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In the active spreading center of the Guaymas Basin, several Leg 64 holes struck basaltic sills intercalated with very young, soft, and still partially hot sediment. This discovery and the measurement of mass physical properties provided a unique opportunity to study the mechanism of sill intrusion in statu nascendi. From a mechanical point of view, it can be shown that, in general, the upwelling magma must form sills as soon as it reaches soft sediment. The vertical buildup of an alternating sill-sediment sequence is controlled by the thickness of the sill contact, the interval (and volume) of magmatic pulses, and the sedimentation rate. Sill intrusions are formed continuously, so long as sediment accumulation is at least as rapid as vertical buildup by sills—including their upper contact zone. Lateral building of oceanic crust by dikes must be much faster, however, because the spreading rate exceeds the accumulation rate by a factor of 30 to 60. As many as several tens of meters below and above the sills, all mass physical properties of the sediment are substantially changed by the sill load, by dissolution of opaline silica and by partial dissolution of carbonate, and possibly by shrinkage in conjunction with a steam zone that occurred at an early stage of the intrusion. From the reduction of porosity at the contact of the uppermost sill, height of expelled pore water is about 30 meters. Thus, space is created for the intruding magma, and the huge amounts of escaping pore water create a hydrothermal system, precipitating part of their dissolved constituents on the seafloor.

Mechanism of Sill Intrusion into Soft Sediment and Expulsion of Pore Water