"Rock of the Month # 108, posted June 2010" --- a sample of black basaltic scoria, a froth of congealed gas-rich lava, from Mount Fuji, Yamanashi prefecture, Honshu island, Japan. Such bubbly ("vesicular") material has been known to float on water, at least for a short time, particularly when the magma is siliceous, the resulting frothy rock known as pumice. Denser material with this same texture, richer in magnesium, iron and other elements, and typically basaltic in nature, are generally termed scoria rather than pumice. The rounded form of the cracked outer surface of the main piece suggests that this piece was once part of a large (>50 cm) "volcanic bomb" which was ejected forcefully from the crater, assuming a rounded form during flight. The main sample, the highlight of each photograph, is 12x12x6 cm in size and, at 1616 grams, was NOT likely to float away! The four samples, clockwise from lower right, are 1438 (the bomb fragment), 1439, 1437 (pale) and 1436.

Sample 1438 is basaltic scoria, like the other other dark specimens, but differs from them in preserving part of the outer surface of a volcanic bomb. These rocks were collected on 03 September 1992, from the crater rim of Mount Fuji, at an estimated altitude of 3,730 m (the summit is at 3,776 m, 12,388 feet above sea level). 1438 and 1439 (see below) were collected loose from the top of a small peak on the northwest extremity of the crater rim, west of the northern descent line to Kawaguchi-ko.

This rock is a shard of a large volcanic bomb. It is composed of fresh, black scoria with mm-scale phenocrysts of glassy plagioclase feldspar. The cracked "bread-crust" texture on the outer surface (left photo) is attributed to differential cooling between the interior and the outer crust. The interior displays pronounced flow-banding and layers marked by variable vesicularity. The bands lie subparallel to the outer crust (right photo). The exact age of the samples is not known for certain --- the last major eruption was in 1707 A.D.

Mount Fuji is an iconic mountain and a classic volcano, with its unmistakable profile (Bueler, 1970; Dahlby and Kasmauski, 2002). Earth tremors may reflect magma circa 10-20 km below the surface. The volcano has erupted at least 10 times since 781 A.D. Mount Fuji is the largest composite stratovolcano in Japan (Miyaji et al., 1992). The 3,776-m volcano has a base diameter of 50 km.

A polished thin section prepared from sample 1439, collected on the same visit at the same site as 1438, confirms that the sample is a vesicular basaltic scoria with small plagioclase phenocrysts. The vesicles in 1439 range from <1 to 7 mm in maximum dimension. The rounded vesicles exhibit very variable size and shape. The smaller voids (as small as 0.1 mm) tend to be perfectly rounded (spherical, presumably a surface tension effect), whereas the largest examples are more irregular and elongate, flattened in flight / flow / compaction. Two pieces of 1439 are seen at the left foreground in the photo at left, and one piece supports the scale bar in the right-hand photo.

In thin section, the feldspars display oscillatory zoning and melt inclusions. The rock is rich in volcanic glass, with scattered plagioclase and olivine microphenocrysts. The estimated modal proportions of the thin section, by area, include vesicles (50%), dark iron-rich volcanic glass (33%), feldspar phenocrysts and microphenocrysts (4 and 8%, total 12%), fine-grained magnetite grains (0.03 mm and smaller, 3%) and small olivine grains of magnesian composition (Fo₇₅, 2%). The complex zoning and inclusion patterns in the plagioclase phenocrysts indicate disequilibrium conditions during rapid ascent of the parent magma. Some of the coarser phenocrysts are crowded with dark brown glassy melt inclusions, of similar colour to the glassy groundmass.

The hand specimen is appreciably magnetic. Studies showing elevated gold contents (4-25 parts per billion) in titanomagnetite concentrates from basalts and basaltic andesites in Japan indicate magmatic partitioning of gold into the oxide phase as the magma evolved and rose beneath the volcano (Togashi and Terashima, 1997). The hand-specimen magnetic susceptibility (in 10^-3 SI units) was measured six times on each of the four samples: sample averages ranged from 3.7 (the pale, highly vesicular scoria or pumice 1437) to 6.7 (sample 1439), 7.0 (1436) and (1438) 14.4. The magnetism of the well-preserved volcanic bomb, in particular, is sufficient to attract a pen magnet. Notably, three readings of the interior averaged 10.2, the cracked exterior averaged 18.5. These values are in the range typical of the hypabyssal, intrusive equivalent of basalt (diabase / dolerite). Is the more-magnetic exterior crust a reflection of preferential, rapid quenching of the exterior of the basaltic lava to nucleate more tiny magnetic oxide grains during the short flight of the bomb? It's an intriguing possibility.

Acknowledgements: With thanks to James Macdonald and Richard Cresswell for companionship on the ascent, and damp and chilly nocturnal vigil, which preceded collection of the samples!

References

Bueler,WM (1970) Mountains of the World: A Handbook for Climbers and Hikers. The Mountaineers, Seattle, WA, 279pp.

Dahlby,T and Kasmauski,K (2002) Fuji, Japan's sacred summit. National Geographic 202 no.2, 26-45, August.

Miyaji,N, Endo,K, Togashi,S and Uesugi,Y (1992) Tephrochronological history of Mt. Fuji. In `Volcanoes and Geothermal Fields of Japan' (Kato,H and Noro,H editors), 29th IGC, Kyoto, Field Trip Guide Book 4, 276pp., 75-109.

Togashi,S and Terashima,S (1997) The behavior of gold in unaltered island arc tholeiitic rocks from Izu-Oshima, Fuji, and Osoreyama volcanic areas, Japan. Geochim.Cosmochim.Acta 61, 543-554.