Three-dimensional (3-D) seismic imaging was used to reveal >2.5-km-wide and >150-m-deep craters at the basal surface of 64 mud volcanoes out of a suite of 86, offshore Egypt. The craters were infilled soon after they formed by successive mud extrusions that combined to build mud volcanoes, as evidenced by onlap fill geometries of the earliest mud flows. We propose that the craters formed as the earliest manifestation of mud volcano formation. We infer that the energy required to excise in situ clays and sands buried and consolidated to depths over 150 m below the seafloor was provided by the highly vigorous venting of a dominantly gas and water mix during the initial eruption, in which gas column height was the critical factor. This primary phase of mud volcanism is rarely observed, and the findings presented here have significant implications for interpretations of the dynamism during this fundamental stage of mud volcano genesis.

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