Final answer:
A lava flow becomes slower as it cools due to an increase in viscosity which hinders its ability to flow, eventually leading to the solidification of the lava into rock.
Step-by-step explanation:
A lava flow becomes slower as it cools. The temperature of lava has a direct impact on its viscosity, with hotter lava being less viscous and therefore able to flow more quickly.
As lava cools, it becomes more viscous and its flow rate decreases.
For instance, basaltic lava, which is very hot, can flow quickly and travel over long distances, as seen in basaltic eruptions where lava flows can move over 20 kilometers. Conversely, as the lava begins to cool, the rate of flow diminishes markedly. This cooling leads to an increase in viscosity and a gradual solidification of the lava into igneous rock.