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

大规模的火山碎屑流构成了广泛的沉积地貌，即后续解剖的初始形态。最初的形式被认为在形成后立即遭受密集的解剖，因为松散的未焊接材料形成了个别沉积物的上部，并且没有植被覆盖。这一点得到了伊藤火山碎屑流沉积研究的有力和典型的支持。伊藤火山碎屑流沉积是大约22000年前发生的一次大喷发的产物，构成了日本九州南部火山口周围广泛的褐云母高原。沿九州南部的久田河、木津河、马诺斯河等河流阶地保存完好，其流域大多被伊藤火山碎屑流沉积所占据。保存下来的还有许多古河谷网络，它们与河流阶地形成于同一时代。河流阶地和古河谷都纯粹位于伊藤火山碎屑流沉积未焊接的上部，并均匀地覆盖着厚达几米的苔藓层，考虑到最古老的(底部)盖层的年龄超过1万年，未焊接沉积的侵蚀性极高，且形成后立即在沉积面上缺乏植被，解释了河流阶地和古河谷网络在伊藤火山碎屑流侵位后的短时期内，在早期剥离中形成得非常快。这表明目前大多数伊藤火山碎屑流沉积地貌的形成是通过集中的初始解剖完成的。早期解剖的时间被认为不超过100年，可能不到10年。

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.