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[MIS12-03] Stepped forms on snowy volcanoes in the Tohoku Region of Japan: A potential case of middle latitude relict cryoplanation terraces
Keywords:snowy mountains, cryoplanation terraces, periglacial landforms, volcanic landforms, nivaton
Discontinuous occurrences of distinctive stepped forms are observed on Middle Pleistocene volcanoes, such as Gassan, located in the heavy snow climate of the Tohoku Region (Fig.). These stepped forms are distributed on vegetated lava slopes and consist of prominent risers that extend roughly parallel to contour lines, accompanied by tread surfaces that spread across the front and back of the risers. For instance, more than 180 such risers have been identified on the eastern slope of Gassan (Shirahata, 2025MS). The scale of these stepped forms is significantly larger than that of periglacial patterned ground and solifluction lobes.
Some portions of the stepped forms were previously interpreted as lava wrinkles (Moriya, 1984). However, compared to typical lava wrinkles formed during the Holocene or Late Pleistocene epochs, the risers of these stepped forms are notably longer and exhibit greater curvature (Fig.). Additionally, they lack convex features extending downslope, such as lava levees. Therefore, it is highly likely that these stepped forms are not merely lava wrinkles but have developed through alternative geomorphological processes.
In the stepped forms located southeast of Gassan's summit, the late-lying snow accumulates at the base of the risers, forming debris-covered slopes (i.e., snowpatch rubble slopes). The late-lying snow enhances weathering, erosion, transport processes, and mass movement; this intensified denudation process is referred to as nivation. Depressions covered with debris, observed both within and outside the stepped forms, are nivation hollows currently under formation. In contrast, most of the stepped forms and nivation hollows are vegetated and considered relict landforms where active formation has ceased.
These stepped forms resemble cryoplanation terraces (CTs), which are prevalent in high-latitude permafrost regions such as northern Canada. CTs are understood as periglacial landforms that develop under the combined influence of nivation and geological structure, leading to slope planation. Although the morphology of the stepped forms in the Tohoku Region are not perfectly similar to that of typical CTs, they may represent CTs formed during cold periods such as Marine Isotope Stages (MIS) 2, 4, and 6. Even if the original form of the stepped forms was lava wrinkles, repeated glacial and interglacial cycles could have subjected them to nivation, leading to the development of CTs, which were later vegetated in the Holocene epoch. If this hypothesis holds true, these stepped forms represent previously unreported fossil periglacial landforms in Japan.
Future research must evaluate nivation processes and establish precise chronological frameworks to determine whether these stepped forms can be classified as fossil CTs. Even as relict landforms, the heterogeneity in snow distribution across the stepped forms likely influences variations in snowmelt timing, which in turn affects local soil moisture, soil temperature, and mass movement. These microenvironmental factors are expected to have constrained the vertical migration of vegetation zones and the establishment of new plant communities. Understanding the developmental history and temporal context of these stepped forms presents a new challenge for discussing the Quaternary natural history of snowy mountain in Northern Japan. Additionally, ripple-like microtopography, smaller in scale than stepped forms, has been discovered on slopes composed of pyroclastic density current deposits in various volcanoes in Japan. Their formation is under debate in relation to pyroclastic surges. Exploring potential connections between ripple-like micro features and CTs also warrants further investigation.
(Figure: Typical features of stepped forms expressed by shaded and slope images by the Geospatial Information Authority of Japan. The scales of all map images are same.)
Some portions of the stepped forms were previously interpreted as lava wrinkles (Moriya, 1984). However, compared to typical lava wrinkles formed during the Holocene or Late Pleistocene epochs, the risers of these stepped forms are notably longer and exhibit greater curvature (Fig.). Additionally, they lack convex features extending downslope, such as lava levees. Therefore, it is highly likely that these stepped forms are not merely lava wrinkles but have developed through alternative geomorphological processes.
In the stepped forms located southeast of Gassan's summit, the late-lying snow accumulates at the base of the risers, forming debris-covered slopes (i.e., snowpatch rubble slopes). The late-lying snow enhances weathering, erosion, transport processes, and mass movement; this intensified denudation process is referred to as nivation. Depressions covered with debris, observed both within and outside the stepped forms, are nivation hollows currently under formation. In contrast, most of the stepped forms and nivation hollows are vegetated and considered relict landforms where active formation has ceased.
These stepped forms resemble cryoplanation terraces (CTs), which are prevalent in high-latitude permafrost regions such as northern Canada. CTs are understood as periglacial landforms that develop under the combined influence of nivation and geological structure, leading to slope planation. Although the morphology of the stepped forms in the Tohoku Region are not perfectly similar to that of typical CTs, they may represent CTs formed during cold periods such as Marine Isotope Stages (MIS) 2, 4, and 6. Even if the original form of the stepped forms was lava wrinkles, repeated glacial and interglacial cycles could have subjected them to nivation, leading to the development of CTs, which were later vegetated in the Holocene epoch. If this hypothesis holds true, these stepped forms represent previously unreported fossil periglacial landforms in Japan.
Future research must evaluate nivation processes and establish precise chronological frameworks to determine whether these stepped forms can be classified as fossil CTs. Even as relict landforms, the heterogeneity in snow distribution across the stepped forms likely influences variations in snowmelt timing, which in turn affects local soil moisture, soil temperature, and mass movement. These microenvironmental factors are expected to have constrained the vertical migration of vegetation zones and the establishment of new plant communities. Understanding the developmental history and temporal context of these stepped forms presents a new challenge for discussing the Quaternary natural history of snowy mountain in Northern Japan. Additionally, ripple-like microtopography, smaller in scale than stepped forms, has been discovered on slopes composed of pyroclastic density current deposits in various volcanoes in Japan. Their formation is under debate in relation to pyroclastic surges. Exploring potential connections between ripple-like micro features and CTs also warrants further investigation.
(Figure: Typical features of stepped forms expressed by shaded and slope images by the Geospatial Information Authority of Japan. The scales of all map images are same.)