Viscosity control of the composition of ocean floor volcanics

Abstract
The fact that volcanic rocks from the ocean floor cover a restricted range in composition does not necessarily imply that only this narrow range of magmas is available in this environment. Instead it is postulated that the viscosity exercises a control by limiting the range of magmas (to those with a sufficiently low viscosity) which is capable of penetrating the upper part of the oceanic crust to emerge on the ocean floor. On land, this control can be over-ridden by the generation of gases.