I studied the active vulcanism in Hawaii with the United States Geologici Service (U.S.G.S) in January of 1983 only a month after the eruptions had begun again after 20 years of quiet.

Lava tubes are not a "form supported" structure. Lava tubes have no shape unto themselves but are only a void, or empty channel within an existing, cooled lava flow. The are tunnels within the lava flow and which surve as conduits to future lava flows which serve to discharge the flow very far down slope via a direct path. These tubes are the remnants of the main hotter lava flow as the older flow cooled around them.

You would have more luck saying the Tubes on Mars are the result of a process similar to "lava TREES" but still you'd be wrong.

Take a look at my site's discussion of the Tubes on Mars and you will see why they cannot be natural and, per force of logic *MUST* be artificial.

http://www.geocities.com/marsunearthed/tubes/mars_tubes.html