By Than Htun(GEOSCIENCE MYANMAR)
This article is the continuation of Episode 53 provided the explosive craters on both banks of the Chindwin River.
The Explosive Craters occur on both banks of the Chindwin River around the village of Shwezaye.
Olivine-Basalt: The predominant rock type in the ejected blocks from the Twintaung crater is a well-marked and highly characteristic olivine-basalt. It is compact and consists of a pale-grey matrix containing numerous large dark-green phenocrysts of augite. Besides these, there are a few small phenocrysts of olivine. The felspars show every gradation from large sizes to the small stumpy ones of the groundmass. They are markedly zoned and show some resorption along the edges. Nearly all the felspars show lamellar twinning but with a wavy extinction. The other minerals occurring as granules interstitially are mainly augite and magnetite.
Other ejected blocks from the Twintaung crater include pyroxene-hornblende-andesite, hornblende-andesite, augite-peridotite, biotite-hornblende-peridotite, hornblendite (hornblende-perknite), augite-rock or pyroxenite and hornblende-schist.
Craters on the West Bank of the Chindwin River.
About 2 ½ miles south of Shwezaye Taung is a group of three craters lying contiguous to one another. The high ground from the river bank to the edge of Taungbyauk crater, the northernmost of the three, is covered by an ash deposit.
Taungbyauk Crater: Sloping up to the crater’s edge from the north, west, and east are ash accumulations similar to those described above in the case of Twintaung. Mixed everywhere with the layers of ash as well as strewn about the surface, are small quartz pebbles. These beds are also coated with a calcareous sinter and emit the same hollow sound when traversed. They exhibit, near the crater’s rim, a slightly undulating surface traverse to the direction of the slope.
The crater’s inner walls are precipitous and usually composed of ash deposits with lava fragments and blocks. The rim is highest to the northeast, where a solid wall of lava is exposed. The lava is an olivine-basalt and differs further from the basalt and andesite of Twintaung in that large phenocrysts are not evident.
The crater measures approximately half a mile in diameter from rim to rim and averages about 200 feet in depth. The floor is now partially covered by a small lake which is fed by fresh-water springs at its edge.
Ejected blocks of lava from this crater consist of a greyish rock in which the flow structure is well marked by lines of elongated vesicles. A thin section shows numerous phenocrysts of olivine which are altered along the edges and cracks to serpentine. The phenocrysts of augite are smaller, and, in contrast to the majority of rocks from the Lower Chindwin region are a brownish-green, while in others, of a normal greenish colour. The groundmass is coarser-grained than in most of the lavas of the district, and there is a marked fluxional arrangement of the long, lath-shaped felspars. Occurring interstitially are abundant crystals and granules of augite and numerous grains of magnetite. The residual material resembles that of the rocks from Kyaukkadaung. It is colourless, with a feeble double refraction, and shows wavy extinction. The lava from this crater is very similar to the picrite-basalt of Kyaukkadaung.
Taungbyauk: The southern rim of Taungbyauk crater is no longer intact but has been partly obliterated by ash deposits thrown out from the middle crater of the group, into which the ashes dip at an angle of 30°, and it would appear, therefore, that the middle crater was formed after the outline of the first had been established. This middle crater is not so clearly defined as the others. No lake covers the floor, which is partly silted and now under cultivation.
Twintaung: Between the middle and southern craters of the group of three is a hill marked as point 812 on the one-inch map. This hill rises some 550 feet above level and is the highest point in the neighbourhood. It has been built up by a great thickness of ash accumulations, which do not appear to be associated with the deposits from the existing craters and must, therefore, represent the remnants of the ash deposits that were originally spread over the neighbourhood. This hill, Twintaung, must not be confused with Twintaung, the craters on the other side of the River Chindwin.
Twinywa Crater: This crater is the southernmost of the group of three. It is the largest in diameter, measuring as much as three-quarters of a mile across. It is very shallow compared with its width, averaging not more than 150 feet in depth. No wall of lava is exposed. The ash beds, comprising a thickness of from 30 to 50 feet, are spread out over Irrawaddy sands and form the rim of the crater. The soft sands are well seen below the ash deposits and from the lower slopes of the sides and floor. The usual fragments and blocks of lava are strewn about the slopes and outer surface. The inner wall of the crater slopes gradually inwards to the edge of a lake, nearly half a mile across, which now covers the greater portion of the floor.
The chemical analysis of the olivine-basalt from the Twinywa crater with its molecular proportions. According to Professor Conrad Burri, the analysis shows higher alkalis than ordinary basalts, and the rock may well be compared with nepheline-bearing lavas (basanites, berondrites, etc.) which is confirmed by the norm. The actual rock, however, contains no nepheline and also no glass. The augite must, therefore, contain some alumina and alkalis, and the plagioclase, perhaps some carnegieite molecules. The occurrence of calcite and the “reversed” zoning of augite from calcium-poor towards calcium-rich suggest probable assimilation of limestone by the rock, most probably from the Pegu Series.
Leshe Craters: Two miles to the southwest of Twin Crater are the compound craters of Leshe, which exhibit the same characteristics as those described above and are surrounded on all sides by a superficial covering of ash deposits that slope gradually up to the rim. No wall of lava is exposed in the crater, and the ash beds are seen to rest upon unaltered Irrawaddy sands, which form the lower slopes and the floor of the depression.
Banbwe: Immediately north of Banbwe village, which is situated about four miles to the southeast of Leshe, there is an isolated ridge of ash beds with a precipitous face to the north. This occurrence lies to the south of the Yinbo Chaung, which has produced a plain to the north of the village separating this ridge from the main ash deposits to the northwest. It would appear that this isolated occurrence of older volcanic tuff of a basaltic nature is a remnant of the deposits which were once scattered over the neighbourhood rather than derived from a crater additional to the above. There are several isolated occurrences in the immediate neighbourhood which were all derived from a common centre. The original deposits were spread out over soft Irrawaddy sands which are rapidly removed by sub-aerial denudation, and thus it is that the existing remnants of today are usually confined to the higher levels.
The bulk of the volcanic activity appears to have been centred about Twintaung and Taungbyauk, especially the former, and it is clear that these two craters mark the sites of older vents, now obliterated, which were in activity for a considerable period following upon the close of Irrawaddy times. It would appear, further, that the existing craters were formed subsequently to the cessation of former activities after the original deposits had been much modified in form by denudation.
The accumulations of ash beds with lapilli four and a half miles distant from the centre of the eruption (Twintaung), the quantity of debris strewn about the outer surface of the cones, all bearing jagged edges as though they had been torn away from a larger mass without the smoothing action of heat, and, finally, the fragmental nature of the cones themselves, all point to an explosive type of eruption.
The successive layers of coarse lapilli, several inches in thickness, between thin layers of fine material, throughout a thickness of some 50 feet of deposits, confirms the suggestion advanced above that the craters were not formed by one explosion but were gradually built up by a series of explosive eruptions which must have extended over a considerable period.The absence of evidence of any extensive lava outpourings from these craters is peculiar, especially as there occur in the vicinity thick sheets of andesitic and basaltic lavas that have been poured out from some vent, but which are not accompanied by ash accumulations. It would appear that the eruptions in the region varied from violently explosive at one extreme to gentle outpourings of sheets of lava at the other.
At Natlabo village on the east bank of the Chindwin River and three miles south of Wuntho, four miles south of the Chindwin River and three miles south of Wunbo, four miles south of the Twintaung crater and 3 ½ miles east of the Taungbyauk crater, a sheet of lava, 10 to 20 feet thick, resting upon Irrawaddy sands, can be observed from the river.
On the east bank of the Chindwin River opposite Natlabo North (22 º 18’, 95 º 2’) a cliff of basaltic tuff, similar to that of the other craters, stands conspicuously over the river, accompanied by huge basalt blocks. The latter suggests the proximity of the source, but no crater exists in the vicinity.
The Southern Volcanic Occurrences on the West Bank of the Chindwin River.
Minma: One mile south of the Leshe craters at Minma in the Yinbo Chaung masses of lava are exposed. Rising from the Chaung on the south side of the village are two plateau-like elevations marked on the map as the Myetthadaung, which are capped by a sheet of lava (vesicular dark olivine-basalt), resting apparently on quartzites. The rock is composed of phenocrysts of olivine, augite, hornblende and plagioclase, embedded in a groundmass of the same minerals with abundant laths of felspar and a little glass occurring interstitially. In the Yinbo Chaung, half a mile to the west of Myetthadaung and west of Minma village is an outcrop of granite. This granite is coarse-grained with small flakes of yellow mica. It is very much decomposed at the surface and crumbles in the fingers.
Silaung Taung: Three and a half miles southeast of Minma and three miles south-south-west of Banbwe is an elevated plateau of about one and a quarter mile in length and some three-quarters of a mile in width, known as the Silaung Taung. This plateau rises about 150 feet above the level of the surrounding country and is capped by a horizontal sheet of lava, at least 50 feet in thickness, which presents a vertical cliff at its extremities. It is composed of a dark grey olivine basalt. Pink calcareous sinter with embedded fragments of basalt is met someway down the slope of the hill. There is no evidence of a vent in the immediate vicinity. It appears to be a very recent date and must have been poured out from a neck, which is now concealed by the flow. There are a few disintegrated blocks around the immediate edges of the flow, but they do not extend outwards for any distance.
References: Chhibber, HL, 1934: The Geology of Burma, Macmillan, and Co Limited, St Martin’s Street, London.
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