By THAN HTUN (GEOSCIENCE MYANMAR)
THE Younger Andesites and Basalts, with Associated Tuffs and Conglomerates.
This article is a continuation of Episode 51 on the Mount Popa Region, extracted from Chapter XXX of The Geology of Burma by HL Chhibber (1934).
The Taungbaw Flow.
Acid Olivine-Basalt: This type is from one of the minor flows of the present Mount Popa and is found to the north of the crater, where the wall has been entirely blown away. It is probable that the flow was originally far more extensive and that the final disruption of the mountain destroyed the greater part; the mass now remaining forms a small hill immediately to the east of Taungbaw village. The rock is purplish-grey, with numerous small phenocrysts that form a distinctive feature of the freshly broken surface.
The rock contains a few phenocrysts, apparently pseudomorphous after hornblende, which was described in connection with the rock of the Taunggala neck.
The Ngayangon Flow.
Hornblende-Augite-Andesite: Like the Taungbaw type this comes from one of the minor flows of the present Mount Popa and is also found to the north of the crater where the greater part of the floe was probably destroyed in the final disruption of the mountain. The remnant forms a small hill just to the east of the village of Ngayangon. A similar rock was observed north of Kanzatkon village. This lava is a fine-grained rock with a whitish and purplish mottled appearance, the mottling being on a fine scale. There are numerous small phenocrysts of black or reddish-brown colour.
This rock is very similar to the one from the Taungbaw flow. However, many of the hornblende phenocrysts have not been completely replaced and consist of bright brown hornblende, very strongly pleochroic, of the basaltic hornblende type. Associated with the hornblende crystals are a few flakes of biotite. The felspar, both as phenocrysts and in the groundmass, is partly corroded and filled with specks of calcite, etc.
The Old Popa Bungalow Flow.
Hornblende-Augite-Andesite: This flow occurs on the western flank of the main mountain about 500 yards down from the crater wall. The rock’s specific gravity is 2.48. Messrs D Waldie & Co Calcutta determined that the rock has 59.9 per cent silica. It is a pale-grey, rough-textured rock in which small white felspar crystals are conspicuous, and has a trachytic appearance in hand -specimens. In some specimens, dull phenocrysts of a dark colour are visible.
Hill “4,801” Flow.
Glassy Hypersthene-Augite-Andesite: This flow forms one of the highest peaks of the crater rim and dips steeply down towards the southwest. The rock is usually pale purplish-grey with numerous small white feldspars often showing a fluxional arrangement. No dark phenocrysts are visible. Numerous phenocrysts of feldspar, usually water-clear, but between crossed nicols notably zoned, are to be seen. A considerable proportion would seem to be orthoclase, but acid plagioclase is also abundant. There are numerous smaller phenocrysts of augite and many large grains of magnetite.
Hornblende-Augite-Andesite: This lava is represented by blocks near the Taungbaw Rest House which probably come from a flow in the eastern wall. The rock is a handsome pale-grey colour, with numerous dark-brown elongated crystals of hornblende. The specific gravity is 2.48.
The Western Flow.
Olivine-Basalt: By the “western flow” it means an important lava which now forms the boundary wall of the disrupted crater on its western side. Perhaps “North-Western Flow” would be a better name, for it probably formed the north-western wall of the crater before its disruption.
The flow was examined at its northern end in a deep ravine to the north of the village of Ngayangon. The rock varies but is generally dark grey to almost black in colour, with numerous very large phenocrysts of very dark green augite. The dark colour of the groundmass renders these phenocrysts less conspicuous than in the rock of the eastern flow.
The Eastern Flow.
Olivine-Basalt: This is a very extensive flow and appears to form the whole of the hill due east of the Taungbaw Rest House, from which point the flow seems originally to have left the crater. From this hill, it extends down eastwards and northwards and forms the lava wall on the eastern side of the disrupted portion. It is a pretty and characteristic rock of pale grey or pinkish-grey colour, marked by numerous large squarish dark-green phenocrysts of augite. The surface of the flow is darker in colour and vesicular. Blocks of this lava are scattered all round Mount Popa, and the original flow must have been more extensive than it is now. In appearance, the rock closely resembles the Twindaung lava of the Lower Chindwin.
The North-Western Outer Flow.
Hornblende-Augite-Basalt: The end of this flow is well-exposed by the side of the main motor road from Welaung to Kyaukpadaung, near milepost 33 Mile 5 Furlong.
At the end of a flow, lava, as is well known, breaks up into partially cooled blocks that roll over one another, some becoming squeezed and flattened, others rolling on some distance ahead of the main mass and becoming hardened before being covered up. It is evident that the flows of Mount Popa behaved similarly, and it is the “nose” of such a flow that is exposed by the roadside.
As one would expect, a considerable variation in rock type in a tiny area exists. In general, the rocks are dark grey but often purplish and contain numerous phenocrysts of greenish augite and dull-brown hornblende, etc.
Iddingsite-Basalt: The rock from near Legyi village, Popa road, is light-grey in colour, finely vesicular, and seems to represent the surface of the flow. Fewer big phenocrysts of dark green augite characterize it.
The Plantation Flow.
Enstatite-Basalt: Near the foot of the waterfall, ¾ mile north of Hill “4,502” and ¾ mile south-west of the Taungbaw Rest House, a large number of huge fallen blocks occur in the midst of a banana plantation stretching right to the foot of the crater wall. They consist of enstatite-basalt.
The Flows of the Western Crater Wall.
Several flows comprising hornblende-augite-andesites and hornblende-andesites are exposed in the western wall of the crater itself, which is east of Hill “ 4,502”. Many specimens were collected from fallen, and they exhibit a certain community of characters. The rocks from the western wall of the crater are characterised, in the main, by an absence of large or conspicuous phenocrysts. They thus contrast with the eastern flows, where very large and conspicuous phenocrysts of augite and the general rule.
The Later Ashes, with Bombs.
The later ashes are unconsolidated and of light grey colours. They often fill up existing valleys, showing that the present drainage system was well-developed before the final outbursts of the volcano. In these later ashes, bombs from a few inches to two feet in diameter are common, and distinct from broken boulders of lava because of their more or less rounded appearance.
They are very abundant on the north-west and north of the crater near the villages of Kanzatkon, Gyaingywa, Ngayangon, etc., and probably were thrown out when the north-western wall of the crater was blown away, and the basic andesites and basalts with well-developed phenocrysts of augite erupted. Vesuvianite-Basalt: A bomb of vesuvianite-basalt was collected from Mile 32-2 on the main Popa road. The thin section consists of a mass of augite and felspar phenocrysts cemented together by brown glass with many vesicles.
Vesuvianite-Hypers thene-Olivine-Basalt: Another specimen, in addition, contains hypersthene in prismatic sections with inclusions of iron ores and well-marked cleavage traces. The minerals, especially the felspar, occur in rounded grains. Specimens of hornblende-pyroxene-andesite are also represented in these bombs. As with the flows it will be noted that both andesites and basalts, sometimes with vesuvianite, constitute the volcanic bombs.
Ejected Blocks from Mount Popa.
The ejected blocks described below are found as boulders, sometimes associated with volcanic material in stream valleys elsewhere. The rocks are all coarse-grained and holocrystalline and are plutonic representatives of the lavas on the surface. Hence, they are important in revealing the nature of the roots of the volcano. They appear mostly to be specimens of diorite and gabbro, which in some cases have been altered to epidiorite afterwards. Specimens of hornblende -and epidote-rock also occur as ejected blocks in the area.
Volcanic Detrital Alluvium.
Volcanic alluvium has partly covered the Peguan on the east and both the Irrawaddian and the Peguan towards the north and the south. There is not much volcanic alluvium on the west, as few large streams drain the volcanic rocks on that side. Where the covering of alluvium has been removed, the Tertiary rocks crop out from below, especially in the streams and road cuttings, etc.
Age
As Sir Edwin Pascoe has noted, some of these rocks, as, for example, the black tuffs and ashes, are interbedded with the Irrawaddian, proving that activity began in that period and continued subsequently in paroxysmal eruptions. The older andesites constitute the oldest rocks in the area, and have undoubtedly flowed over the Peguan rocks. The commencement of the volcanic activity was, therefore, post-Peguan. FJ Warth has calculated 413 years as required to erode one foot of rock in Mid-Burma from Mandalay to Prome. Multiplying 413 by 786 we get 324,618 years (or about 1/3 million years in round figures), which gives approximately the time which has elapsed since the Pliocene period and since Popa first erupted.
There may be some truth in the legend recorded by Bell that volcanic activity ceased only in historic times. He states: “According to tradition, there was a great earthquake in 442 BC, during which the great cone of Popa rose from the plains, but the native chronicles leave no record of how long it was active and when it became extinct.” The author could not find any clue as to the date of the last eruption in man’s memory.
The author’s notes on the age of Mount Popa Volcano
Although HL Chhibber and earlier geologists estimated the age of Mount Popa Volcano as previously mentioned, today’s geologists and volcanologists have refined these estimates using advanced age-dating methods. Among these studies, Takashi Sano et al. provided a comprehensive analysis of the petrogenesis and the age of lava flows in the Mount Popa region (refer to the enclosed map) in their 2022 publication, as cited in reference 2. This is the end of the geology of Mount Popa.
References:
1. Chhibber, HL, 1934: The Geology of Burma, Macmillan, and Co Limited, St Martin’s Street, London.
2. Takashi Sano, Kenichiro Tani, Shigekazu Yoneda, Hla Min, Thaung Htike, Zin Maung Maung Thein, Osamu Ishizuka, Nao Kusuhashi, Reiko T Kono, Masanaru Takai & Chris E Conway, 2022: “Petrogenesis of isotopically enriched Quarternary magma with adakitic affinity associated with subduction of the old lithosphere beneath central Myanmar”, www. nature.com/scientificreports
