岩石氷河-氷河説と周氷河説-

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Recent progress in rock glacier studies is reviewed with some emphases on the competition between the glacial and periglacial hypotheses. Rock glaciers are tongue-shaped or lobate bodies composed of angular boulders that resembles a small glacier, usually accompanied by multiple transverse ridges resulting from a compressive flow. Rock glaciers are classified, in terms of the origin of surface materials, into talus and morainic rock glaciers, and in the light of the activity status, into active, inactive and fossil ones. The distribution of active rock glaciers are delimited by the regional glacier equilibrium line and lower limit of mountain permafrost. The internal structure of rock glaciers has been approached by direct observations and indirect geophysical soundings. In some rock glaciers, natural outcrops exhibit a massive ice body with debris bands beneath the surface boulder layer, which has encouraged the glacial hypothesis. Massive ice was also found in boreholes penetrating through a rock glacier permafrost in the Swiss Alps, despite being considered to originate from snow avalanche or refrozen meltwater. In fact, deformation occurred mostly in the frozen debris layer beneath the massive ice, indicating the periglacial origin of the rock glacier due to permafrost creep. Geophysical soundings, including seismic, geoelectric and gravimetric measurements, have provided useful information on the three-dimensional structure, stratigraphy and ice contents of rock glacier bodies, although authors preferring the glacial hypothesis tend to reject such indirect results. The origin of any rock glacier is thus equivocal without detailed analyses of internal stratigraphy and ice composition. Most of the active rock glaciers are moving at a speed of 101 cm yr-1, two orders of magnitude slower than ice glaciers. The periglacial model attributes such a slow movement to permafrost creep. A possible consequence of this is that active rock glaciers usually have ages of several thousand years ; that inactive ones were activated repeatedly during the colder periods of the Holocene; and that fossil ones moved during the Late Glacial. In contrast, the glacial model explains that many rock glaciers originated from a debris-covered glacier during the Little Ice Age and have been loosing their ice content and speed rapidly with the 20th-Century warming. Only a few rock glaciers have been identified from Japanese mountains. Although locations favorable for active rock glaciers are restricted to the northern side of some high mountains, the mountain permafrost belt must have been wide enough to form a number of rock glaciers during some past cold periods. Subsequent permafrost melting would have fossilized these rock glaciers, some of which may have been misinterpreted as glacial moraines or protalus ramparts.
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松岡憲知
筑波大学

岩石氷河-氷河説と周氷河説-

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