CONTROLS ON EXTRUSION AT MIDOCEAN RIDGES

A magma lens can erupt to form extrusives only if it is under greater pressure than the static pressure in a column of magma reaching from t he lens to the surface, The excess pressure results partly from overbu rden pressure caused by the presence of high-and low-density rocks (di kes and extrusives, respectively) above the lens. The thicker the pile of low-density extrusives, the lower the average overburden density. Thus, extrusion above a lens should be self-regulating, in that thicke ning the extrusive layer reduces the driving pressure for subsequent e ruptions. Flexural stresses may affect extrusion by altering the press ure on a magma chamber. For ridges lacking an axial valley, we predict that deeper magma lenses should correlate with thicker extrusive laye rs, consistent with recent observations.