Many studies have investigated how channels and tubes form within basaltic flows. The partitioning of a broad lava flow into narrow regions of enhanced flow velocity (which we refer to as preferred pathways) can affect lava emplacement, and allow flows to reach great lengths on Earth and other planets. In this study, we investigate the role that a dynamic instability, driven by large changes in viscosity due to small changes in temperature, can play in the formation and propagation of low-viscosity preferred pathways. We use a fluid dynamics-based model of laminar lava flow emplacement that examines this process exclusively. Analysis of this model shows that preferred pathways will initiate when the temperature dependence of the lava's viscosity is sufficiently strong. If a preferred pathway develops, it will form and stabilize over a distance that reflects the competition between the cooling rate through the flow's surfaces and the flow velocity within the preferred pathway. Using velocity and cooling rates of typical basaltic flows on Earth, this model is able to produce preferred pathways within lava flows with reasonable dimensions and timescales. The model also fails to produce such pathways in situations where tubes and channels are not found in nature (e.g., in basalt sheet flows and silicic flows < 1 km in length). These results are sufficiently positive to demonstrate that this thermodynamic instability may play a role in the formation and propagation of preferred pathways within some flows, and thus merit further investigation.