Basalts erupted along oceanic zones of spreading and on intraplate seamounts, at depths where hydroexplosions are not likely, follow various styles of eruption. ''Quiet'' eruptions produce a variety of forms, including lava lakes and sheet, pillow, and pavingstone lavas. ''Reactive'' eruptions occur instead when the lava is thermally shattered as in the formation of hyaloclastites. The stresses induced in a rapidly cooling lava flow are important in determining the different lava forms, with lava lakes and sheet flows resulting from low stresses, pillows from higher stresses, and hyaloclastites from even higher stresses which cause rupture. Calculations show that if we consider basaltic lavas of the same composition, the intensity of the stresses induced in the cooling lava increases proportionally to the initial viscosity and inversely to the temperature and rate of discharge of the lava at eruption. The viscosity is strongly dependent on the crystal/liquid ratio of the magma at eruption. The temperature and viscosity of the lava at eruption are related ultimately to the thermal regime of the lithosphere below; viscosity tends to increase going from fast to slow spreading ridges and to intraplate seamounts. Within a given ridge segment, temperature tends to decrease and viscosity to increase approaching a transform or overlapping boundary. Thus the model predicts that going from fast to slow spreading ridges (i.e., from East Pacific Rise to Mid-Atlantic Ridge), the probability of forming lava lakes and sheet flows decreases and the probability of forming pillow flows increases. Moreover, within a given ridge segment the areal sheet to pillow ratio should decrease as a transform or overlapping boundary is approached. Sheet flows should be significant only in the earliest stages of the growth of large intraplate seamounts, pillows and hyaloclastites in later stages. The probability of forming hyaloclastites should be low in spreading ridges, higher in intraplate seamounts.

Recent high-resolution observations on the East Pacific Rise, the Galapagos Rift, the Mid-Atlantic Ridge, and the Red Sea are consistent with the model, because they show that (1) the areal ratio of lava lakes and sheet flows over pillows decreases going from fast to slow spreading ridges, (2) within a given ridge segment it decreases approaching a transform, and (3) hyaloclastites are important on seamounts only. ¿ American Geophysical Union 1988