Ineed with these: 1. what determines the viscosity of lava? silica and feldspar content silica and gas content. gas content and feldspar content 2. what effect do feo and mgo content have on the viscosity of lava flows they tend to break the chains of silica polymerization they tend to make the magma more viscous they tend to cause more violent eruptions 3. the slow, darwinian evolution through time is termed punctuated equilibrium phyletic gradualism evolutionary tree