Fig. 1: Two pieces of obsidian from a moraine near the south coast of Iceland. The material is very fine grained to glassy, with subconchoidal fracture and a satiny to subvitreous lustre. Both pieces readily attract a pen magnet.
Sample I-7, collected 31 July 2022.
Larger: 10 x 7 x 5 cm, 587.5 grams, magnetic susceptibility (n=3) averages 9.63x10-3 SI units. Specific gravity measured as 2.76.
Smaller: 7 x 7 x 4 cm, 284.4 grams, magnetic susceptibility (n=3) averages 23.03x10-3 SI units. Specific gravity measured as 2.78.
"Rock of the Month # 259, posted for January 2023" ---
1. Obsidian in Iceland: Obsidian is a glassy volcanic rock of broadly rhyolitic (granitic) composition. In Iceland, it is of localised occurrence, related to the more siliceous volcanic centres. An occurrence of "textbook" material with conchoidal fracture is found in the Krafla volcanic system, northeast of Lake Myvatn in northeast Iceland (Saemundsson and Gunnlaugsson, 2014, p.67; Thordarson and Hoskuldsson, 2014, pp.182-184). The immediate sources are stratovolcanoes, which display styles of volcanism more explosive than the gentle effusions of basalt noted in the Reykjanes peninsula in 2021-2022 (see "pahoehoe basalt lavas").
The Sample. As seen in Figure 1, obsidian is black and glassy. In some cases, scattered crystals (phenocrysts) of feldspar may be visible in the dense groundmass, or incipient crystallization in spherulites, but these features are absent here. Given the silica-rich bulk chemistry of obsidians, it may seem surprising that this rock is a) black, unlike the white, grey or pink tones typical of granites, and b) exhibits distinct magnetism (seen in some but far from all granites). It is important to note that obsidian is largely glass, and so lacks the pale quartz and feldspars that define most granitic rocks. A thin slice, under the microscope, would likely reveal incipient crystallites too small to see with the hand lens, and probably some magnetite (iron oxide) as the carrier of the magnetism.
The Locality. These samples were collected loose from a prominent glacial moraine near the coast of south-central Iceland, west of the port town of Hofn. The locality lies just south of the extensive Vatnajokull ice cap, under the southern margin of which lie a number of volcanoes, including Oraefajokull (Thordarson and Hoskuldsson, 2014, pp.109, 128-129). The moraine is readily accessible by vehicle from the national ring road known as Highway 1. To the east are the glacial lagoons of Fjallsarlon and Jokulsarlon and the attractions of Hofn, and stretching far to the west is the glacial outwash plain of Skeidararsandur. Nearby is the hamlet of Oraefi. To the north is the ice-shrouded volcano Oraefajokull, which includes the highest point on Iceland, the mountain peak of Hvannadalshnjukur (2,110 metres; 6.923 feet).
Oraefajokull, a stratovolcano, is the presumed source of the felsic volcanic rocks scattered through the moraine, including the black obsidian and some flow-banded pink rhyolite. The rocks around the caldera rim are dominated by rhyolite. There were two large historic eruptions, in 1362 and 1727-1728 (see illustrated guide by Guttormsson, 2011, pp.46-51).
Fig. 2: A view along the crest of the moraine. It is - naturally enough in this part of the world - a mixture of volcanic lithologies. The obsidian is quite easy to find and collect, but certainly <1% of all the pebbles in the pile.
2. Notes on Obsidian: As we shall see, obsidian gets all the attention in the popular press, but there is a related rock, with a higher water content and a more waxy lustre, named pitchstone (see "Scottish examples"). In practice, not all obsidian is strictly rhyolitic, the major-element chemistry sometimes veering toward the more alkalic trachyte and phonolite. Macdonald et al. (1992) compiled geochemical data on obsidians from many locations, from five tectonic settings: primitive island arcs, mature island arcs, continental margins, continental interiors, and oceanic extensional zones (such as Iceland). In the MINLIB bibliography, "obsidian" appears seven times as often as "pitchstone", and one-third of the obsidian references relate to archaeological contexts. Obsidian - mostly when devoid of phenocrysts and with a nice glossy lustre - is something of a minor celebrity in the gemstone world, and features in jewellery and lapidary creations.
Gemstones. Obsidian is an inexpensive raw material for jewellers, obtained as rough or in beads. Black obsidian is normal, but there are exotic variants such as lace obsidian, red or brown obsidian, sheen obsidian, iris or rainbow obsidian, flame or fire obsidian, and snowflake obsidian (Thompson, 2010). Phenocrysts or inclusions may account for much of the variation. In the case of fire obsidian from Glass Buttes, Oregon, observations at very high magnification revealed thin layers composed of nanometric crystallites of magnetite, the optical character explained in terms of thin film interference effects (Ma et al., 2007).Archaeological significance. Because of the utility of obsidian for manufacture of stone tools and ornaments, archaeologists have long been interested in scientific provenance studies, that can throw light on ancient trade routes by suggesting the likely source of an artefact. Examples are numerous, including the Mediterranean region (Barca et al., 2019), where obsidian from the Lipari Islands was mentioned by Theophrastus (Caley and Richards, 1956). Obsidian use was however worldwide, in western North America, Mesoamerica, the Pacific, New Zealand and East Africa (Cann. 1983). Hansen (2020), in her excellent review of the rise of global trade routes, cites the trade goods available in the late Woodland period, circa 1050 A.D., at the city of Cahokia in the Mississippi valley of modern Illinois The goods included mica from North Carolina, native copper from Lake Superior and a unique obsidian from Pachuca in Mexico. The Aztecs used obsidian mirrors for divination, a chemical provenance study indicating that at least one source of the raw material was also Pachuca (Weiss, 2022). Pachuca, incidentally, lies some 90 km NNE of Mexico City, in modern Hidalgo state. The colonial mining and smelting centre lies a small distance northwest of Teotihuacan, the old Aztec capital and ceremonial centre.
Tektites. Rochette et al. (2022) carried out a very different kind of provenance study on glassy materials, to distinguish some unusual obsidians and other volcanic glasses from tektites, found around the world as stratigraphic markers of major impact events (years earlier, the tektites, readily distinguished by shape from most obsidians, were shown to be melted sedimentary target rocks and so, unlike the impacting body, of terrestrial origin).
Barca,D, Crisci,GM and Miriello,D (2019) Obsidian and volcanic glass shards: characterization and provenancing. In "The Contribution of Mineralogy to Cultural Heritage" (Artioli,G and Oberti,R editors), EMU Notes in Mineralogy 20, European Mineralogical Union, 448pp., 393-409.
Caley,ER and Richards,JFC (1956) Theophrastus on Stones. Ohio State University, Columbus, OH, 238pp.
Cann,JR (1983) Petrology of obsidian artefacts. In "The Petrology of Archaeological Artefacts" (Kempe,DRC and Harvey,AP editors), Clarendon Press, Oxford, 374pp., 227-255.
Guttormsson,H (2011) Vatnajokull National Park - a guidebook. Friends of Vatnajokull, Reykjavik, translated from the Icelandic by Jeffrey Cosser, 152pp.
Hansen,V (2020) The Year 1000. When Explorers Connected the World - and Globalization Began. Scribner, 308pp.
Ma,C, Rossman,GR and Miller,JA (2007) The origin of color in "fire" obsidian. Can.Mineral. 45, 551-557.
Macdonald,R, Smith,RL and Thomas,JE 1992) Chemistry of the Subalkalic Silicic Obsidians. USGS Prof.Pap. 1523, 214pp.
Rochette,P, Bezaeva,NS, Beck,P, Debaille,V, Folco,L, Gattacecca,J, Gounelle,M and Masotta,M (2022) Obsidian and mafic volcanic glasses from the Philippines and Vietnam found in Paris Museum Australasian tektite collection. Meteoritics & Planetary Science 57, 1460-1471.
Saemundsson,K and Gunnlaugsson,E (2014) Icelandic Rocks & Minerals. Mal og Menning, Reykjavik, 2nd edition, 232pp., translated from the Icelandic by Anna Yates, with photography by G. Eiriksson.
Thompson,SE (2010) Obsidian. Lapidary Journal Jewelry Artist 64 no.4, 26-27, July.
Thordarson,T and Hoskuldsson,A (2014) Classsic Geology in Europe, 3. Iceland. Dunedin, Edinburgh, 2nd edition, xv+256pp.
Weiss,D (2022) Reflecting the past. Archaeology 75 no.2, 16, March.
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