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The volcanogenic, or effusive-sedimentary type of deposits includes the unique accumulations in the vicinity of volcanic structures and on adjacent parts of the ocean floor. The effect of volcanism on sedimentation is complex. Liquids, solids and gases are produced by volcanic eruptions. When eruptions occur underwater all the volcanic material including the liquid and gaseous products remain in the water.

Many authors think that explosive eruptions in the oceans are impossible at depths greater than 2000 m (Rittman, 1961) or even 500 m (McBirney, 1963). These figures, based on theoretical considerations about the critical pressure of water vapor, appear confirmed by hydro-acoustic data of the “SONAR” Service which, in more than 10 years of activity in tsunami warning and underwater sound recording in the Pacific Ocean has registered no explosive eruptions at great depths, whereas the shallow-depth eruption of the Medsin volcano was recorded simultaneously by several stations. Evidently, most seafloor eruptions are rather quiet. At first glance this conclusion appears to contradict the wide distribution of pyroclastic material near underwater volcanoes. Palagonite, or hydrated volcanic glass, is especially widespread in the Pacific.

Current ideas about the composition and quantity of liquid and gaseous volcanic products are incomplete and discordant. It is quite clear, however, that most of these soluble compounds enter the dynamic chemical reservoir of the ocean and lose their volcanogenic identity by mixing with compounds derived from land. The concentration of some elements in ore quantities occurs only where thermal water accumulates, such

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