The volcanic rocks of Easter Island (Chile) and their use for the Moai sculptures

摘要

Easter Island (Chile) is a volcanic island made up entirely of volcanic rocks, which are represented by lava flows and domes as well as cinder and scoria pyroclastic cones, covering the entire compositional range from basalts to peralkaline rhyolites. Apart from representing a cultural heritage of worldwide importance, the megalithic Moai statues of Easter Island are an exquisite example of the utilisation of a variety of volcanic rocks in sculpture. This work illustrates the spectrum of volcanic litho-types available to the islanders through new chemical and pétrographie data and presents chemical, mineralogical, petrographie and physical data on the rocks from the same volcanic deposits used for fashioning the megalithic sculptures. The stones used for the statues and their topknots are represented by volcanic rocks with different depositional mechanisms and chemical compositions. They include tuffs deposited in water, subaerial welded scoriae, lava flows and lava domes, with compositions ranging from basalt to trachyte. Despite such variations, they all share rather high total porosity, and are all easily workable stones. Their low apparent density seems to be the critical factor in enabling such large statues to be fashioned. The rest of the volcanic rocks on the island were unsuitable for building megalithic sculptures for various reasons: they are either too heavy, as is the case of the basaltic lavas, or too fragile, as the obsidian, or too loose, as the unconsolidated tuff and scoria cones. The rocks used for the platforms and altars are lavas whose compositions vary moderately, from basaltic to mugearitic, but which share similar physical and mechanical properties, well suited to building stable bases for the statues. The results of the study on the Rano Raraku tuff, employed in the great majority of the statues, show that it consists of a hyalotuff, in which volcanic glass was altered by interaction with sea water. The products of alteration vary in composition from nearly amorphous palagonite to crystalline smectite. The magma that erupted at Rano Raraku was originally mugearitic, and the high loss on ignition (LOI) and low alkali content of the rock, as well as its high clay content, are characteristics stemming from glass-sea water interactions during the deposit formation. The samples taken from the base, middle and top of the cone flank hosting the quarries show very similar composition of the bulk rock, the fresh glass and the palagonite products, suggesting homogeneous rock characteristics throughout the quarries.