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Deep-sea drilling and dredging have shown that the oceanic crust consists predominantly of basaltic lava flows. The frequent occurrence of glassy selvages, observed in some cores — but absent in others — shows that at least two volcanological types are common: pillow lava sequences and more homogeneous lava flows. If the basaltic crust consisted of a continuous sequence of homogeneous lava flows or pillow, then relatively constant physical properties would be encountered during drilling. Results of DSDP cruises show that this is generally not so, and that although diagnostic measurements are lacking, considerable variations occur in the basaltic sequences; easily drilled sections alternate with hard rocks that drilled slowly. On Leg 49, we could in several cases correlate slow drilling with compact flows and fast drilling with either interlayered sediments or volcaniclastic products (hyaloclastites or breccias). (See site summary chapters, this volume.) The aim of this paper is to emphasize the possible importance of these volcaniclastic layers in the oceanic crust. This has consequences for volcanic processes operating in submarine environments, geophysical interpretations of the oceanic crust (in terms of density, velocity), and geological processes (circulation of fluids, alteration, etc). Such layers are particularly well developed in Hole 410, where the whole sequence consists of volcaniclastic products. Detailed observations of this core allow us to define the geological processes responsible for the formation of these breccias. Comparison with other occurrences, either outcropping in some areas (volcanic islands, Afar) or found by drilling in some atolls (e.g., Mururoa or Fangataufa in the Pacific), show that the occurrence of such products is not uncommon in the oceanic environment and may be rather frequent, not only in volcanic island substrata and in guyots or seamounts, but also in normal oceanic crust.

Autoclastic Submarine Breccias in Hole 410, Leg 49, and Other DSDP Sites