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In this picture, you see a simple cross-section of a typical upwelling of lava through a magma tube, as in a volcano. The molten rock (the red stuff) oozes up until it spills over the lip of the volcano, and a river of lava runs down the side of the cone.
Especially on the surface of the moon, we don't need a volcano to create a lava flow. Every large impact should have created some massive lava flows, which behaved the same way.
The outer surface of the lava cools fastest, and hardens. (That's the dark stuff.) Molten lava will continue to flow through the resulting tube as long as there's more flowing from the source; but even then, most will likely fall away from the level of the tube.
Once the supply of lava is cut off, the molten lava continues to flow out of the tube, leaving a void behind it. In the illustration, the source of lava cuts off as soon as the level of magma upwelling from the volcano drops below the solid lip. You see a lava tube (indicated by the light gray area) forming as the lava runs out.
In this animation, you see a lava dome forming right after the lava tube is done. Lava domes form in a very similar manner, except that the dome is usually the void left when a solid crust forms in the cone and the molten magma retreats.
Finally, in this animated scenario, further flows from surrounding geological events bury the lava tube and the lava dome. On the moon, these caverns might be many miles beneath the surface. We will learn a lot about the geological history of the moon when we find these things, so they would be worth looking for even if they weren't a very convenient place to build a human habitat.
The vertical scale is exaggerated for effect. I wanted to emphasize that when we find lava tubes in the moon, they will most likely slope toward a low end where we encounter a solid plug. If we build a terrestrial atmosphere in such a cave, moisture will condense out of the air and form a lake at the low end. We can take advantage of this in the design of a bioregenerative life support system.
Additional References in the Artemis Data Book
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