Hawaiian volcanoes consist of rocks of tholeiitic suite, alkalic suite and nephelinitic suite. Variation of chemical compositions of rocks in time and space and physico-chemical condition of the magma such as temperature, viscosity and oxygen fugacity studied both in field and in laboratory are briefly summarized. The petrologic model of the structure under the Hawaiian volcanoes proposed by the study of the distribution and the petrology of mafic and ultra-mafic inclusions in lava flows is also presented. Combining with the experimental data of the element partition between olivine and magma, the compositions of the rocks with 13-14% MgO in tholeiitic suite and 15-16% MgO in alkalic suite may represent the compositions of original magmas for each suite.

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Nyiragongo volcano (1°31’S, 29°15’E, 3,470 m in ht.) is located in the Birunga volcanic field, one of the most notable provinces of highly potassic rocks, in the Western Rift of the Eastern Africa Rift Valley. At 10 : 20 a.m. on January 10, 1977, Nyiragongo volcano erupted abruptly and ejected lava in the manner of fissure eruption from 4 flank rifts; Mudjoga, Shaheru, and Baruta rifts in an approximate N-S direction through the summit, and the other rift in the west flank of the summit. The lava flows spread over 34 km² in total area. Only a few hours after the eruption, lava lake, which had been active in the summit caldera for the past 50 years, was lost completely. Before the present activity of Nyiragongo, an eruption had started on December 26, 1976, at Murara and Harakandi on the southern flank of Nyamuragira, an active volcano neighboring to the west of Nyiragongo. The activity lasted for 5 months and formed two spatter cones surrounded by new lava. The lava of the present eruption of Nyiragongo is aphyric nephelinite, which contains very small amounts of melilite, nepheline, Ieucite, and augite in phenocryst. The chemical composition of the lava is 39 wt.% SiO₂ and 5.5 wt.% K₂O and contains 25% lc, 24% ne, and 14% cs (CIPW normative minerals). This composition is the same as the lava of the lava lake, lost in the present eruption of Nyiragongo. It is clearly different from the new lava of Murara and Harakandi, which is porphyritic basanite, containing plagioclase, augite, and nepheline in phenocryst. The lava contains 45 wt.% SiO₂ and 3.6 wt.% K₂O, and contains 21% or, 8% an, and 11% ne. This composition is very similar to that of the lava in the 1957 eruption of Nyamuragira. The large difference in chemical composition of the above two lavas shows clearly that the present simultaneous eruptions in Nyiragongo and Murara-Harakandi, Nyamuragira, were caused separately by two different magmas.

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The directions of horizontal maximum compression (σ_Hmax) are determined from the volcanic surface features which indicate the trend of zones of concentration of dikes underground at 16 volcanoes in the Sunda Arc. The directions ofσ_Hmax coincide well with those of p-axis obtained from the faultplane solutions for shallow earthquakes in each region of Sunda Arc. This result provides evidence that the direction of σ_Hmax yields that of the maximum principal stress of tectonic origin. Near the bend of trench such as Sumatra-Jawa and Sumba-Timor regions, the state of tectonic stress differs from the regional trend of convergence between the Indian and Eurasian plates.

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