Process of dissection of initial form of large-scale pyroclastic flow deposits

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Large-scale pyroclastic flows constitute extensive depositional landform, i.e. initial form for the subsequent dissection. The initial form is considered to suffer intensive dissection immediately sfter its formation, because of loose unwelded material forming upper portion of individual deposits and absence of vegetational cover. This is strongly and typically supported by the study in the field of Ito pyroclastic flow deposit, the product of a big eruption which occurred about 22,000 yr B.P. and constitutes extensive ignimbrite plateaus around Aira caldera in Southern Kyushu, Japan.River terraces are well preserved along rifers such as Hishida river, Kimotsuki river, Manose river in southern Kyushu, the drainage basins of which are mostly occupied by Ito pyroclastic flow deposit. Also preserved are many paleovalley networks, which were formed at the same age as the river terraces. Both river terraces and paleovalleys stand purely within the upper unwelded portion of Ito pyroclastic flow deposit, and are uniformly covered with tephra layers of several meters in thickness.Taking it into account that the age of the oldest (bottom) tephra layer is older than some ten thousand years, the erodibility of the unwelded deposit is extremely high, and the vegetation was absent on the depositional surface immediately after its formation, it is interpreted that the river terraces and the paleovalley networks were formed very rapidly during incipient dissection in a short period immediately after he emplacement of Ito pyroclastic flow. This indicates that the formation of most of the present landforms of Ito pyroclastic flow deposit was accomplished by the concentrated incipient dissection. The time for the incipient dissection is considered to have been no more than 100 years and probably less than 10 years.