毘沙門岳火山は単斜輝石—斜方輝石安山岩∼デイサイトと比較的少量の角閃石安山岩からなる。Sr/Y>40, SiO₂≈63%, Al₂O₃≈18%の組成を有するこれらの岩石はアダカイト質であり，近隣の火山の岩石と比べてK₂O, Rb等の液相濃集成分に乏しく，基本的には，広域的安山岩マグマと当火山に固有の珪長質マグマの混合物と考えられる。珪長質端成分マグマは，Mgに富む斜方輝石（Mg/(Mg+Fe)≈0.80），単斜輝石（Mg/(Mg+Fe)≈0.82）恐らくマントル由来の橄欖石（Fo≈90）を特徴的に含み，その化学組成（SiO₂>65%, Na₂O≤3.2%, K₂O≤1.2%, Rb≤20 ppm, Y<10 ppm, Nb≤6 ppm,全FeO/MgO≤1.6,およびSr/Y>60）はアダカイトの特徴をより明瞭に示す。断定するには未だデータ不足だが，この端成分アダカイトマグマは沈み込みつつあるフィリピン海プレート海洋地殻の部分溶融作用で生じた可能性が高い。

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⁴⁰Ar/³⁹Ar年代測定で得られるJ-値を用いて京都大学原子炉（KUR）での照射中性子密度の均一性の評価を行った。φ16×18の大きさの円柱状試料ホルダーにつめた0.3 mm程度のHb3gr角閃石試料をレーザーを用いて溶融して得られた結果は垂直，水平方向ともほぼ均一な値を示した。KURは, ⁴⁰Ar/³⁹Ar年代測定にとってはほぼ均質な速中性子束密度を実現できるといえる。カリウムとカルシウム塩試料から得られる補正因子は，Hb3gr角閃石標準年代試料にはほとんど影響しない。

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The K-Ar ages and paleomagnetic stratigraphy of the Plio-Pleistocene volcanic rocks from the Ariake Sea region have been studied to clarify the evolution of volcanism in the western part of Beppu-Shimabara graben, west Kyushu, Japan. This region has seven domains of volcanic rocks. We determined K-Ar ages of ten volcanic rocks from five domains: the Misumi volcanic rocks=4.51 Ma and 3.97 Ma; the Kunisaki andesite=1.45 Ma; the Ibinoodake andesite=3.97 Ma and 4.52 Ma; the Uki volcanic rocks=1.63 Ma; the Yushima basalt=0.82 Ma; the Otake volcanic rocks=1.82, 1.77, and 1.72 Ma. Paleomagnetic measurements were made on volcanic rocks collected from all seven domains. Magnetic property of lavas from these domains can be classified into four magnetic polarity epochs: Brunhes Chron (Ninotake volcano, Takesaki basalt and Yushima basalt), Gauss Chron (Nogamajima volcano), Gilbert Chron (Ibinoodake andesite and Hidake lava dome), and Matsuyama Chron (the other volcano). The age of Ohya tuff, which belongs to the Kuchinotsu Formation, corresponds to Gilsa event. We synthesized our new data together with previously reported ages and stratigraphic data, and examined the volcanic activity in this region in terms of a spatial and temporal distribution. There is no correlation between spatial distribution of the volcanic rocks and eruption ages, but we revealed that the volcanic activity in this region is episodic during the last 5 million years. We can identify the magmatic activities as five episodes: 5-4 Ma, 3.2 Ma, 2.0-1.4 Ma, 1.0 Ma, and <0.9 Ma. The intensity of volcanic activity in the Ariake Sea region increased with time.

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