Final answer:
Molten rock flows through fissures and fills deep basins during the widespread mare-like volcanism stage of a terrestrial planet's development. This stage follows the solid crust formation and is pivotal in shaping geological features.
Step-by-step explanation:
In the development of a terrestrial planet, molten rock flows through fissures and fills deep basins during the stage of widespread mare-like volcanism. This stage occurs after the formation of a solid crust and heavy cratering, when the interior of the planet is still hot and molten materials can easily breach the surface. On Earth, this process has been integral in the formation of the planet's basaltic ocean basins and other geological features. It is important to note that such geological activities differ from planet to planet. For example, Venus never experienced plate tectonics, and its volcanic plains formed in a manner similar to the lunar maria, with widespread lava eruptions.