LANDSCAPE EVOLUTION AND PRESERVATION POTENTIAL OF CULTURAL HORIZONS IN RANO RARAKU CRATER, SITE OF ANCIENT QUARRIES, EASTER ISLAND

Geologic mapping and coring were undertaken in Rano Raraku crater, the site of moai (statue) production on Easter Island ca. A.D. 1200-1400, to address three questions: Was the interior lake larger in the past? Could buried quarries exist lower on the interior slope, below known quarries? What is the preservation potential for cultural horizons down slope of the quarry locale? Easter Island is well known for its moai and the potential for human-driven ecological catastrophe. Previous studies include geochemical analyses of some volcanic units, including the Rano Raraku tuff used in all moai production, and three research teams have cored the crater's interior lake, producing three different paleoenvironmental interpretations. Island-wide erosion histories have also been developed; however, patterns of erosion and deposition and landscape change have not received investigation on the crater's interior south slope, the quarry area itself.

Our mapping demonstrates that the basin is a composite landform; the high south wall comprises inward dipping lapilli tuff within which all quarries are developed. The tuff constitutes the remaining flank of an eroded Plio-Pleistocene volcano, while the rest of the basin formed when a small eruption through the flank produced a tephra cone.

A core transect was taken down the slope, from quarries to lake edge. Cores reveal a basal tephra with a well-developed paleosol that is interpreted as the pre-quarry slope, dominated by palm forest. Incomplete preservation of the paleosol represents deforestation and increased erosion. A quarry debris unit containing stone tools overlies the paleosol. This is in turn overlain by a presumably post-quarry period debris fan of colluvium containing abundant decomposed lapilli and reworked stone tools. A weakly developed paleosol on the colluvium suggests revegetation and landscape stability between the end of quarrying and the recent use of the basin for grazing. The uppermost unit is a thin colluvium post-dating ca. A.D. 1920.

To date we can state that the interior lake has not been appreciably larger, and given the tephra blanket on the tuff, the likelihood of quarries much lower on the slope is virtually zero. A cultural horizon related to quarry activity is well preserved below a debris fan and will be targeted in our upcoming excavations.