Stripping of Keanakakoi Tephra on Kilauea Volcano, Hawaii

• -* I Stripping of Keanakakoi tephra on Kilauea Volcano, Hawaii MICHAEL C. MALIN Department of Geology, Arizona State University, Tempe, Arizona 85287 DANIEL DZURISIN U.S. Geological Survey, Cascade Volcano Observatory, Vancouver, Washington 98661 ROBERT P. SHARP Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 ABSTRACT Morphological characteristics, erosional processes, and effects of burial and exhuma tion by debris mantles on basaltic volcanic landforms have been evaluated through field study of the Keanakakoi Formation, a I basaltic tephra formed in 1790 by phreato km magmatic eruptions from Kilauea caldera, Hawaii. The upper coarse lithic, interme N I diate fine vitric, and lower mixed members of this formation play different roles in the creation of micro-terrain elements during 1 stripping of tephra from the underlying I bedrock. Of the seven micro-terrain ele ments defined, bedrock, scabby upland surfaces, and lag gravels are the most dis tinctive and widely distributed. Different proportions and combinations of micro f j terrain elements define five zones of pro l gressive deterioration of the Keanakakoi tephra blanket southwestward from Kilauea caldera into the Kau Desert. Fluvial and eolian processes operate on different time scales and at different locations, governed by blanket thickness, debris caliber, and the formation of case-hardened crusts. Strip ping of an entire mantle is probably not v possible; however, materials trapped within depressions form the only clearly discerni SYMBOLS: Benchmarks ble morphological expression of previously /--' Fault scarps, bar and ball more extensive debris blankets. on downthrown side ~ Cracks 0 Conspicuous lava flows INTRODUCTION Figure 1. Sketch map of Kau Desert, Kilauea Volcano, Hawaii, showing five zones of Burial of surface terrains by a friable degradation of the 1790 tephra blanket. I. Complete cover by tephra. II. Areas of small debris blanket and their subsequent exhu bedrock outcrops. III. Areas of continuous but fractional bedrock outcrop. IV. Bedrock mation may have been widespread on Mars surface with small insets of debris. V. Bedrock and reworked accumulations and dunes. (Soderblom and others, 1973; Malin, 1976), Abbreviations: FPT = Footprint Trail, CP = Cone Peak, SH = Sand Hill, H = Halemau with important effects on the appearance of mau, HVO = Hawaiian Volcano Observatory, VH = Volcano House, KI = Kilauea Iki, K features perceived from orbit. Martian Keanakakoi (pit crater), AK = Ahua Kamokukolau, MIT = Mauna Iki Trail. Geological Society of America Bulletin, v. 94, p. 1148-1158, II figs., October 1983. 1148 I STRIPPING OF TEPHRA, KILAUEA VOLCANO, HAWAII 1149 blanketing material may include weathering Kau Desert is a 350-km2 wedge apexing at the entire unit (> 5 m) to the 1790 event. His products, rocks comminuted by meteorite Kilauea caldera with a longitudinal axis interpretation was supported by recently impact, and tephra that was formed by vol along the Southwest Rift Zone (Fig. I). Sur acquired 14C dates which indicated a basal canic eruptions and emplaced (Binder and face materials are mostly thin, young basalt age younger than 350 ± 60 yr (Kelley and - others, 1977; Mutch and others, 1977) and flows, bedded tephra, and deposits of others, 1979). partly removed by wind (McCauley, 1973). reworked tephra debris. After nearly a century of lava-lake activ Objectives of the present study were (I) to A major phreatomagmatic eruption ity, Halemaumau erupted explosively on 91125 examine morphological characteristics of a within Kilauea caldera in 1790 A.D. dis May II, 1924, and continued to eject frag partly eroded tephra blanket on a fresh vol tributed tephra over the Kau Desert, mental lithic material and copious steam for canic landscape, (2) to identify the erosional creating the Keanakakoi Formation of 14 days. During the peak of the 1924 erup processes, and (3) to determine the effects of Wentworth (1938). A less violent phreatic tion, rhythmic explosions ejected blocks burial and exhumation on the surficial eruption within Halemaumau during 1924 weighing several metric tons, ash-laden appearance of the underlying lavas. Lavas formed a thin tephra layer at the caldera steam clouds rose at least 2 km, and more in the Kau Desert are primarily pahoehoe, rim. Reports concerning the 1790 eruption than 30 cm of lithic tephra accumulated and so attention necessarily focuses upon consist of sketchy retrospective narratives locally on the south rim of Halemaumau. A that type of surface, although tephra has by natives as related to missionaries. An few centimetres of finer lithic debris barely locally mantled aa lavas, and such relation eruptive column several kilometres high was cleared the southwest edge of Kilauea cal ships are treated briefly. visible, and a group of Hawaiian warriors dera (Powers, 1948, PI. 3D; Macdonald, were suffocated 9 km downwind from the 1949, p. 72; Macdonald and Abbott, 1970, Setting caldera. This tragedy and many charact&is p. 315). An eyewitness account by Stearns tics of the deposits have been interpreted to (1925) recorded a heavy fall of ash at Kilauea is an active basaltic shield vol support the hypothesis that base surge Pahala, 34 km southwest, and of sharp, cano constituting the southeast part of played a significant role in emplacement of angular, sand-sized fragments, as large as I Hawaii Island. Major orographic features Keanakakoi tephra (Swanson and Christ mm, at nearby Wood Valley. Of especial include the 3 by 5 km summit caldera iansen, 1973). Powers (1948) regarded only interest are showers of accretionary lapilli with its nested pit crater, Halemaumau, the uppermost layers « I m) of the Keana (also called pisolites) reported by Stearns and prominent east-west-trending and kakoi Formation as a proQuct of the 1790 (1925, p. 202), 2.5 km southwest of Hale southwest-trending radial rift zones. The eruption, but Christiansen (1979) attributed maumau 2 hr after an explosion. " KEANAKAKOI TEPHRA BLANKET Stratigraphy Keanakakoi Formation consists of ash, lapilli, and blocks of accessory material as well as essential vitric ash and pumice (Wentworth, 1938, p. 92-102). In Kau Intermediate Desert, this tephra blanket is heterogeneous, Vitric although well stratified. R. L Christiansen Unit (U.S. Geological Survey, 1979, written commun.) has studied in detail the stratig raphy, emplacement, mode of origin, and age of these deposits. For our purposes, a Lower simplified threefold definition of the forma Mixed tion is adopted: (I) an upper, coarse lithic Unit unit; (2) an intermediate, finer, and richly vitric unit; and (3) a lower, mostly fine, mixed lithic and vitric unit (Fig. 2). The Lava Flows upper unit is predominantly lithic, but IWS the intermediate and lower units contain admixtures of vitric, lithic, and crystal zones of o 2 meters fragments. [';-!',.,I Pumice I I of small Upper Lithic Unit. This is the most Bedrock UC - Unconformity distinctive unit of the three, ranging from Id dunes. I mm to several metres thick and consisting ialemau Figure 2. Simplified section of the three major units of the Keanakakoi Formation: an almost wholly of sand- to block-sized angu a Iki, K 0 upper lithic unit principally sand- to block-sized angular fragments of accessory rock, an lar fragments of dense, nonvesicular acces Intermediate vitric unit ofsilt- to sand-sized volcanic ash fragments (with minor amounts of sory rocks, largely olivine basalt, picrite lithic, crystal, and pumiceous debris), and a lower mixed unit similarly composed of sand basalt, diabase, and gabbro (Macdonald, and silt-sized vitric, lithic, and crystalline materials. Unconformities between each unit. 1949, p. 65). Scattered blocks as much as & 1150 MALIN AND OTHERS I m in size are seen near the caldera rim, orifices around Kilauea, deposit sulfates loose, angular lithic fragments, for the most fragments of 20 to 40 em in diameter are and opaline silica (Naughton and others, part about 0.5 em in diameter, interrupt the abundant within I km of the caldera rim, 1976), and similar substances might be sequences of finer beds. Occasional lithic and particles from 0.5 to 1.5 em prevail derived from seepage waters. Crust forma blocks as large as 10 em are seen in such throughout. This unit contains a few thin tion is a continuing process, for it occurs on layers. Pumice fragments, 0.5 to 2 em in (2-4 em) layers of brown, silt-sized fine vol erosional facets of various ages cutting diameter, are particularly abundant in the canic ash locally rich in accretionary lapilli. across bedding and upon the walls of recent lower half of the intermediate unit, and Bedding, although crude, irregular, and gullies and cracks. A thinner, but somewhat some layers as much as 10 em thick are discontinuous, is nonetheless prevailing. less coherent, crust has already developed nearly pure pumice. The most obvious crys Primary dips to 15° are abundant owing to on 1924 tephra, indicating rapid formation. tal fragments are olivine, although feldspar scour fillings and to draping over an ir Intermediate Vitric Unit. This unit can be also may be present. regular surface. Cross-bedding occurs in as much as 2 m thick near the caldera rim, In many exposures, the uppermost part some layers, on a 10- to 20-cm scale, and in although extremely variable and sometimes of the intermediate unit is a massive, yellow I· channel fillings near the caldera rim. entirely missing. Fine grain size, high vitric brown, silt-sized fine ash, rich in accretion The upper lithic unit rests unconformably content, and a khaki brown color are char ary lapilli in its uppermost part. Tracing 01 upon the intermediate unit, as shown by acteristic. Fine lamination on a millimetre strata through the Kau Desert to the Foot local angular truncation of bedding (Fig. 2). scale is common, and layering is generally print locality (Swanson and Christiansen. or1 Near the caldera rim, this contact is scoured well defined throughout.

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