ABSTRACT

The ages of, submarine rocks dredged from seamounts along the Japan Trench were determined by K-Ar method. In order to obtain more reliable age, the effect of acid leaching on an altered submarine rock was examined. Some samples treated by acid solution gave K-Ar ages up to 20% older than the original untreated sample. This result suggests a possibility that, if suitable acid teratment conditions are found, they would result in more reliable ages for submarine rocks. Some submarine rocks dredged from seamounts along the Japan Trench on the oceanic side gave K-Ar ages of about 70-80 m.y, whereas a sample dredged from a seamount on the Japanese Islands side gave a substantially younger age. Considering the present result as well as previous data, the last volcanic activity along the Japan Trench seems to have occurred about 70-80 m.y ago or older.