I’d like to consider continental rifts on massive scales. I’m trying to imagine one about the length of California that is so deep, a long “river” of lava flows along its length. The rift walls would be mostly vertical but would slope, slightly, such that it gets narrower at the bottom: from a few miles across at the top to about 100 to 200 yards across the lava river. I figured it would need to be at least as deep as the Marianas Trench, which is 6.825 miles, so perhaps 8 miles deep?
Gigantic fallen boulders wedged between the two rift faces, near the bottom, would provide an imperfect “roof” over the lava rivers, and floor to the chasm.
I expect surface rivers would flow into the rift, but having so far to fall, would turn mostly to mist before reaching the lava to be vaporized into steam. Combined with airflow due to the heat and gasses released at the bottom of the rift, this could create various weather conditions such as eternal mists that vary from water vapor to clouds of poisons, in some places, and powerful up/down drafts in others.
But I’m not sure such a feature would even be geologically possible for any length of time. For example, would the lava just rise up to the surface, like in a volcano? I know there are currents of magma beneath the surface of Earth; would these be sufficient to drive the river of exposed lava fast enough that it wouldn’t solidify- even with rivers pouring in?
I’m really hoping to find some way to make this work, as I can imagine some interesting ecologies and civilizations taking advantage of the various extreme environments it provides. Any natural planetary state (including uncommon/rare) is an acceptable condition to make this feasible, but it needs to remain a habitable world, and magic should be excluded.
I’m looking for answers that explain what events could create such a rift (secondary), and what conditions are required for it to remain active for thousands of years?