Oq((- 3
THE SUMMARY
OF THE
GEOLOGY
OF
WESTERN SAMOA
Aleni Fepuleai
c
Regional Setting
The Samoa group of island is situated at 14 °S, 172°W (Fig. 1). There are four
main islands, Savaii, Upolu, Tutuila and Manua, which together with a number of
islets, atolls and submerged reef banks and seamount which form a linear feature
which rises steeply from the Pacific Ocean floor. The island chain has a total length
of about 1000 kilometers from Rose Atoll in the east to Wallis Island on the west and
varies between 130 to 220 kilometers in width (Hawkins, 1987). The Samoa Islands
consist of western group referred to as Western Samoa and an eastern group called
Tutuila and Manua, which is also known as American Samoa.
Rose Atoll on the eastmost end of the island chain, is a low carbonate bank
(Keating, 1992). Other than the alignment of Rose with the remainder of the chain,
no evidence exists to directly link the atoll to the Samoa group. Sachet (1954),
Setchell (1924) and Mayor (1924) have suggested it may be an old seamount of
unrelated origin. Savaii, on the western end of the chain, is still volcanically active.
Seamounts and reef banks between Savaii and Wallis Islands indicate that the linear
seamount -island chain is approximately 1100 kilometers long (Glasby and Katz,
1975). Age dating of rocks dredged from these banks suggests that they are a
continuation of the Samoa seamount group.
Bathymetrically the Samoa islands are located at the end of the 7000 to 9000
meters deep Kermadec -Tonga trench (Hawkins and Natland, 1975). The Samoa
Islands are aligned along a trend of 288 °, nearly parallel to the 283° trend of the
Tonga Trench and to the vector of relative plate motion of the Pacific Plate with
respect to a fixed Australia Plate. This vector trends 278° at a point near the sharp
bend in the Tonga Trench. Hawkins and Natland (1975) have presented data to
support their hypothesis that the Pacific Plate is sharply flexed and ruptured at the
bend in the trench. They proposed that the Samoa chain be aligned along the axis of
this flexure and attribute the location and origin of the chain to magma leaks
controlled by complex flexure of the lithosphere plate.
Physiography
Upolu and Savaii are much the largest and most mountainous islands of the
Samoan Archipelago. The volcanic cones of Upolu are distributed along the crest of
the island and Savaii is in the form of chain over a broad convex plain. The island of
Savaii represent the uppermost part presently exposed above sea level of Savaii
Volcano a huge pile volcanic rocks built upon the surrounding deep sea floor from a
depth of present sea Ievel, The island presently rises to a height of 1,858 meters at
Mountain Silisili making the total height of the volcano nearly 6,000 meters.
A prominent feature of the Upolu island is its high central ridges, which rise to a
maximum of 1100 m. Although much of the topography is relatively gentle, the island
has regions of deep canyons and precipitous slopes. On Upolu there is a marked
difference between the high relief topography at the eastern end of the island and the
flatter regions to the west.
Erosion is very high in the eastern part of Upolu Island, mostly between Tiavea
on the west and the south of Falefa village on the northern coast. This is indicated
1
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Figure 1
Position of the Samoa Islands and their location relative to the Kermadec-T
(from Hawkins, 1987)
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by some streams and rivers, which merge to produce a steep (about 700 to almost
vertical) rugged terrain or narrow razor -backer ridges and sharp peaks.
Sea cliffs, where the protective coral reefs are absent and the sea cuts against
the rocks and erodes the headlands commonly occur between the western part of
Tiavea to the east of Falefa Harbour. Some of these sea cliffs reach a height of 150
meters or more, mainly to the northwest of Sauano bay in the Fagaloa district. The
headlands between Samemea and Uafato expose several notches which indicate
that the sealevel has dropped possibly as a result of some sort of tectonism in this
part of the island.
Fringing reefs are quite rare in the area between Tiavea and Falefa while in the
northern part they are only about 200 meters or less in width compared to the
western part of Upolu and Savaii where they have a width of more than 3 kilometers
(Fig. 2).
Regional Geology
The predominant rocks of the Samoa Volcanism are olivine -rich basaltic lava
flows of aa (volcanic breccia) and pahoehoe type in irregular sequence. These lava
flows are associated with marine tuffs (volcanic ashes), volcanic breccia and alluvial
deposits. Most of these younger basaltic lavas have been erupted from a great
number of volcanic cones distributed along the crest of both Upolu and Savaii.
Kear
and
Wood
(1959)
identified
six
geological
units
based
upon
geomorphology and the degree of weathering and these units were named
geographically (Table1.1). The Fagaloa
Samoa stratigraphic sequences. They are unconformably overlain by the younger
rocks of the Salani, Mulifanua, Lefaga' Puapua and Aopo Volcanics. These younger
units partially filled valleys in the Fagaloa volcano. The Aopo Volcanics the youngest
unit is mostly abundant on the northern part toward the western end of Savaii. In the
(study, 1997) these young volcanic units are all referred to as post -Fagaloa
formations.
UNITS
FEATURES
NAME COME FROM
Aopo Volcanics
Fresh or slightly weathered with very thin soil or
Village on the northwest
none, pahoehoe and aa flows common only around
of Savaii about 8
cones which rill older valleys and spill out over
kilometres inland.
coasts to fill lagoons and cover the barrier reef.
Puapua Volcanics
Slightly wcuhered with thin soil, lava flow offshore
Village on the northeast
and form rocky (or ironbound) coasts, ubiquitous as
Savaii.
artd pahoehoe structure form broad domes.
Lefaga Volcanics
intermediated weathered and thin soil and only
District on the southeast
narrow fringing reefs present offshore of these
Upolu.
outcrops.
Mulifanua Volcanics
Intermediate weathered and soil and the existence of
Village on the
wide barrier reef existing offshore.
southwestern part of
Upolu.
Salani Volcanics
Fagaioa Volcanics
Thick soil and deeply weathered, reef is quite far
District on the southeast
offshore and gorges cut in flanks
of Upolu.
Very thick soil and deeply weathered. reef is closed
District on the northeast
to the inshore or none in some pan, deeply eroded
Upolu.
and dissected volcanics terrains and lava associated
with series of dykes.
Table I. f Sir strotigraph c unis of die Western Samoa sequence and ¡heir diss,nguishrns fearures (from Kear
and Wood. 1959).
4
0
PETROLOGY
Most of the volcanic rocks are holocrystailine (coarse) and fine grained to
porphyritic. Generally they are less to highly vesicular lava with very dark grey and
almost black in appearance in most of the places. Phenocryst phases are commonly
pyroxene, plagioclase and olivine; phenocryst sizes range from 0.5 mm to 2 cm
diameter.
Fagaloa Formation
Fagaloa Volcanic extends, with only minor coverings, over the whole of the
north- eastern quarter of Upolu from Apia eastwards (Fig.2). They form the buried hill
south of Magia, and the hilly country between Falelatai and Lefaga Bay in south -west
Upolu. In Savaii more doubtfully, the rocks outcropped along the Vaipouli valley are
also referred to this group.
The thickness of the Fagaloa Formation is variable and it is very difficult to
determine in the field. Fagaloa Formation is divided into two main parts (study,
1997; the upper Fagaloa and the lower Fagaloa, based on stratigraphic position and
also on some factors such as the thickness of lava flows, vesicularly, density and
mineral content.
The lower Fagaloa Fagaloa consist of dark brown to green to grey less
vesicular lava. It is made up of thicker pahoehoe flows with a thickness varying from
0.5m to 3.5m intercalated with volcanic breccias between one centimeter and up to
meters thick. The lavas are porphyritic rocks containing very coarse (1 cm -3cm)
phenocrysts of pyroxene and olivine. They are well exposed at Papaboa Point east of
Fagaloa Bay and Cape Utumauu to the east of Apia. A highly weathered part of this
lowest unit has an andesitic like colour, which commonly occurs, at the Fagaloa bay.
The upper part of the Fagaloa Formation is commonly comprised of highly
vesicular thin lava flow (5cm -2cm). They are porphyritic lavas and are intercalated
with volcanic breccia. It also consist of lensing and discontinuous thin pahoehoe
flows which are exposed at the northeast of Musumusu village, in the western part of
Fagaloa Bay in an outcrop about 28 meters high. Phenocryst of olivine ranges from
coarse to fine; pyroxene is not very common.
Sa lani formation
The Salani Volcanics are widespread in Upolu, forming most of the southeastern quarter of the island, and extending southwards over a wide area from Apia
to the Vaiee Peninsula on the south coast (Fig.2). A great deal of the eastern half of
Savaii is formed of Salani rocks, which are exposed, for examples, in the gorges of
the Vanu River, north of
Sili,
around Vaiola, and south and east of Ologogo
Plantation.
The Salani formation fills some of the deep valleys eroded in the Fagaloa
Formation for example areas to the south of Apia township on Upolu Island or along
the Vanu Valley to the east of Gataivai village on Savaii Island. Well developed
horizons of Vini tuff with a thickness of about 15 centimeters separate the Fagaloa
Formation from Salani formation. These are exposed on the western part of the
Alaoa Power Station to the south of Fagalii Bay or southeast of Apia (Kear and
Wood, 1959). Like the Fagaloa Formation the Salani lavas are deeply weathered
and highly eroded in some areas, for example the road cutting section to the
southwest of Alaoa Power Station south of Apia. The formation consists thickness
(0.5m --2m) with dark grey, fine- grained, porphyritic pahoehoe flows intercalated with
volcanic breccia beds 20 cm up to thickness. The more weathered nature of the
outcrops and the presence of few large, pale green olivine phenocrysts (like those
common in Fagaloa rocks) serve to distinguish these rocks from the lithological
similar, but younger Mulifanua basalt.
Mulifanua formation
In Upolu the Mulifanua Volcanics are exposed over most of the western part of
the island about 10 to 13 kilometers (Fig.2). A broad strip of Mulifanua volcanics
about 5 kilometers wide elongated in a northeast to southwest direction on the
northeastern part part of Upolu. It unconformably overlies the Fagaloa Formation,
which exposed along the Falefa River to the northwest of the Fagaloa Bay. On
Savaii island these rocks constitute most of the western part of the island
present in the eastern half near Fagamalo and Tuasivi.
,
and are
Mulifanua flows reached the sea through valley gullies or even lava tunnel.
Lava blisters vary from one meter into many meters in size. A huge blister formed an
island known as Albatross Island in Saluafata Harbour. The circular to elliptical dome
shape of the island is 45 meters or more in diameter.
The formation is highly vesicular, grey brownish red, jointed, porphyritic, non porphyritic and moderately weathered. The porphyritic variety is the most common
and contains phenocrysts of olivine (2 mm -3 mm in size). Pahoehoe flows with 5cm
to 30cm in thickness are intercalated with 2cm to 4cm thick volcanic breccia beds
and commonly the contacts are very irregular. Boulders are more common above
the surface than is the case of Salani rocks and they differ from those of the Salani in
that onion -skin weathering has not usually commenced to develop and hence the
boulders are angular (Kear and Wood, 1959). Mulifanua may be also distinguished
from the Salani formation largely on their lesser erosion and weathering.
Lefaga formation
The Lefaga rocks are well developed in the western third of Upolu (Fig.2). They
extend along the south coast from Sataoa almost to the head of Lefaga Bay, and
descend almost to the north coast near Saleimoa. The formation has not been
recognised in Savaii, but some of the western Mulifanua rocks there are very
scoriaceous, and might be close to Lefaga in age.
The surface expression is similar to that of Mulifanua rocks except that as at
ground surface is more common, the lavas appear to have flowed out into the
lagoona! area, and the reef is relatively close inshore.
Onion -skin weathering occurs locally, due possibly to the high feldspars
Lefaga rocks are dark -grey and black feldspathic porphyritic basalts with
much greenish and red scoria in thick irregular beds, and with many bombs and
lapilli. Flow surfaces at the coast are highly scoriaceous. The main lithological
content_
6
difference from Mulifanua rocks is the predominance of scoriaceous aa in the Lefaga
Voicanics.
The Lefaga are presumed to be younger than the Mulifanua Volcanics since the
width of the offshore reef is less. This may imply that the Lefaga basalt flowed into,
and partially filled a pre -existing lagoon, or that the reef commenced to grow at a time
of higher sea level change.
Puapua formation
This formation occurs like finger -like flow elongated parallel to the streams
exposed on the eastern part of Laulii Bay and Alaoa East to the south of Apia (Fig.2).
The ropy lavas of the Puapua formation have covered pre- existing flows, filled gorges
and flowed over high scarps cut in pre -existing rocks. They generally have a fairly
low gradient, and even surface in some areas to the south of Laulii village.
It
consists of columnar jointed with extremely broken and angular surface. The Puapua
lavas are dark grey to black pahoehoe flows, slightly to moderately vesicular,
porphyritic with common phenocrysts (2cm -3cm) in size of olivine and some
plagioclase.
Aopo formation
The Aopo formation is restricted to Savaii Island (Fig.2). In about 1760 lava
from Mauga Afi flowed past the west side of Aopo 'to the sea, and covered large
areas to the south -west, west, north -west and north of the village. In 1902 relatively
small eruptions occurred from fissures at Mauga Mu on the island summit to the
south of Aopo. Lava from the 1905 -1911 eruptions of Matavanu flowed northeastwards to the sea at Mauga, and thence spread north and south in the lagoonal
area toward the Saleaula village to the west part.
The rocks are extremely fresh and almost without soil, and in places are almost
devoid of vegetation. The surface of the flows is ribbed and puckered in very diverse
ropy flow patterns, studded with knobs, blades, and spines, and covered with a brittle
glassy skin of flattened bubbles and flakes that crackles as one walks on it. The
rocks consist of ropy, vesicular, porphyritic (feldspar and olivine) basalts, with but
little aa. Such aa as seen is blocky, loose, and appears to have been transport on
the surface of the flows and to have accumulated along the margins.
Sedimentary formation
The Holocene alluvium covers most of the exposed terraces of the older lavas
with very thick deposit along the eastern and northeast of Upolu. Other areas on
both Savaii and Upolu have an only a thin deposit of this type.
Most of the sedimentary units along the coast from Apia toward the northeast
end are pocket deposits. They occur mainly from the east of Falefa village toward
the Afulilo area on the west. These deposits are mainly talus of older volcanics unit.
The fringing reefs are very narrow or rare in most locations probably subsided as a
result of fault slumps, which drown the reef.
7
GEOLOGICAL STRUCTURE
The major faults in Western Samoa are approximately parallel the direction of
the volcanic alignment (Fig.2) on the western part of Savaii (Kear and Wood, 1959).
Many of the minor faults in the Fagaloa Formation on the northeast Upolu could be
associated with numerous slope failures (Gudge and Hawakin, 1991). However,
there is a lack of information as to whether the minor faults in the Fagaloa have a dip
slip or strike slip component in their movement (Hawkins, 1987). The exposures of
volcanic breccia (with clasts of coral and volcanic pebbles) about 8 meters thick on
the southeastmost part of Upolu (cape Tapaga) to the west of Tiavea indicates an
upthrow of a normal fault approximately parallel to the coastal line and elongated in
an east to west direction.
A seriesof faults have been identified from aerial photographs in areas on the
eastern and western sides of the Fagaloa Bay. A circular fault mirrors the shape of
Fagaloa Bay and is probably associated with a northeast to southwest fault and it
dissects the bay approximately in the centre. Other lineations (faults) observed to
the south of Saluafata Harbour are oriented approximately in the same direction.
Overall the Fagaloa Formation outcropped northeast of Upolu slopes gently
toward the NNE at an average of 12 °. A series of dykes with variable orientations
intrude the Fagaloa Formation and they dip from shallow to very steep angles or are
vertical. Their width varies from several centimeters to meter, Some are quite
vesicular, indicate proximity to the surface at the time of solidification. Mostly they
are highly jointed with commonly two jointed pattern, one parallel to the orientation of
a dyke and the other perpendicular. The orientation of these intrusive bodies are
more or less along a northwest to southeast alignment probably in the same
orientation as the volcanic cone alignment. The degree of weathering is very high
particularly where surface water is abundant for example at Falevao village to the
west of Fagaloa Bay.
Columnar joints are very common in the pahoehoe flows; columns vary from
several centimeters to a meter in diameter. Lava domes form vertical parallel joints
in some areas for examples Lemafa quarry to the southwest of Fagaloa Bay and
Falealupo on the western end of Savaii. The contact between pahoehoe and
volcanic breccia is commonly wavy. Lava channels vary from 6 meters to 12 meters
in diameter with a thickness of about 5 to 8 meters for examples the road cutting
section west of Laulii village and the Piula swimming pool to the norhtwest of Falefa
village on Upolu Island.
The Age of the Volcanism in Western Samoa
Kear and Wood (1959) used geomorphologic criteria such as erosion, extend of
soil profile and relation to offshore reefs to define the relative ages of lava formation
both Upolu and Savaii. Barrier reefs around Upolu were drown during the rise of
sealevel that followed the last glaciation, Chamberlin (1924) and Stern (1944). This
feature is common in area to the east of Falefa village toward Tiavea on northeast
Upolu Island. Exposures of lava along the northeast Upolu coast appear to be
considerably older than the drowned reef complexes that built upon them and are
now shallow banks far offshore. The exposures are deeply eroded and have
irregular coasts produced by drowning of river valleys; they stand as rugged
topographic highs above considerably younger basalts. A few of these younger
8
basa ts may be as the younger of the drowned reef (Kear and Wood, 1959) but most
are younger. The summary of the ages of these units is shown by (Tablel .2).
Unit
Age
Aopo Volcanics
1905 - 1911
Puapua Volcanics
Middle to Late Holocene
Lcfaga Volcanics
Early Holocene
Mulifanua Voicanics
Last Glaciation
Salani Volcanics
Penultimate Glacial to Last Glacial
Fagaloa Volcanics
Pliocene to Middle Pleistocene
Table 1.2 Western Samoa volcanic formations and their ages (from Kear and Wood,
1 959).
K -Ar ages have established that the Fagaloa Formation of Upolu was active
between 2.8 Ma (lower Fagaloa) and 1.5 Ma (upper Fagaloa) (Natland and Turner,
1985). This is based on five dated samples from northeast Upolu (Tablel.3).
Lithology
Basalt flow
5
Age (Ma)
Location
Masina
village
in
Fagaloa Bay
1.5 ± 0.05
(northeast Upolu)
Basalt flow
Southern end of Lufilufi (northeast
1.82 ± 0.06
Upolu)
Basalt flow
Between Falefa falls and Tapuivi
2.45 ± 0.07
(northeast Upolu)
Basalt flow
Cape Utumauu (northeast Upolu)
2.65 ± 0.07
Basalt flow
Cape Utumauu (northeast Upolu)
2.8 ± 0.20
Table 1.3 Age of basaltic flow from northeast Upolu Island or the northern pan of the
study area (from Natland and Turner, 1985).
9
1
History of Volcanism
The progression of volcanic activity in the Samoa chain is long thought to be
from east to west, that is opposite to the trend first observed for the Hawaii Island hot
spot (Dana, 1949). Savaii was noted earlier in this century for being the largest
single outpouring of basalt on any island in the Pacific outside of Hawaii (Kear and
Wood, 1959). An underwater eruption in 1866 (Fiedlander, 1910) to the northwest of
Manua group at the eastern end of the chain (Fig 1.1) raised the confusing prospect
of a Pacific island chain active at both ends.
Hawkins and Natland (1975) suggested that the deformation of the Pacific plate
along the Tonga Trench system was a possible explanation for nearly simultaneous
volcanism along about 1500 kilometers of the Samoa chain (American Samoa Wallis Island). Shaw (1973) proposed, however that linear island chain volcanism is
a consequence of melting in due to shear resulting from vertical or horizontal motions
in the asthenophere; shear melting is therefore a plausible explanation for Samoa
younger volcanism. However the geochemical signature of the post Fagaloa lavas
from the (study, 1 997) is incompatible with this interpretation.
Natland (1980) proposed that plate bending provide the principal zone of
dilatancy allowing younger volcanism lavas a route to the surface. The first of the
older Samoa volcanoes, therefore appears to have formed to the north and east of
the region of pronounced plate bending, probably at least as far away as the present
Manua group.
Most older Samoa volcanoes formed in short strings of volcanic centers.
Submerged bank west of Manua forms one such string ( Natland, 1980). The older
volcanoes of Tutuila form another while the east -west Fagaloa ridge perhaps forms a
third.
Eruption Types
What determines whether a volcano extrudes magma violently or "gently "? The
primary factors include the magma's composition, its temperature and the amount of
dissolve gases it contains. To vary degree, these factors affect the magma's
mobility, or viscosity. The more viscous the material, the greater its resistance to
flow. Note that a magma's viscosity is directly related to its silica content. In general,
the more silica in magma, the greater is its viscosity. Consequently, because of high
silica content, rhyolitic lavas are very viscous and tend to form comparatively short,
thick flows. By contrast, basaltic lavas which contain less silica, tend to be quite fluid
and have been known to travel distance of 150 kilometers or more before congealing.
In Hawaiian type of eruptions, (eg. eruption on Upolu and Savaii Island) the
magma are hot and basaltic, so they are extruded with ease. By contrast, highly
viscous rhyolitic magmas (eg. Mount Ruapehu 1995 -1996) are more difficult to force
through a vent. On occasion, the vent may become plugged with viscous magma,
which results in build up of gases and great pressure increase, so a potentially
explosive eruption may result. However, a viscous magma is not explosive by itself.
It is the gas content that puts the bang into a violent eruption.
10
Lava samples of both Fagaloa and post -Fagaloa from western Samoa are
mainly concentration in a range between 41 and 51 percentage of silica content
which compare to a violent eruption (rhyolitic -type) of over 70% in silica content (eg.
Mount Pinatubo in Philippines -1991 and Taupo eruption 1800 and 26,500 year ago).
Field observation together with geochemical data interpretation of lavas from Samoa
resulted of three main types of eruption which occurred in the western Samoa which
as followed;
Lava -dominated eruptions
Lavas are flows of coherent magma that are erupted at the earth's surface
during effusive volcanic activity which is essentially non -explosive, or, for some
basaltic lavas, that are fed by lava fire fountains. Due to their low silica content,
basaltic lavas are usually very fluid and flow in thin, broad sheets or stream -like
ribbons.
These types of eruption are well known from the Hawaiian volcanoes and tend
to be relatively non -explosive, but produce large lava flows which can inundate broad
areas for example the broad lavas flow from 1905 -1911 eruption to the northern part
of Savaii. These eruptions are derived from long fissure zones, some of which may
be a kilometer or more in length. At the vent fissure there is a fire- fountaining of
scoria and highly fluid magma up to several hundred meters high. This builds up
small scoria cones around the vents which are commonly outcropped to the southern
part of Itu -o -tane (Patamea-Sasina), southwest part (Taga-Falealupo) and to the
western part of Faasaleleaga district (Salelologa- Puapua) on Savaii, and also to the
eastern end part of Upolu Island (Aleipata district -Falealili district). Associated with
this is a continual discharge of highly fluid magma causing lava flows. The lavas
flows are controlled by topography near the vent and paths of the initial flows can be
predicted once the vent location is known. Later lava flows tend to overtop earlier
ones and can flow into different areas. Many of the lava eruption on Upolu and
Savaii Island developed from a series of aligned fissures, and this may cause lava
flows in several different areas at once that may be several kilometers apart. Lava
flows travel at varying speeds, depending on the viscosity of the lava and the slope
over which it travels. On the more steeply sloping parts of the islands some flows
may travel faster than human running pace. Lava flows may are very hot, with those
in Samoa likely to be in excess of 800- 900 °C.
These types of eruptions may continue for months to years. Several past
eruptions (for example the Mulifanua and Lefaga formation on Savaii and Upolu) of
this nature have inundated areas approximately over 10 -30 km2 with lava flows,
whilst lava flows from at least one eruption covered more or less 18 -26 km2 with up to
10 -15 m of lava.
Cone -building eruptions
These eruption are more explosive due to a greater amount of gas within the
eruption magma. They may form from an elongate fissure or from central vent. Such
eruptions on the islands can produce hot columns of rising volcanic ash and gas that
reach up to 9 -12km above the vent. Volcanic ash and gases are directed by the
wind, with heavy ash falls occurring in downwind areas for example at the village of
Mauga on the north of Savaii and also road cutting outcrop between Lata and
Matautu on the south east end of Upolu. Large scoria cones may be constructed
11
around the vent area. Lava flows are also generate during these eruptions but tend
to be confine to total areas of <5 -10 km2.
Cone -building eruption are more or less may last for months to years, with small
ash falls being distributed over different areas depending on wind directions. Most of
very thin ashes deposit observed on the northeast and other locations to the eastern
end of Upolu Island are well welded, with some indication of how hot were these flow
during the time of eruption. Some cone -building eruptions may have many periods of
activity, building up a series of adjacent cones or nested cones over a series of
months or year, which properly referred at the inner part ( Mauga Silisili area) and to
the eastern part of Savaii at Faasaleleaga district.
Water -magma eruptions
This type of eruptions which really need more work to identify their wide
distribution in the field. Water -magma eruption have a possibility of less common on
both Upolu and Savaii compared to the other two styles of eruption above, but are
the most explosive types of eruption that are ever likely to occur on islands. These
eruptions occur where rising magma encounters water, either swamps, lakes or
shallow sea water. Magma at up to 1100 °C causes super- heating of water it meets,
and an explosive chain reaction occurs. These eruptions, generate radial blasts of
water, gas and rock particles which move at extremely high velocity (55 -100 krn /hr)
up to 3 km from the vent location (eg. Moore, 1987).
An examples of this type of eruption occurred on the Apolima Island situated
between Savaii and Manono Island also Fanuatapu, Namua and Nuutele Island to
the east of Upolu and of Cape Tapaga on the southeastmost part of Upolu. All
exposures of this formation are of marine ash rings in various stages of erosional
destruction. In general these rings of ash slope radially outward, presumably away
from the center of eruption at an angle more or less between 30 and 40 °. These
deposits consist of hard, thick -bedded of calcareous ash which associated with
scoria, coral and other marine organism fragments.
Genetic link between Upolu, Savaii and
oceanic lavas
other
In the (study, 1997) geochemistry was the main method which used to present
new petrological data of these mafic lavas of Samoa. Whole rock and mineral
chemical analyses are the main types of data used in this study to interpret the origin
and relationships of lavas in both Upolu and Savaii Island. Major and trace element
characteristics of the lavas provide information on nature and efficiency of melt
extraction from the mantle source of the island. (Major elements; Si02, T102, A1203,
Fe203, MnO, MgO, CaO, Na20, K20 and P205, Trace elements; Ba, Rb, Sr, Pb, Th,
U, Zr, Nb, Y, La, Ce, Sc, V, Cr, Ni, Cu, Zn, and Ga).
Genetic link between Fagaloa, post -Fagaloa Formation and other oceanic
basalts are suggested by way of similarities in geochemical properties. Such a link,
is able to be established on a number of grounds. Firstly major and trace element
data show a consistent overlap of trends associated with Fagaloa Formation lavas
12
and other oceanic basalts, and thus provide an independent corollary to the
hypothesis of shared magma batches (Fig.3a -i). Secondly is the similar Zr /Nb ratio
represented by parallel trends (Fig.3j).
These genetic links between basaltic lavas of Upolu, Savaii and others can be
summarised as followed: Lava's generated on Savaii, seamount on the northwest of
Savaii and Upolu represent a continuum of same magmatic processes, heralded by
plot of Zr vs Nb. Geological data obtained by previous worker also corroborate the
foregoing approach. No conspicuous geochemical difference is observed on lavas of
Upolu and Savaii with changing age which leads us to infer that the whole suite of
western Samoa lavas, both Fagaloa and post- Fagaloa are petrogenetically related.
A question needed to addressed (Is whether the stratigraphie subdivision which
based on geomorphological features by Kear and Wood, 1959 are still appropriate ?).
13
(a)
.
(b)
ow ¡ d
54.
e
m111411014
iO
u
°
o
1S
(d)
SFOz (Vt7G)
(c)
m
49.
a
°
o7 r
34
44
54 p
70
N
711
Mj N>,rLer
(9)
55
se
30
40
(e)
47
M
Sioi(wt%)
SiO2 (+%)
1J
42 n 34 A
1N
1114
74
43 w
,a
14
3r
45
it Si N if
1M
140
180
73
M= Number
260
340
Tr (ppm)
II ff
75
13 n
()
(h)
1115 MamDer
33
Mg Hamlet
Mt Manlier
420
75
15
Ms Number
500
Fagaloa of northeast Upolu (this study)
Savaii (Cibik, 1997)
c Northwest seamounts of Samoa (Johnson et al, 1986)
Tutuila (American Samoa) (Nadand, 1985)
o Cook Island chain (Thomson, 1997)
o
Figure 3 Variation diagrams of the Fagaloa Formation with respect to other oceanic basaltic lava.
Previous Work
Most of the earlier geological publication on Samoa are written in German.
Dana (1849) reported on geological exploration which was a part of the US
Expedition under Lt. Charles Wilkes. He suggested that Upolu and Savaii were
formed from two volcanic fissures, one of age equivalent to Tahiti (in the Society
Islands) and Kauai (in the Hawaiian Islands), and a second contemporaneous with
the present reefs.
Isotopic, geochemical and petrographic studies of the basaltic lava flows have
been reported by Hauri et al (1993) and Hauri and Hart (1993), Pored and Farley
(1992), Wright (1987), Wright and White (1986), Johnson et al (1986), Natland and
Turner (1985), Palacz and Saunders (1985), Peterson and Tilling (1980), Hawkins
and Natland (1975), Hawkin (1975), Hedge et al (1972). Geologic and tectonic
studies of the islands include those by Gudge and Hawkins (1991 and 1988),
Hawkins (1987), Okland et al (1986), Brocher (1 985), Stice (1 968), Kear and Wood
( 1959), Sterns (1944) and Thomson (1921).
Geophysical studies include a
paleornagnetic study by Keating (1985a) and Taring (1962; 1955).
Further Study
Further work on Upolu and Savaii Island may help to determine the relationship
between the upper and the lower Fagaloa or with post -Fagaloa lavas in more detail,
Isotope work would undoubtedly be useful in further constraining the petrogenesis of
the Fagaloa and post -Fagaloa Formation. Geophysical method would be very
importance to investigate the internal structure and eruption style of each volcanoes
(gravity and aeromagnetic). It is also used to determine bulk magnetisation
parameters, especially related to anomalous magnetisation direction, which provide
temporal link between volcanoes.
15
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2l