Introduction to Landforms and Geology of Japan


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Accretionary Lapilli

Accretionary lapilli
Photo 1: Accretionary lapilli

Gray roundish stones in the center of Photo 1 are accretionary lapilli found in the southern Boso Peninsula to the south of Tokyo (map below). Accretionary lapilli are pellets of very fine volcanic ash formed by accretion attributed to surface tension of waterdrops or electrostatic force of ash in eruptive columns. They are commonly less than 1 cm or so in diameter and sometimes have a concentric (onion-like) internal structure. Although the lapilli in the photo have a core, some accretionary lapilli do not contain it. Because accretionary lapilli are generally distributed within about 20 km around a crater, the location of a vanished volcano may be presumed.

The largest accretionary lapillus in Photo 1 is about 1.5 cm in diameter with a scoria core. Black stones around the accretionary lapilli are scoria. A red tiny object to the right side of the lapillus is oxidized scoria. Red scoria is much less than black scoria in the Miura Group, suggesting the eruption near the surface of the sea because red scoria is generally produced by slow cooling under conditions rich in oxygen. Small snails are periwinkles (probably “Granulilittorina exigua [DUNKER]”). These snails dislike seawater, so they move away from the seawater when the tide comes in. The photo was taken on a wave-cut bench (Photo 3).

Layer of accretionary lapilli 
Photo 2: Layer of accretionary lapilli

Accretionary lapilli in the Boso Peninsula range from several millimeters to two centimeters in diameter and have concentric internal structure. Their cores are scoria, rock fragments or minerals. These accretionary lapilli are found with scoria (black stones in Photo 2) in formations consisting of deep-sea sediments and an abundance of tuff (11.9 to 2.8 million years old, Miura Group) distributed in the Boso Peninsula and the Miura Peninsula on the other side of Tokyo Bay. It is thought that the eruption producing the lapilli was phreatomagmatic explosion that occurred in a shallow sea or near the surface of the sea and fine ash accreted to scoria to grow accretionary lapilli in eruptive columns under wet conditions. The distribution of the lapilli suggests that the volcano was situated in the sea to the west of the Miura Peninsula.*  However, the lapilli were also found at least 60 km to the east of the presumed volcano.**

Wave-cut bench
Photo 3: Shooting location




* Descriptions of accretionary lapilli in the Boso Peninsula: Aihara, 1990; Kanagawa Prefectural Museum of Natural History and Yokosuka City Museum, 1999
** Hirano et al., 1995

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