The Burma Circle of the Geological Survey of India and their Contributions to the Geology of Myanmar
By
THAN HTUN
(GEOSCIENCE
MYANMAR)
EPISODE:58
Volcanoes of
Narcondam and Barren Island
This article continues from Episode 57,
highlighting key points from HL Chhibber’s 1934 account of Volcanoes of
Narcondam and Barren Island.
The farther south, though outside Burma,
is situated on the same line, with the volcanic islands of Narcondam and Barren
Island, which deserve special mention for several reasons. Firstly, their rocks
bear a very close similarity to those of Burma, and in reality, the volcanoes
themselves form a southern continuation of those of Burma. Secondly, they are
the only volcanic islands in the Bay of Bengal, while Barren Island, perhaps,
constitutes the only instance of an active volcano in these seas. Both of them
rise abruptly out of comparatively deep water and may furnish material for the
modern petrologist who desires to study the nature of magmas that have not been
affected by surface contamination and assimilation. It is regrettable that
being situated in rather out-of-the-way places, they have not yet received the
full treatment they deserve.
Narcondam.
Topography: Narcondam lies
about 75 miles NNE of Barren Island. Like the latter, it rises abruptly out of
deep water, with depths of about 450, 565, and 562 fathoms being noted on the
chart within about one kilometre from the island in various directions. Its
area is 2.78 square miles, and it has an oval outline. Its greatest length,
northeast-southwest, is about 2.5 miles, and its greatest width is about 1.75
miles. It has a roughly conical form, with sides deeply scored by ravines. The
summit, 2,330 feet above the sea, is rather to the south of the centre of the
island, and it bears no crater. The culminating portion of the volcano includes
three hummocks, of which the northern is the highest. The shore is cut away by
wave action into cliffs, up to several hundreds of feet high.
The lavas that build Narcondam,
according to HS Washington, are very uniform in composition. Mallet described
them as hornblende-andesite, but Washington, under modern terminology,
designated them dacite, as his analyses showed that the silica content is much
too high for andesite, although little or no quartz is visible.
Hornblende-Andesites:
The
specimens of lava collected from the highest point of the mountain and
different localities on the slopes and around the coast are all compact or very
slightly vesicular rocks, in which crystal of white translucent felspar and
black or dark-brown hornblende, with occasional biotite crystals, are
disseminated through a groundmass, which is generally light-grey in unweathered
specimens. According to H S Washington, the felspar phenocrysts comprise
orthoclase, andesine, and calcic-plagioclase, about labradorite Ab1 An2. The
first two minerals are mainly untwined and show central clusters of minute
inclusions. The crystals of dark yellowish-brown hornblende are about as large
as those of the feldspar but are far less numerous. An occasional dark-brown
biotite is seen, and large sections of hornblende and biotite show very dark
margins. Magnetite and apatite also occur. The groundmass is microcrystalline
and mainly felspathic, being composed of andesine with specks of augite,
hornblende, magnetite, and biotite.
Agglomerates: Volcanic
agglomerates composed of rounded and angular fragments of lava of every size up
to three or four feet in diameter, embedded in a finer matrix, are exposed in
sections along several parts of the coast. As stated, Washington, and Mallet
believed, because of the “acid” and very uniform character of the rocks and the
absence of a crater and ash, that the island was a normal eruption.
Barren Island.
Barren Island lies approximately 12°
15’N Lat and 93° 50’E Long, and is about 100 kilometres east of the southern
end of Middle Andaman Island. Oldham states, “Soundings show that the cone
rises from 800 fathoms below the sea.” Barren Island is almost circular, with
an area of 3.07 square miles. Most writers on vulcanology have regarded these
islands as situated in the continuation of that great zone of volcanic energy,
which stretches for some 2,000 miles along the Sunda group of islands, the
terrible activity of which was illustrated by the explosive eruption of
Karakatoa in 1883.
The island consists of an almost
circular ridge with heights varying from about 600 feet above sea level on the
northwest to 1,160 feet on the southeast. On the western side, there is a deep
breach, giving entrance to the interior of the crater. It has a diameter of
about a mile, and its floor has altitudes above sea level of about 80 feet near
the landing place to 319 feet east of the central cone. Towards the southeast,
the slopes, rising to a height of 700 and 900 feet above the floor of the
crater, or some 1,100 above the sea, are well wooded. On the north, they are
much less steep and high, with rounded outlines towards the top, and are largely
covered with loose, black, naked ash.
Central Cone: In the centre of
the crater, the central cone rises to a height of about 1,015 feet above the
sea, or about 700 to 900 feet from its western foot with, as seen from the
west, an almost perfectly symmetrical outline, the sides preserving an even
slope of 20°. The truncation at the summit marks the site of the crater, and
here, a thin column of stream rises slowly into the air.
The Recent
Crater: The
recent crater is ovoid, with the major axis lying about S by W-N by E. The
highest points of the edge, to the north and south, are 78 feet above the
floor. The material inside the crater is loose blocks of lava, scoriae, and
ash, but to the southwest of the bottom, there is a mass of solid lava, which
is probably in connection with a flow down the side of the cone, which has been
covered over by the latest eruption of fragmentary material. The floor has a
diameter of 60 feet.
Streams of Lava:
Almost
encircling the base of the cone are streams of lava, one of which has poured
through the breach into the sea. They are all covered with rugged black and
scoriaceous crusts and with fissured and hummocky surfaces.
The crater rim is higher on the
southeast. Mallet ascribed the fact that the northwestern half of the volcano
is of lower elevation than the other half to a bodily sinking of that side.
But it may equally well have been
lowered by one of the explosive outbursts, during which the northwestern side
was partially blown away. Similarly, the northern side of Mount Popa is of
lower elevation than the other sides and also has a breach in that lower wall,
and probably the same explanation applies. Moreover, the evidence of the Akyab
coast and the Andaman and Nicobar Islands points to a recent uplift of land and
not to a subsidence.
Lavas of the
Ancient Cone: The
ancient cone of Barren Island is largely made up of lava. The specimens
described by Mallet are more or less vesicular, with small crystals of white
translucent felspar and small granules and crystals of bottle-green,
translucent augite disseminated through a dark-grey groundmass.
Brownish-yellow olivine is frequently present also. The felspar is triclinic
and contains glass enclosures and numerous black opaque particles, many of
which have the shape of magnetite crystals. The groundmass is microcrystalline
and is made up of minute crystals of plagioclase felspar, with augite and
nearly or quite opaque specks, which appear to consist of more or less
decomposed augite with some magnetite.
Fragmentary
Ejectamenta of the Ancient Cone: Fragmentary ejectamenta interstratified
with the lavas of the ancient cone are exposed in some good sections along the
coast. In some places, the scoriae are rudely interbedded with irregular layers
of lava. One hundred yards northwest of the landing place, a somewhat contorted
bed of black volcanic ash is interstratified with a similar alternation of
scoriae and lava. They show miniature faults of a few inches throw by which the
bed has been dislocated.
Recent Ash on
Ancient Cone: A
considerable part of the ancient cone, including both the walls of the crater
and the exterior slopes, are covered with ash ejected from the central cone.
The greater proportion occurring to the north and northeast may be ascribed
partly to the influence of the southwest monsoon and partly to the crater rim
being lower there. The newer ash differs from most, the older in being nearly
or quite black instead of dull red.
Flows of the
Central Cone: According
to Mallet, there are three distinct flows of quite recent lava, which may be
distinguished as the eastern, the southern, and the northern streams. In
general appearance, they are all very similar to one another, presenting the
same exceedingly rough, hummocky, and fissured surfaces with the black
scoriaceous crust broken into pieces of every size up to those many tons in
weight. The lava streams have lateral and terminal banks, so they are
frequently noticed in the case of Mount Popa and other volcanoes.
The recent lavas, like those of the
ancient cone, consist of white, translucent, triclinic felspar crystals and
bottle-green translucent augite with frequently a little olivine in a
microcrystalline groundmass composed of triclinic felspar, augite, magnetite,
apatite, etc. This description compares perfectly with the recent olivine
basalts of the Twintaung type from the Lower Chindwin and Mount Popa areas.
Southern Flow: The southern
stream broke out a little more than halfway up the cone, at a point now marked
by a slight projection. After pouring down the side of the cone, it followed
the course of the valley into the sea, which has since then cut the face into a
cliff 10 or 15 feet in height. This was the largest flow of all, a considerable
part of the area covered by it being obscured by the sand which has been down
from the old crater walls to the south.
Northern Flow: The northern
stream broke through at about 250 feet below the crater; close to the point of
issue, there is a solfatara with an opening eight or nine inches in diameter.
From the mouth, steam issued copiously. From the point of issue the molten
rock, after reaching the base of the cone, flowed down the valley and over the
previous stream from the south, the termination of the later flow being marked
by a bank a little to the west of the cone. Mallet believed that the lava had
been emitted within a quite recent period, almost certainly within the last
century.
Recent Aqueous
Deposits: In
the lower part of the main gorge, which debouches to the south of the alluvial
plain, there is an agglomerate composed of rounded and angular lumps of volcanic
rock in the finer matrix which appears to be a deposit of the stream itself. At
the mouth of the gorge to the north of the anchorage, another aqueous deposit
may be seen, and doubtless, there are many others on the island.
Hot Springs: There are springs
of hot water, with a temperature of 116° F. in 1884, which gush forth on the
beach at the landing place. The shore between the cliffs of ancient lava and
the recent stream has a length of about 40 yards, and along it the hot water
issues in numerous places from the shingle. The level of the springs rises and
falls with the tide, and when first observed in 1832 the rocks were so hot that
the “stones on the shore, and the rocks exposed on the ebbing of the tide, were
smoking and hissing and water was bubbling all around them.”
Physical and
Geological History of the Volcano: The physical and geological history of
the volcano can be divided as follows:
(1) The first and important phase must
have formed the volcano, which is now represented by the outer amphitheatre. It
is made up of scoriae, ash, agglomerates, and basaltic lava.
(2) The second stage consisted of a
great paroxysmal eruption when part of the crater was blown off, and the
present breach was formed. Probably, the sea invaded the inner part of the
volcano at the same time.
(3) The third stage built up the central
pile, which consists of newer fragmentary ejectamenta, ash, scoriae, agglomerate,
and lava flows.
(4) Lastly, the flows of recent lava
welled out, some of which Mallet believed were not even a century old.
(5) Since the welling out of lava
streams, the volcano has been seen in activity by several observers, ejecting
red-hot stones, flames, smoke, steam, and Sulphur vapour. The last is
responsible for the deposits of Sulphur seen in the solfataras.
References: Chhibber, H L, 1934: The
Geology of Burma, Macmillan and Co Limited, St Martin’s Street, London.
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