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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.

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    $\begingroup$ Lava tubes form as result of a surface flow of lava, therefore they are always parallel to the surface and not vertical. I think your question will better point to what you are looking for if you add something like "why do vertical conduits carrying lava out of volcanoes do not form lava tubes?", I guess part of the answer is because the deep lava transport is trough diapirs, not continuous conduits. But there are definitely quite deep vertical conduits (maybe not as deep as the Kola borehole), and despite I got some ideas, I'm not sure why those vertical conduits never form lava tubes. $\endgroup$ – Camilo Rada Apr 13 '18 at 3:38
  • $\begingroup$ Thanks @CamiloRada. I recognize lava tubes are probably not the type of lava conduit I'm most interested in so I tried to keep the question broader than that. However, the vertically deepest conduits I found mention of on Wikipedia were lava tubes! Not being an expert and Wikipedia not being perfect, I could be missing something for sure. $\endgroup$ – cr0 Apr 13 '18 at 14:39
  • $\begingroup$ Reading more about diapirs does help clarify how volcanism works, and why there won't be a pathway beyond the zone of ductile rock where diapirs form and rise. However, could lava conduits lead all the way to the magma chamber through brittle rock of the lithosphere, terminating at the threshold of the asthenosphere? (See en.wikipedia.org/wiki/Diapir#/media/File:Subduction-en.svg) $\endgroup$ – cr0 Apr 13 '18 at 14:42
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    $\begingroup$ Bear in mind that the wikipedia image you linked is a very schematic diagram. Don't think that is exactly what all volcanic systems look like. In the last 10 years or so there has been a big change in the way petrologists think about magma chambers, to the extent that we now believe that the classic view of a magma chamber as a vat of crystal poor liquid is actually a rather rare situation. What is much more common is large volumes of crystal rich, liquid poor mush zones distributed through the crust below volcanic systems. $\endgroup$ – bon Apr 14 '18 at 9:31
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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.

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