One can estimate the age of pyroclastic fall unit by means of the observation of soil profile on it. At first each fall unit must be distinguished, and next the age of each unit should be determined based on the following principles.
1) When living trees are found on the uppermost pyroclastic soil unit, the age of this unit can be estimated by means of the dendrometry.
2) The age of pyroclastic fall unit can be roughly estimated by the degree of soil profile development on it as shown by the following standard.
Age of weathering Soil horizon sequence
Within 100 years C or (A)/C
100-500 years (A)/C, A/C or A/(B)/C
500-1500 years A/(B)/C or A/B/C
Over 1500 years Complete A/B/C profile
3) The humus content of the volcanic soils must be responsible for the weathering age of it, so the humus content or the value of the humic acid extracted from the humic layer may be useful to estimate the weathering age of pyroclastic fall unit, though the mass-wasting and the artificial disturbance of soil materials should be kept in mind.
4) When the pyroclastic fall unit is intercalated within peat layer, one can estimate the age of the unit by determining the depth of the peat layer on it, supposing that the accumulating rate of peat is constant (1cm per 10 years). According to the same way, the age of the pyroclastic unit overlain by the aceolian soil layer can be estimated.
5) When any carbonized materials or prehistoric implements including obsidian tools. made by aborigines are found within the volcanic soil unit, the age of the pyroclastics can be estimated by means of 14C dating, the thickness of hydration layer of obsidian tool and the cultural chronology of the implements, etc.
6) Moreover, if one can obtain the ancient record of volcanic activity, the chronology of the pyroclastic fall unit determined by the above mentioned methods can get undoubtedly more accuracy.
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