Study of Lava Lake at Nyiragongo Volcano

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The Nyiragongo volcano in the western rift valley of the East African Rift System had a long-lived lava lake in the summit crater for about 100 years. We reviewed this lava lake actibities based on the old documents and the recent our field observations. Some results of the present study is summarized as follows ;(1) Based on the old documents and our field observations, the temporal variations of lava lake activity in the summit crater are summarized for the last 100 years.(2) Based on the observatin of temporal changes of lava-lake elevation, the average magma supply rate is estimated to be 7,000 kg/day. Total magma suplly is 86 million cubic meters in the period of 1994-1996.(3) the volcanic earthquakes were classified into two types on the basis of their waveform. The seismic activity of the A-type events shows ratherconcentrated features at the depth of about 15 km just beneath the Nyiragongo crater.(4) Whenever volcanic tremors are observed the eruptive activity in the lava-lake has been always reognized by an eye-witness. After the initial episode of eruption in the lava-lake, we observed at least 36 eruptive episodes in 1994 - 1996. We found that there is a dear reverse correlation between the occurrence of the A-type earthquakes at a depth and the lava fountain on the surface of lava lake. This feature can be interpreted by the increase of advective overpressure in the conduit system.