Mihara Volcano, Izu-Ooshima Is., erupted violently on 15th November in 1986, and after six days the fissure eruption took place along a eine toward the northwest from caldera rim to somma slope of the volcano.
By a advanced helicopter, aerial environmental gamma ray measurements over Izu-Ooshima Is. were carried out on 4th December in the same year and on 19th in next January, in order to clarify a behaviour of radon and thoron as bed gas in such volcanic eruptions. As a result, it can be stated that, on the occasion of volcanic eruption, the ratio of potassium/scattered gamma ray increases but instead the ratios of radon/potassium and thoron/radon decrease.
The following eruption model may reasonably be interred on the behaviour of bed gases.
1) On the first stage, the magma reservoir and all around basement layer are applied lateral pressure to.
2) As the result, fissure in this layer contacts and then ascension of magma begins.
3) Decreasing of gap volume in the fissure and supply of magmatic heat cause increasing of pressure of pore water in the fissure and the layer.
4) In the continuous process, ascension of magma along the weak fissure keeps up and finally it results in eruption.
5) Owing to this eruption, the comparable stress based on some pressures decrease.
6) After this stage, magmas descend, the fissures in basement layer gap, and the pressure of pore water is restored.
And more, the author believes that this model will be of great value in the prediction of volcanic eruption.