274 IGNEOUS ROCKS/Obsidian Figure Two obsidian flows and several obsidian domes, Glass Mountain, California The northern of the two flows emanates from an obsidian summit dome with a minute axial orifice for gas escape, and there is a succession of very small pancake like effusions from a chain of centres in the upper left hand corner of the photograph The pressure structures on the flows, their glassy nature, and their tongue like form, with steep sides and fronts on account of their viscosity, are features clearly illustrated Reproduced with permission from Green J and Short NM (1971) Volcanic Landforms and Surface Features: A Photographic Atlas and Glossary New York, Heidelberg, and Berlin: Springer Verlag at different times in the magma chamber crystallization, and so they too will show differences in minor or trace element ratios Each obsidian flow thus has a unique minor and trace element ‘fingerprint’ Geochemical matching begins with obtaining a number of different minor and trace element values for each source flow, as a baseline Having identified the minor trace element values of the likely source flow, the geochemist can with confidence match it with obsidian artefacts from archaeological sites and delineate the trade routes In J Glover’s Geological Journeys, three cases were described in which this has been done successfully In various northern and central locations of what is now the United States are found the Hopewell sites, which are remarkable burial sites built by the ancestors of Native Americans between 100 bc and ad 500 Obsidian artefacts are found there, but there are no developments of the volcanic rock nearby Possible sources on the Pacific Coast or in Alaska, Yellowstone National Park, New Mexico, and Mexico have been investigated by JB Griffin and colleagues Neutron activation analyses confirmed two groups with Na/Mn ratios of 150 and 90 These results fingerprinted two flows at Yellowstone National Park in Colorado Figure shows the path over which the obsidian had to travel, over 2400 km eastward, to the Hopewell sites The process may have involved intermediate stages of bartering across this route Artefacts in the south-west Pacific have also been analysed Emission spectroscopy analysis results on flakes from the Santa Cruz Islands, part of the nation of the Solomon Islands (one of the most northern group of the chain previously called the New Hebrides, now Vanuatu), show that the chemistry matches obsidians cropping out on New Britain, not the basaltic volcanic rocks of Santa Cruz Radioacarbon dating shows that the flakes were deposited in Santa Cruz about 1000 bc, so the population of the islands 3000 years ago must have carried obsidian in small craft by sea east–southeastwards over at least 2000 km (Figure 10) Work in 1996 by R Service has shown, further, that obsidian from New Britain and the Admiralty Islands (a group close to New Britain) was transported earlier to a