How vertically deep do lava conduits go?

Question

I'll preface this with the truth and hope it doesn't disqualify the question but rather give context: after reading Journey to the Center of the Earth by Jules Verne, I am amazed at the prospect of venturing into volcanic conduits to explore the interior of the Earth. How deep do lava conduits go, why don't they go deeper, and what kinds of formations or environments exist at their terminus?

I am not new to Earth science but am pretty new to in-depth geology (pun intended) and volcanology. From what I can tell, the type of lava cave which goes the deepest is referred to as a lava tube. Apparently these tubes can be fairly long (e.g. a tube from Mauna Loa goes 50km before terminating[?] at the ocean!) but not very deep (Wikipedia on lava tubes lists a mere 15m!) Why do these tubes not go far deeper?

The deepest conduit from surface to inner Earth is apparently the Kola Superdeep Borehole which terminates at ~12km deep upon reaching higher-than-expected temperatures. I think of magma - liquefied hot rock erupting out of volcanoes - as coming from even deeper than that depth. Therefore, why doesn't lava tubes or other volcanic conduit continue to such deep depths? Do they, but it is too difficult to explore? Do they during eruptions, but soon after cool and fill with solidified rock or eventually collapse and thereby fill with rock?

As noted I'm new but interested to the subject, so any info to help me understand the processes and environment involved in this interface between inner Earth and the surface would be much appreciated.

Answer 1

As mentioned in the comments, lave caves and tubes are surface features. You get a lava tube when there's a lava flow and the external part in contact with air (or water) cools down and solidifies while the interior is still flowing. National Geographic made an excellent feature on it, all with diagrams and photos and stuff.

Why do these tubes not go far deeper?

Because lavas are liquid, and they flow downwards. If there's a hole that goes deep, the lava will fill it. It's possible to explosively eject lava out of a hole (as commonly happens), but these are violent events that will cause the collapse of the conduits, forming breccia pipes, with collapsed calderas.

I think of magma - liquefied hot rock erupting out of volcanoes - as coming from even deeper than that depth

...terminating at the threshold of the asthenosphere?

Let me clear a common misconception here: there is no magma ocean in the interior of the Earth. There is no large reservoir of magma in the deep earth, and volcanoes are not just holes through which this (non existent) magma is coming out. Both the crust and the mantle are essentially solid rock. The asthenosphere is solid rock.

Melting is an uncommon occurrence in the Earth's crust and mantle. It needs exceptional circumstances to happen, which mostly happen above subduction zones (i.e. the Pacific Ring of Fire) or above hotspots (East Africa, Hawaii, Yellowstone). The erupting magma can come from anywhere that's as shallow as less than a kilometre below the surface (which can be above sealevel if you're on a high volcano) to several tens of kilometres. Very rarely these magmas come directly from asthenospheric depths. These magma chambers can have complex plumbing systems beneath them and all sorts of morphologies of feeders.