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

大规模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。由于个别沉积物上部松散的未焊接材料和缺乏植被覆盖，初始形态被认为在形成后立即遭受强烈的解剖。伊藤火山碎屑流沉积领域的研究有力而典型地支持了这一观点。伊藤火山碎屑流沉积是发生在距今22000年左右的一次大喷发的产物，在日本九州南部的艾拉火山口周围形成了广泛的火成岩高原。九州南部的志田河、木月河、马乃斯河等河流沿岸保存完好的河流阶地，其流域多为伊藤火山碎屑流沉积所占据。许多古河谷网也被保存下来，它们与河流阶地形成的年代相同。河流阶地和古山谷都完全位于伊藤火山碎屑流沉积的上部未焊接部分，并均匀地覆盖着几米厚的火山层。考虑到最古老（底部）的温层年龄在一万年以上，未焊接沉积的可蚀性极高，且沉积表面在形成后立即没有植被，可以解释为在伊藤火山碎屑流侵位后不久的短时间内，河流阶地和古谷网在初期解剖过程中形成得非常迅速。这表明伊藤火山碎屑流沉积现今大部分地貌的形成是由集中的初期解剖作用完成的。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.