With increasing exploration of the seafloor, it is becoming apparent that high-silica lavas are an important component of the mid-ocean ridge system.  While volumetrically small compared to mid-ocean ridge basaltic lavas, andesites and dacites have erupted at several different spreading centers; including, the Juan de Fuca Ridge, Galápagos Spreading Center, and East Pacific Rise.  These lavas are typically associated with ridge discontinuities and/or regions of episodic magma supply. Although the high-silica lavas have erupted at different ridges, they show remarkably similar major element trends and incompatible trace element enrichments, suggesting that similar processes control their formation. MOR dacites are geochemically characterized by 1) elevated U, Th, Zr, and Hf; 2) relatively low Nb and Ta; 3) Al₂O₃, K₂O, Cl, H₂O concentrations that are higher than expected from fractional crystallization; 4) relatively low δ¹⁸O glass values of ~5.6 compared to values ~6.9 ‰ expected from fractional crystallization.  This suggests that crustal assimilation is an important process in their petrogenesis.  Petrologic modeling using AFC formulations suggest that low degree partial melting and assimilation of altered basaltic crust into crystallizing magma chamber can explain the geochemical signatures of the MOR dacites.  The role that this process may have in typical MOR environments and in the formation of MORB lavas, however, is the subject of continuing debate.