthe location of the Hawaiian and the Louisville hot spots. However, no specific sign
was obtained for hot spots in the south and western Pacific which have been remained
controversial including that of the Caroline hot spot.
One strong piece of evidence which poses question on the hot-spot origin for the
Caroline Islands is the presence of basal dike system with columnar joints (indicating
extensional environment) in Chuuk (Fig. 5) and Pohnpei. These features cannot be
simply explained by a hot-spot model.
Alternate to the hot-spot model is the fracture-induced volcanism. If the volcanic
chain is on a broad zone instead of a narrow and straight line, the seamounts might
have formed from a volcanism along fractures which were produced by forces acting
along the plate edges. In such cases ridges would be parallel as can be seen in the
Pacific Plate near the Caroline Ridge. However, in such case the volcanism will
likely occur all along the fracture system and there is no need to have a progressive
increase in age in a certain direction. The progressive increase in age towards west in
the Caroline volcanic islands, therefore, led the authors (JARRARD and CLAGUE 1977,KEATING et al. 1984a, 1984b, SPENGLER 1990) to propose a hot-spot model.
It is already known that on the western side of the subducting Pacific Plate half
graben (LEE 2004) exists which extend in east-west and northwest-southeast trend.
These fractures might have provided space for the volcanism along the broad Caroline
Ridge. Due to westward motion of the Pacific Plate and slow subduction along the Yap
Trench, the crustal portion of the Pacific above the unknown hot spot (if ever existed)
increased the crustal thickness and caused the young and weak hot spot to die in a shortperiod of time i.e. less than 14 Ma.
Based on our observations in the field and literature study, we believe that fracture
model is much realistic (Fig. 6A&B). The reason why the volcanic activity ceased
earlier in the west and remained active until < 1 Ma in the east could be explainedby the thickening of the Pacific Plate’s crust progressively as shown in Fig. 6. As

REHMAN et al.: Geological Origin of the Volcanic Islands of the Caroline Group

115

discussed above, there is almost negligible convergence along the Yap Trench and thePacific Plate motion towards northwest is > 8 cm yr-1. This would imply to increase
the thickness of the Pacific Plate’s crust near the convergent boundary. When the crustis thickened enough, it would have been difficult for the magma to escape which will
ultimately cease the volcanism in the west. As the Pacific Plate moves progressively
westward, the fractures formed in the forthcoming eastern portion caused next eventof volcanism, therefore the rocks on Pohnpei and Kosrae are younger than thoseon Chuuk. The presence of numerous dikes (cross-cutting the breccias and older
volcanic units), and columnar joints in Chuuk and Pohnpei also indicate extensional
environment, which could be only possible if there is a fracture system propagating
eastward from the subduction front.
The formation of the Caroline Islands by a hot spot might be correct but the age
overlap of about two million years in volcanism on Chuuk (ca. 14 to 4.3 Ma) andPohnpei (8.7 to 0.9 Ma), and the presence of younger (ca. 0.9 Ma) volcanic rocks
on Pohnpei than the rocks on Kosrae (ca. 2 Ma) suggest that volcanism was active
between 6.7 and 8.7 Ma on Chuuk and Pohnpei, and the post-erosional event on
Pohnpei (ca. 0.9 Ma) coincides with the shield-building event in Kosrae ( < 2 Ma). If
we propose a simple hot-spot model for the Caroline Islands similar to the Hawaiian
chain (Fig. 6C), this cannot be possible to have rocks of the same age on these different
islands which are several hundred km away from each other. If that part of the plate
moved upon a stationary hot spot (once occurred there), there could not be age overlap
on these islands. The large time span from 8 to 0.9 Ma for the volcanism on Pohnpei
would imply that the motion of Pacific Plate during this time was almost negligible,
which is not realistic. Moreover, the presence of volcanic rocks <1 Ma on Pohnpei
and 2 Ma on Kosrae would also imply that the hot spot was almost active below bothof the above islands in a same time. This also seems questionable. Therefore, ournotion that the Caroline seamounts were formed by fracture-induced volcanism due tosubduction-related tectonic activity is plausible. The presence of numerous dikes whichcross-cut the older sequences and the pyroclastic flow deposits and breccias indicatethat volcanism persisted on these islands for long time which contradicts the hot-spotmodel hypothesis. If the hot-spot model was the only cause of producing the Caroline
Islands, then the geochronological results might be erroneous. Systematic geochemicaland geochronological work on each representative unit of the volcanics and the young
dikes intruding these sequences is required to clarify the origin further.

Conclusions and Future Target
The Caroline volcanic islands are geologically important for understanding the
hot-spot versus fracture-induced subduction-related tectonic model. Geochronological
data from the volcanic islands (Chuuk: 14.8 to 4.3 Ma; Pohnpei: 8.7 to 0.92; andKosrae: 2 to 1 Ma), indicate a progressive increase in age towards west which led a116South Pacific Studies Vol.33, No.2, 2013

number of authors to propose a hot-spot origin for their formation. However, overlap
in age of the volcanic islands (Fig. 4), absence of a hot spot east of Kosrae, and theoccurrence of these islands along a broader zone in the Pacific (Fig. 6B), instead ofa volcanic chain along a line (Fig. 6C), contradict the hot-spot only origin. Based on
geological field and literature survey we propose that the volcanism on the Caroline
Islands was not only due to the hot spot activity but the volcanic islands were mostprobably formed in a fracture-induced subduction-related tectonic environment.
Further, geochemical and geochronological work on individual lithological units isrequired to elucidate the origin of these islands.

Acknowledgements
We are thankful for the cooperation by the Chuuk State Government for giving us
the permission to conduct field survey in the area. Due thanks to Mason Fritz (Chuuk
Visitor’s Bureau), Marsae Morsolo (Tol), and our efficient and punctual boat-driverKenner. We also thank Noda S., Nagashima S. and Yamamoto S. for their fruitful
discussion and help in the field. Undergraduate students of the Intensive Science
Education Program (Fujiwara Y., Itou M., Takayoshi K., Moriyama I. and NakakuraN.) helped in sample preparation and petrography. A critical review by two anonymous
reviewers greatly improved the earlier version of the manuscript. This research waspartly funded by the grants from the Kagoshima University Research Center for the
Pacific Islands.

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