Rapa Nui Journal: Journal of the Easter Island Foundation Volume 11 Article 6 Issue 2 June 1997 1997 Evolution of the Southwestern Edge of the Poike Volcano, Easter Island A. Boven M. DeDapper P. DePaepe R. Langohr P. Pasteels Follow this and additional works at: https://kahualike.manoa.hawaii.edu/rnj Part of the History of the Pacific slI ands Commons, and the Pacific slI ands Languages and Societies Commons Recommended Citation Boven, A.; DeDapper, M.; DePaepe, P.; Langohr, R.; and Pasteels, P. (1997) "Evolution of the Southwestern Edge of the Poike Volcano, Easter Island," Rapa Nui Journal: Journal of the Easter Island Foundation: Vol. 11 : Iss. 2 , Article 6. Available at: https://kahualike.manoa.hawaii.edu/rnj/vol11/iss2/6 This Research Paper is brought to you for free and open access by the University of Hawai`i Press at Kahualike. It has been accepted for inclusion in Rapa Nui Journal: Journal of the Easter Island Foundation by an authorized editor of Kahualike. For more information, please contact [email protected]. 1 Boven et al.: Evolution of the Southwestern Edge of the Poike Volcano Evolution of the Southwestern Edge of the Poike Volcano, Easter Island A.Boven, M. De Dapper, P De Paepe, R. Langohr andP Pasteels Introduction New K-Ar data A joint Belgian-Chilean biological mission, conducted by Radiometric ages of rocks coming from all over Easter Prof. H.Dumont of the Department of Morphology, Systemat­ Island were published from the middle of the seventies on­ ics and Ecology of the Gent University (Belgium), visited wards (Baker et aI., 1974; Clark and Dymond, 1974; Easter Island in the summer of 1990. During this expedition, Gonzalez-Ferran et aI., 1974; Bonatti et aI., 1977). They made which lasted a few weeks, cor­ it clear that Poike is a relatively old volcano as compared to Table I ing operations took place in the other main volcanoes (Rano Kau, Maunga Terevaka) Chemical composition of the crater lake of Rano Raraku responsible for the construction of the island. two lava flows from in view of biological and The earliest phase in the subaerial development of Poike Easter Island palaeoecological research. goe back to about 3 Ma ago (Baker et al., 1974). It was (major elements in wI. %; In parallel with the scien­ followed by repeated eruptions of thin basaltic and hawaiitic trace- and rare-earth ele­ tific activities of the biologists lava flows and rare outbursts of pyroclastics that built up a ments in ppm) (Anal.I.Vergauwen) of the project, three co-authors ymmetrical cone, probably approaching 5 km across, with a of the present paper (M.D.D., hallow, small summit crater that at the present rises 370 m RN67 RN83 PD.P. and R.L.) made field above sea-level. The three trachytic domes located on the observations and proceeded to northern slopes of Poike are thought to be representatives of ISi02 49.10 48.50 a limited rock and soil sam­ the later stage of activity of the volcano. Unfortunately, the Ti02 3.27 3.70 pling in the easternmost part of highly altered state of the rocks forming these parasitic cones A1203 14.95 14.58 the island. This reconnais­ precludes any reliable K-Ar age determination. sance survey provided new in­ Both the decline of the volcanic activity and the insularity Fe203 14.10 14.82 sights into the geomorphologi­ of Poike gave a strong impulse to marine erosion. The latter MnO 0.21 0.23 cal, pedagogical and environ­ resulted in the formation of high, near-vertical seacliffs on all mgo 4.68 4.39 mental evolution of the south­ sides of the volcano and a severe reduction of its landmass. western, inland-facing side of CaO 9.44 8.73 Wave action is still going on all but the west and southwest the Poike volcano since the sides of the volcanic edifice. Here, several lava flows issued Na20 3.23 3.38 youthful lava flow that encir­ from fi ure-controlled lateral centers of Maunga Terevaka K20 0.67 0.97 cles the north and northeast inundated the coastline of Poike bringing about the subaerial P205 0.43 0.54 rims of Rano Raraku was linkage between the two adjacent emerging volcanoes and poured out and protected the protecting the cliffed area between Tongariki and Mahatua H20+ 0.04 0.00 area between Poike and from further marine erosion. Raraku against further marine Rb 12.4 16.5 erosion (Paskoff, 1978a & 1978b). Sr 289 295 The geochronological re­ Ba 142 158 sults presented in the f allow­ H rERCYAKA SC 35.0 33.2 ing account were obtained by v 338 346 the first author (A.B.), in close collaboration with P.P., at the Cr 60 20.0 Department of Geochronology Co 42.0 37.0 of the Brussels Free University Ni 31.0 12.5 (Belgium). Other K-Ar age de­ terminations related to rock y 52 67 samples from the Poike strato­ i...-_--"Km Zr 240 298 volcano are in progress and Nb 29.4 36.1 will be included in a forthcom­ ing paper. Major- and trace­ •• DEAD'· ESCARPMENT _ element abundances were de­ La 28.7 38.3 RANO termined by inductively cou­ RARAKU Nd 39.0 50.4 pled plasma atomic emission + 0 RN1;7 Eu 2.8 3.6 spectrometry at the Depart­ RNB3 ment of Earth Sciences of the Dy 9.5 11.5 Universite de Bretagne Occi­ Er 4.9 6.0 dentale (UBO) at Brest Figure I, Location of the three studied profiles (A, B and C) and Yb 4.2 5.3 (France). rock sample points RN67 and RN83 PublishedRapa byNui Kahualike,Journal 1997 75 Vol II (2) June 1997 1 Rapa Nui Journal: Journal of the Easter Island Foundation, Vol. 11 [1997], Iss. 2, Art. 6 A N S 110m 0.5./. REGOLITH - .... :':.c:'::':""':c..:o._~ . ~ n0"-- CONVEX UPPER SLOPE SAPROLITE. CORES TONES -- "'$:fJ,~:'::::~%~::.'0 BASAL FRONT OF WEATHERING -- -' ",'''''-'- -'-' ~ '" !'!'- ~ - UNWEATHERED ROCK SHORE PLA TFORM 0 7 / ~""":::~---=,,=-=--- SEA LEVEL B 105m 0.5.1. REGOLlTH_ i.:,.~,..:.;7~ _ ~ ° -CONVEX UPPER SLOPE '0''-0' GJ' '0 22 S BA S AL Af,[6~L;.rgF ~tllJR:'~~ -:; .~:~:. ';'0:': :~:.. ~. -''- CLIFF UNWEATHERED ROCK WITH BASALT BLOCS ____SEA LEVEL C ENE WSW 85m 0.5./. CONVEXO-RECT~/NEAR-CONCAVESLOPE L, i' =-_ ___...:.5_'-,6' 6' REGOLIT H ..... ,". 7' 'a' '1' SAPROLITE. CORES TONES .: :: . '. .'. IJ' BANKS OF UNWEATHERED BAsr;;:; IwR..uN..u6..uZcu..ucU:ill~6'~'r-16']" BASLALT BLOCS I " n' SOm RN63 16'1" aUATERNARY LAVA FLOW IS· Il- (RN67) ;I ~Z~L9 ~",. 5 ~~~~~:...;z~''r.m!°'TT·ms·i'· /'--SAPROilTE .... 6h,'"";':7'7' 4 :"" i']' 1,5' l' " + DETAILED PROFILE COLLUVIUM? I 10 10 F,C,9] Figure 2. Schematic representation of the profiles: (A) represents an active sea-cliff. (B) a semi-active cliff, and (C) a "dead" escarpment. RN62, RN63 and RN67 refer to rock samples, Rapa Nui Journal 76 Vol II (2) June 1997 https://kahualike.manoa.hawaii.edu/rnj/vol11/iss2/6 2 Boven et al.: Evolution of the Southwestern Edge of the Poike Volcano Two lava flows con idered to be of utmost importance for The basal front of weathering, which separates the sapro­ our knowlcdge of the recent geomorphological evolution of lite from the underlying unweathered bedrock, gives rise to a both the outhwe tern part of Poike and the Rano Raraku area harp angular convex knick in the profile. On the mid-slope were tudied with the K-Ar technique. Sample RN67, that the unweathered rock i expo ed in a subvertical cliff; by yielded an average age of 0.51 ± 0.07 Ma, come from the unloading and gravity, blocks fall off the cliff side. The latter outhfullava flow which encircle the northern and northea t­ accumulate temporarily on a narrow shore platform, with a ern lopes of Rano Raraku and c ntributed greatly to the general lope of 7°, that i located at the foot of the cliff. By fo dizatlon of the marine e carpment developed along the wave action the block are broken down, rounded and eventu­ outhwe t ide of Poike. Rock pecimen R 83, that gave an ally wa hed away. For that reason, there is no net accurnula- ab olute age of 0.22 ± 0.0 Ma. I repre entative of the lava flow of M. namarama thatoccupie the plain extending from MToa Toa to Rano Rarakuand definitively stopped the ma­ nne ero Ion along its southea tern liffed border. The location f both lava ample is hown on Fig. We t of Rano Raraku, the younger flow (R 83) overlies the older one (RN67). Both K-Ar dated rock samples are porphyritic-textured hawalitic basalts. They contain variable proportions of phe­ nocry ts and/or microphenocrysts f plagioclase (up to 6 mm in length) and olivine set in a groundmas of plagioclase laths, clinopyroxene. opaque grain. olivine and rare volcanic gla . The major-. trace- and rare-earth clement chemistries of both peclmen are presented in Table 1. Plate 2. View on profile A and Hanga Ta1caure (1, on the foreground) (photo: M.D.D., September 1990) Geomorphological evolution of the southwestern part of Poike The cliffs that bound the eaward- and landward- ide of tion of debri at the base of the cliff so that the latter retreats Poike dIu trate how an actlve ea-cliff gradually pa se into In a parallel way through a proce of backwearing. an inactlve or "dead" manne escarpment. Three profile Profile B (Fig.2B) how a emi-active cliff-escarpment.