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RW JOHNSON Volcanic Globule Rock from Mount Suswa, Kenya Abstract

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RW JOHNSON Volcanic Globule Rock from Mount Suswa, Kenya Abstract R. W. JOHNSON Volcanic Globule Rock from Mount Suswa, Kenya Abstract: A volcanic rock consisting of globules of lava is present on Mount Suswa, a Quater- nary volcano in Kenya. The rock is found on surfaces of sodalite phonolite lava flows ("globule- surface" lavas), where it seems to have formed by vesiculation of the upper parts of the flows. The rock also constitutes the entire volume of thin uniform sheets ("globule flows") which may be globule-surfaces that slipped off the lava flows. Introduction dull gray to dark buff in color, well compacted, An unusual heterogeneous volcanic rock is and easily indented by a blow from a hammer. present on Mount Suswa, a Quaternary phono- Especially characteristic are cellular, sub- lite volcano in the middle of the Eastern Rift rounded or lenticular patches of material which Valley, about 45 km west of Nairobi, Kenya range from microscopic dimensions up to about (Fig. 1). The geology of Mount Suswa has been 5 cm in length (PI. 1, fig. 1). Most of the larger described by McCall and Bristow (1965) and patches are flattened parallel to the flow sur- Johnson (1966), and the volcano occupies the face, and some of their centers are hollow and southern part of an area from which numerous lined with feldspathic and chalcedonic ma- examples of "froth flows" have been described terial. (McCall, 1962a, 1962b, 1962c, 1965). The rock shows columnar jointing, with The heterogeneous rock consists of closely polygonal cross sections ranging in diameter packed, spherical or disc-shaped globules, each from 4 or 5 cm to about 0.5 m (PL 1, fig. 2). with a highly vesicular crystalline core. The Most exposed surfaces have a thin, rust-brown- rock is found in two settings: (1) in the upper to-maroon, lateritic coating. parts of sodalite-phonolitc flows, termed "glob- ule-surface" lavas; (2) constituting the entire volume of a few, thin, uniform sheets, termed "globule flows." 35E 37E Both types of flow were produced toward the end of the earliest period of vulcanicity on \akeVictoria^ \A MountKenya Mount Suswa. During this period a primitive o- shield-shaped volcano was built, the outer \ \ 0 flanks of which were eroded during a later \ phase of quiescence. The inactivity was then is- interrupted by explosive eruptions accom- panying the formation of a caldera which, in Nairobi its northeast part, shows a vertical section through the upper part of the primitive 2S- volcano. Lake Acknowledgments Natron The author is grateful to G. D. Borley, J. G. Jones, G. J. H. McCall, R. L. Smith, G. P. L. Ngorongoro Kilimanjaro Walker, and Howel Williams for their criti- cisms at various stages in the preparation of this paper. main faults of Rift Valley Description of the In hand specimen, the heterogeneous rock is Figure 1. Index map. Ccological Society of America Bulletin, v. 79, p. 647-652, 4 figs., Ipl., May 1968 647 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/79/5/647/3442762/i0016-7606-79-5-647.pdf by guest on 24 September 2021 648 R. W. JOHNSON--VOLCANIC GLOBULK ROCK, MT. SUSWA, KENYA Iii tliin section the jointed rock is seen to fn thin section the rock is also seen to con- consist principally of globules, mainly 0.05 to tain numerous crystal fragments of alkali feld- 3 mm in diameter, with open spaces between spar, olivine, augite, and titanomagnetite. them (Fig. 2). The globules are closely packed Small amounts of debris include: irregular blebs and most of them are molded onto each other. of crystalline lava (up to 1 or 2 cm in diameter); I'.ach globule has a highly vesicular crystal- small fragments of fine-grained, flow-banded line core, and a thin, continuous, enclosing rim lava; and dark-brown-to-black glass cinders, ot dark-brown glass containing numerous rarely more than a few mm in diameter. opaque grains; this glass is often partially, and sometimes completely, dcvitrified. The core Description of the Globule-Surface Lavas consists mainly ol alkali feldspar laths that On the outer flanks of the volcano. The flows typically fringe the inner wall ol the rim. Feld- on which the globule rock is found are termed spars arc also present in clusters that radiate "globule-surface" lavas. They are tabular in from diffuse centers rich in opaque grains. shape with fronts that rarely extend more than Minor amounts of material interstitial to the 7 to 8 km from the caldera rim; flow thick- feldspars include light-brown glass, aenigma- nesses vary between 3 and 25 m. tite, soda-amphibole, and augite. Figure 3 shows diagrammatically the geo- The cellular patches of material visible in metric relationship between the globule rock hand specimens are the cores of larger globules and the rest of the flow. The globule rock (up to 5 cm in length) that contain abundant, covers the central area of the lava flow top, but irregularly shaped vesicles. Some of the larger when traced to the flow margins it abruptly globules have complete or severed, internal gives way, over a distance of about 1 or 2 cm, dividing walls of material similar to that mak- to convoluted, vesicular, glassy lava. ing up the outer rim. This convoluted surface is littered with loose Figure 2. Sketch from a photomicrograph of a globule-surface rock showing globules with continuous glassy rims (black) and vesicular, crystalline cores (hatched); globules less than 0.1 mm in diameter arc shown as open circles. The stippled areas are crystal and lithic fragments. A prominent cinder fragment is shenvn at upper center. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/79/5/647/3442762/i0016-7606-79-5-647.pdf by guest on 24 September 2021 Figure 1. Photograph of a globule-surface rock. Magn. X 1.5. Figure 2. Polygonally-jointed surface of a globule-surface lava. The thin deposit in the top left corner consists of pumiceous pyroclastic material erupted at the time of cauldron collapse. JOHNSON, PLATE 1 Geological Society of America Bulletin, volume 79 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/79/5/647/3442762/i0016-7606-79-5-647.pdf by guest on 24 September 2021 NOTES AND DISCUSSIONS 649 escarpment at the summit of Mount Suswa is mantled by later deposits, and it is only in the northeastern part that lavas of the primitive volcano can be identified with certainty. Here, six flows are exposed in vertical section. They are, from oldest to youngest, 25+, 6, 25, 12, 12, and 17 m thick. Although the surfaces of all six flows have not been examined across their total exposed width (due to inaccessibility and poor exposure), it appears that none show Figure 3. Diagram showing relationship be- tween upper globule-surface (shaded) and lower globule-surfaces. In vertical section the six flows show a con- voluted and folded flow structure, suggestive of movement of viscous lava; all are relatively subrounded fragments of lava and shows crude poor in crystal and lithic fragments compared corrugations parallel or perpendicular to the with the globule-surface rocks. The four thin- direction of flow. In thin section, the lava shows nest flows are highly vesicular and fine grained, no globule texture, but instead consists of a usually with interstitial glass developed continuous, vesicular groundmass of feldspar throughout the entire flow thickness. In con- laths, opaque grains, and dark glass. Crystal trast, the two thickest lavas have non-vesicular and lithic debris in the lava are sparse in com- and completely crystalline centers. In thin sec- parison to the amounts found in the globule- tion, these crystalline rocks show numerous surface rocks. microphenocrysts of sodalite set in a ground- On some flows a few small outcrops of vesicu- mass of alkali feldspar laths, interstitial ferro- lar lava rise from parts of the globule-surface as magnesian minerals, and some nepheline. raft-like masses, up to 1 m high and from 4 to In contrast, sodalite is notably absent from 5 m wide. All of these lie within 15 m of the the globule-surface rocks on the outer flanks, a lava/globule-surface transition and none are deficiency which is characteristic of all the found on the central portions of the flows. The glassy rocks on the volcano. Chlorine analyses base of the masses is always abruptly grada- (by X-ray fluorescence) of 25 phonolite sam- tional into the underlying globule-surfaces. ples from various parts of the Mount Suswa At a few exposures that appear to have under- succession show that the rocks with the highest gone little erosion, the jointed globule rock is chlorine content (from 0.25 to 0.5 wt percent) concealed by a semi-consolidated, yellow or come from the sodalite-bearing, central parts light-brown, carbonate-rich, earthy deposit of thick lavas, whereas those from the sodalite- containing highly vesicular, rounded glass free, globule-surface rocks, flow crusts, and thin cinders. Most of these cinders range from glassy lavas contain less than 0.25 percent microscopic sizes to fragments 20 cm in di- chlorine. Retention of the volatile chlorine and ameter; a few larger, twisted and contorted slow cooling therefore appear to be prerequi- forms exceed 1 m in length. The deposit is sites for crystallization of the sodalite. usually less than 20 cm thick and grades abruptly into the underlying, more compact, Origin of the Globule-Surfaces globule rock. The globule-surfaces seem to have originated The thickness of the globule-surfaces on by vesiculation that disrupted the tops of the most flows is unknown owing to lack of dissec- lava flows.
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