Geologic Map of the Summit Region of Kïlauea Volcano, Hawaii By Christina A. Neal and John P. Lockwood Aerial view of some of the prominent fissures within the southwest rift zone of Kïlauea Volcano. The dark lava erupted from these fissures in September 1971. (Photo- graph by J.D. Griggs.) Pamphlet to accompany Geologic Investigations Series I–2759 2003 U.S. Department of the Interior U.S. Geological Survey DESCRIPTION others, 1988; Moore and others, 1989). This map illustrates a succession of young basaltic The area covered by this map includes parts of four lava flows erupted from Kïlauea and Mauna Loa Vol- U.S. Geological Survey 7.5´ topographic quadrangles canoes, as well as pyroclastic deposits erupted from (Kïlauea Crater, Volcano, Kaÿu Desert, and Makaopuhi). Kïlauea. No interfingering of Mauna Loa and Kïlauea It encompasses the summit, upper rift zones, and Koaÿe flows is known in this area, although interleaving of flows Fault System of Kïlauea Volcano and a part of the adja- is seen outside the map area and should be common at cent, southeast flank of Mauna Loa Volcano (fig. 1). depth in the border region between the two volcanoes. The map is dominated by products of eruptions Because Mauna Loa’s surface area is much larger than from Kïlauea Volcano, the southernmost of the five vol- Kïlauea’s, the rate at which its surface is covered by lava canoes on the Island of Hawaiÿi and one of the world’s (about 40 percent/1,000 years [Lockwood and Lipman, most active volcanoes (fig. 1.) At its summit (1,243 m) 1987]) is less than Kïlauea’s (about 90 percent/1,000 is Kïlauea Crater, a 3 km-by-5 km collapse caldera (fig. years [Holcomb, 1987]), and extensive remnants of 2, on map sheet) that formed, possibly over several cen- older lava flows are thus preserved on the larger volcano. turies, between about 200 and 500 years ago. Radiating Mauna Loa lavas in the map area erupted from the north- away from the summit caldera are two linear zones of east rift zone of the volcano, 10 km north of the map edge intrusion and eruption, the east and the southwest rift (Lockwood and others, 1988). They range in age from zones. Repeated subaerial eruptions from the summit a few hundred years to late Pleistocene. Kïlauea flows and rift zones have built a gently sloping, elongate shield erupted from vents within or just outside of the modern volcano covering approximately 1,500 km2. Much of caldera or along its upper southwest rift zone and east the volcano lies under water; the east rift zone extends rift zone and range in age from historic (about 1790 to 110 km from the summit to a depth of more than 5,000 1982) to approximately 3 ka. Pyroclastic deposits shown m below sea level (Moore, 1971; Holcomb and others, on the map are either from phreatic and phreatomagmatic 1988); whereas the southwest rift zone has a more explosive eruptions or lava fountaining in Kïlauea’s limited submarine continuation. South of the summit summit region. caldera, mostly north-facing normal faults and open fractures of the Koaÿe Fault System extend between the two rift zones. The Koaÿe Fault System is interpreted GEOLOGIC MAPPING OF THE KÏLAUEA as a tear-away structure that accommodates southward SUMMIT REGION movement of Kïlauea’s flank in response to distension of the volcano perpendicular to the rift zones (Duffield, This work includes and updates the geologic 1975). Farther to the south and outside the map area, the mapping of Peterson (1967), who completed the first large normal fault scarps of the Hilina Pali are structures 1:24,000-scale geologic map of Kïlauea Crater quad- related to the seaward subsidence of Kïlauea’s mobile rangle. It also builds upon Walker’s (1969) geologic map south flank (Stearns and Clark, 1930; Swanson and of the Kaÿu Desert quadrangle and the reconnaissance others, 1976). geologic map of the entire Kïlauea edifice by Holcomb The northwest corner of this map covers part of the (1980, 1987). Stearns and Clark (1930) and Stearns and southeast flank of Mauna Loa Volcano, the most volu- Macdonald (1946) made the first generalized geologic minous volcano on Earth. Lava flows from Mauna Loa maps of the Kïlauea summit area. They defined the Puna encroach upon and are diverted by the Kïlauea edifice Volcanic Series, now the Puna Basalt (Easton, 1987), (Lockwood and others, 1988). The summit of Mauna to include Kïlauea’s historic and prehistoric flows and Loa lies 30 km west-northwest of Kïlauea caldera and pyroclastic deposits that rest on top of the Pähala Ash, a reaches 4,164 m above sea level. Like Kïlauea, Mauna complex volcaniclastic deposit exposed on Mauna Loa’s Loa is characterized by a summit caldera and two radial southeast flank and in a few places on Kïlauea’s south rift zones, the northeast and southwest rift zones (fig. flank. For Mauna Loa, Stearns and Macdonald (1946) 1). Additionally, radial fissure vents are present on the defined the Kaÿu Volcanic Series, now the Kaÿu Basalt northwest flank (Lockwood and Lipman, 1987). Of the (Easton, 1987), as lava flows younger than the Pähala two rift zones, only the southwest rift zone continues off- Ash. shore as a conspicuous ridge extending an additional 50 The sequence and map distribution of historic flows km to water depths of about 4,000 m. Major flank fault within Kïlauea Caldera during the period 1823 to 1961 systems include the Kealakekua Fault System on the were previously compiled by Peterson (1967). This western flank, the Kahuku Fault along the distal south- map updates the distribution of historic lava flows and west rift zone, and the Kaÿöiki-Honuÿapo Fault System delineates additional prehistoric eruptive units. We have on the southeastern flank. These structures are related to made only minor changes to the pre-1971 historic flows seaward subsidence of Mauna Loa’s flanks (Lipman and mapped by Peterson (1967). 1 156° 155°30' 155° EXPLANATION Fault Generalized distribution of eruptive vents in rift zones of Kilauea KOHALA and Mauna Loa * Crest of lava shield Caldera or crater identfied in text 20° MAUNA KEA HUALÄLAI Hilo Kailua-Kona MAUNA LOA MLNERZ KÏLAUEA 19°30' MAP AREA KEALAKEKUA FAULT SYSTEM KÏLAUEA CRATER ERZ M E * KÜPAIANAHA T KÏLAUEA IKI S Puÿu ÿÖÿö Y * MAUNA ULU MLSWRZ S * I YSTEM MAUNAIK LT S T AU LI L F PA ÿE A TEM U A IN SYS A KO IL F H LT N AU O SWRZ F A P NA ÿA ILI E U H N C O H O I K I IC ÿÖ F A I K C PA T L 19° U A F U 0 25 KILOMETERS K U H A K Figure 1. Map showing the five volcanoes that form the Island of Hawaiÿi, rift zones and major subaerial structures of Mauna Loa and Kïlauea Volcanoes, and the area covered by this map. SWRZ, southwest rift zone of Kïlauea Volcano; ERZ, east rift zone of Kïlauea Volcano; MLNERZ, northeast rift zone of Mauna Loa Volcano; MLSWRZ southwest rift zone of Mauna Loa Volcano. Geology was primarily mapped on aerial photo- lava flows of prehistoric age in the southeast corner of the graphs during field traverses. Linework was transferred map are obscured by heavy vegetation and contact rela- to a base map by using a Kern PG-2 stereo plotter and, tions are approximately located. Different eruptive units locally, by inspection. In areas of heavy vegetation along were recognized and correlated on the basis of lithology, the east border of the map or thick ash cover on parts surface weathering characteristics, paleomagnetic secu- of Mauna Loa, contacts were determined by mapping lar variation data, vesicle morphology, outcrop pattern, sparse outcrops and by aerial photo interpretation. Due and degree of vegetation cover. Some contacts between to dense vegetation and difficult access, the mapping widespread tube-fed pähoehoe flows of similar appear- north of the pit craters of Kïlauea’s upper east rift zone ance are difficult to follow and are shown as approximate and south of Highway 11 is only approximate. Similarly, on the map. 2 ROCK TYPES huna Ash Member of the Puna Basalt, and sparse expo- sures of the Pähala Ash, a poorly understood deposit of Lava flows, pyroclastic deposits, and intrusive rocks deeply weathered tephras of mostly pre-Holocene age portrayed on this map have physical characteristics that cover large areas southwest of this map area. and chemical compositions that are typical of Hawai- Additional small pyroclastic deposits, including the ian tholeiitic basalt. Flow textures are pähoehoe, ÿaÿä, early 19th century Golden Pumice of Sharp and others and transitional varieties (Swanson, 1973). Pähoehoe is (1987) and the phreatic explosion debris from the 1924 generally smooth or hummocky, having surface rough- explosions at Halemaÿumaÿu pit crater (Stearns, 1925), ness consisting of ropy folds and rounded, billowy toes were not mapped due to their limited extent and poor (Wentworth and Macdonald, 1953). Some pähoehoe preservation. Recent work has tentatively documented flows are marked by tumuli, skylights into lava tubes, and several additional tephra deposits of unknown origin local patches of ÿaÿä. Interior textures of pähoehoe flows surrounding the summit of Kïlauea (Dzurisin and others, can range from dense, poorly vesicular basalt to highly 1995). These deposits are exposed only beneath surface vesicular basalt containing appreciable void space called flows in the map area and are not indicated on this map. shelly pähoehoe. Slabby pähoehoe consists of broken, upended plates of lava and is common in the near-vent area and in the transition region between pähoehoe and ÿaÿä.