"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.