Volcanic and Glacial Landforms on Mount Giluwe, Territory of Papua and New Guinea

D"'"'°" "f Land Research, CSIRO. Canberra. Australia E* LOFFLER B Volcanic and Glacial Landforms on Mount Giluwe, Territory of Papua and New Guinea ABSTRACT were glaciated during the Pleistocene. Other Mount Giluwe is a large extinct dome-shaped examples include Iztaccihuatl, Mexico, and volcano built up mainly of thin flows of mafic Kilimanjaro, in eastern Africa, which have been lava rich in potassium. It rises from 2100 m to described by White (1962) and Downie 4368 m above sea level. During the Pleisto- (1964), respectively. cene, it was covered by an ice cap which ex- Giluwe is situated at 6°2'S. lat., 144°7'E. tended as low as the 3200 to 3500 m level. long. It rises abruptly to an altitude of 4368 m Although the volcano is dissected by a radial from plains 2100 to 2300 m above sea level. It drainage system, cones, craters, lava mounds, is the second highest peak, after Mount Wil- lava flow surfaces, and volcanic debris, slopes helm (4509 m), in the Territory of Papua and are well-preserved below the glaciated area. New Guinea. The basal diameter is about 30 Above this, the original volcanic landforms km. have been destroyed or modified by glacial ac- Giluwe is situated close to the Equator, tivity, and well-preserved lateral moraines, consequently it has an equatorial highland cli- groups of recessional moraines, glacial valleys, mate, with little seasonal variation in tempera- cirques, rock basins, and roches moutonnees ture or rainfall. Frosts are common above 3000 are present. During the period of maximum m at any time of the year, and the maximum glaciation, valley glaciers as thick as 400 m pro- daily temperature seldom reaches 20°C. The truded from the edge of the ice cap down annual rainfall is probably more than 2.5 m. preexisting valleys and terminated at elevations Most of the mountain (Fig. 2) is covered with between 2750 and 3000 m. On the northeast- montane rain forest and grassland. ern side of the volcano, fluvioglacial deposits The flanks of the volcano are dissected by derived from the valley glaciers formed fans closely spaced radial streams but still show extending onto the Kaugel plain, which is in- many well-preserved volcanic features. On the cised 40 m by the present Kaugel River. This summit area, which is taken to comprise all of incision is almost entirely postglacial. the volcano above about 3500 m, the original Palagonitic breccia and scoria overlie, and volcanic landforms have been destroyed by gla- are interlayered with, normal lava flows in the cial action, and well-preserved glacial features summit area of the volcano. They indicate that are present. some subglacial volcanic activity took place and The volcanic nature of Giluwe was first noted that there were probably at least two different by L. C. Noakes in 1939 (unpub. rep.) and was periods of glaciation. The last glacial period briefly described by Rickwood (1955). An ac- (contemporaneous with the Wiirm glaciation of count of some of the volcanic and glacial fea- Europe and the Wisconsin glaciation of North tures is included in a report on the lands of the America) was the more extensive, covering Wabag-Tari area (Perry and others, 1965). A 2 about 190 km . It was responsible for the de- description of the glacial landforms is also in- velopment of the existing glacial landforms. cluded in a recent paper by Leffler (1971). Two Most of the volcanic landforms are older than chemical analyses of rocks from Giluwe are this last glaciation. given in Jakes' and White (1969). Present-day erosional processes on Giluwe are discussed by INTRODUCTION Bik (1967), and some soil profiles are de- Mount Giluwe is a large extinct volcano in scribed by Rutherford (1968). the highlands of New Guinea (Fig. 1), and is The present study of the volcanic and glacial one of several volcanoes within the tropics that features is based on field work carried out in Geological Society of America Bulletin, v. 82, p. 1605-1614, 6 figs., June 1971 1605 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/6/1605/3428352/i0016-7606-82-6-1605.pdf by guest on 26 September 2021 1606 BLAKE AND LOFFLER —LANDFORMS, MT. GILUWE, PAPUA ------ - -------- -- ------------- -- -------- -- ---------------- -- ------- -- --- ---------------- -- -------- -- Figure 2. View of Giluwe from the north, showing the Kaugel plain in the middle distance. July and August 1969, and on air photo inter- which the convex flanks slope at 10° to 20°. pretation. It has revealed that there were proba- The flanks extend down to between 3000 bly at least two major glacial periods on and 2200 m. They are dissected by subparallel Giluwe, and that some subglacial volcanic ac- steep-sided valleys radiating from the sum- tivity took place. mit area. The dissection is most dense on the upper flanks; downslope the many small val- SHAPE OF THE VOLCANO leys join to form a relatively small number of From a distance (Fig. 2), Giluwe is seen as a major valleys. In their lower parts, the major large, nearly symmetrical dome-shaped volcano valleys are narrow and steep-sided but up- with broad concave footslopes. However, in stream they become broader. Prior to glacia- detail, the smooth domical form is marred by tion, they were probably amphitheater-headed several parasitic cones and by deeply incised valleys (Stearns, 1966) similar to those on valleys. other New Guinea volcanoes such as Karimui, The summit area has a gently convex profile, Manam, and Balbi (Blake, 1968). with slopes increasing to about 10° near its mar- A prominent break in slope occurs at the base gins. It is surmounted by two prominent castel- of the flanks, below which are slightly concave lated peaks rising 400 m above the general footslopes of less than 10°. These descend to the summit surface. These are the main peak, 4368 Kaugel plain in the northeast and to the Mendi m, and the east peak, about 4300 m. The north- valley in the west, and they merge with the ern, northwestern, and southern sides of the footslopes of the extinct lalibu volcano in the summit area are incised by U-shaped glacial southeast. valleys, 200 to 400 m deep, which are sepa- rated from one another by aretes. In contrast, GEOLOGICAL STRUCTURE the southwestern and eastern sides consist The volcano is built up mainly of innumera- mainly of broad, slightly dissected slopes with ble thin outwardly dipping mafic lava flows relatively wide glacial valleys less than 100 m (Fig. 3). The lavas are normal subaerial flows deep. The edge of the summit area is marked erupted from numerous vents in the summit by a break in slope at 3300 to 3500 m, below area and on the flanks (Fig. 1). Local angular Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/6/1605/3428352/i0016-7606-82-6-1605.pdf by guest on 26 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/6/1605/3428352/i0016-7606-82-6-1605.pdf by guest on 26 September 2021 PETROGRAPHY 1607 unconformities within the lava sequences are common. The individual flows are generally 5 to 7 m thick but range from less than 1 m to more than 100 m. The thickest flow observed is shown in Figure 3. Most of the lava flows have red scoriaceous tops over dark-grey massive rock which in some cases shows crude columnar jointing. The base of each flow is hummocky and commonly di- rectly overlies the scoriaceous top of the flow below, although in places a separating layer of rubble is present. No soil or weathered hori- zons have been found between flows, and it seems that in most cases the flows succeeded one another too quickly for significant weather- ing. In addition to normal mafic flows, there is also a body of palagonitic breccia exposed on ridges at the head of the Gogon valley, on the north side of the summit area. The only nonmafic rock found on Giluwe is an andesite which forms a large cumulo dome on the summit area north of the main peak (Figs. 1 and 3). Morainic material covers much of the summit area, but no undoubted glacial deposits have been found underlying lava flows. On the eastern side of the summit area, however, in a waterfall exposure at 3650 m, two meters of rubble of possible morainic origin and five meters of stratified sand and silt are interbedded between two lava flows. The upper flow is over- lain by glacial till deposited during the last glaciation. The footslopes of Giluwe are formed of both lava flows, originating mainly from local parasitic volcanoes, and coarse fanglomerates, which are formed of volcanic debris. The latter include some fluvioglacial deposits. The Kau- gel plain northeast of Giluwe is made up of water-laid gravel, sand, and silt, and several lay- ers of peat. Several prominent faults occur on the footslopes of Giluwe (Fig. 1), where they form scarps as much as 70 m high, but none have been recognized on the convex flanks or summit area. On the southeastern side, aligned scoria cones, craters, and small lava mounds indicate the presence of three volcanic fissures; two trend west-northwest, and one trends northwest. PETROGRAPHY The mafic lavas on Giluwe consist of abun- dant small phenocrysts of calcic plagioclase (by- townite-labradorite, commonly with narrow Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/6/1605/3428352/i0016-7606-82-6-1605.pdf by guest on 26 September 2021 1608 BLAKE AND LOFFLER-LANDFORMS, MT. GILUWE, PAPUA Figure 4.

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