Lava tubes are a peculiar type of lava caves formed by a low-viscosity lava flow that can develop for crusting-over, forming a continuous and hard crust, which forms a roof above the still flowing lava stream, or for inflation, slipping between pre-existing lava flows. The resulting structure constitutes among the most efficient thermal structures on Earth, because of their capacity of thermal insulation isolated lava flows can travel over long distances across lava fields.

Lava tubes, which on Earth are typical features of lava fields encountered in intracontinental plateaus and volcanic-shield islands (slopes <2°, e.g. Hawaii, Canaries, Iceland, etc.), have also been recognized on the surface of other rocky bodies of the Solar System such as Mars and the Moon. Due to the similar characteristics of basaltic volcanism on rocky bodies, it is expected that lava tubes have similar morphologies and origin among them. The growing interest in studying large terrestrial lava tubes is motivated in part by their analogy with their extra-terrestrial counterparts.

A study led by the Department of Geosciences of the University of Padova and recently published in JGR Solid Earth focused on the lava tube system of La Corona (Lanzarote, Canary Islands, Spain) in order to identify how pre-existing stratigraphy can govern the evolution of a lava tube.

Only recently it has been possible to perform comparisons between lava tubes on different planetary bodies with implications on the study of planetary volcanology, habitability and astrobiology.

Indeed, in the last decade, high-resolution orbital images on planetary bodies like Mars and the Moon offer the possibility of studying the morphology of these structures, detecting them from the recurrent collapses in the lava tube roof, that shows the presence and the path of the tube itself.

The differences in gravity between Earth and the other planetary bodies and its concurrent influence on the effusion rates result in a significant difference in lava tube dimensions, indeed, terrestrial lava tubes tend to generally be 2 or 3 order of magnitude smaller (10–30 m) than those on Mars (250–400 m) and the Moon (500–1100 m).

Multidisciplinary efforts, combining terrestrial laser scanner technology with field observations and geochemical analyses enabled us to reconstruct the three-dimensional geometry of the lava tube system, the paleo-surface trough which it developed, and the volcanic series into which the tube carved its path. The results of this study show that a pyroclastic layer that characterises the La Corona system played a key role in the development of the lava tube. This layer - derived from late Quaternary Strombolian activity, which preceded the effusive activity - is traceable along almost the full length of the tube path and defines the paleo-topography. The excavation process mostly happens because of the termo-mechanical strength of the substrate, that controls the widening of the growing lava tube. Since weak layers such as regolith are a common feature of extra-terrestrial lava fields, the processes seen at La Corona might be highly relevant to the development of planetary lava tube systems.

Another important aspect is the post-cooling phase, when lava tubes are characterized by a near–constant inner temperature and, on other planetary bodies, they might offer a natural protection against micrometeorites and solar and cosmic radiations making them ideal locations for future planetary explorations.

Within this framework, studying the largest lava tubes on Earth is of interest as they could represent the best planetary analogues. Thus, in order to employ lava tubes as locations for future explorations, it is important to understand exactly how they form and develop not only during the active phase (flow-phase), but also and more importantly during the post-cooling phase.

“The importance of this kind of structure is growing because they are present non only on Earth but also in other planetary bodies of our Solar System, like Mars and the the Moon. There they have bigger dimension, usually two or three order of magnitude bigger than the ones on Earth”

said Ilaria Tomasi, PhD student of the Deparment of Geosciences of the University of Padova and first author of the study.

The peculiarity of La Corona lava tube is not only its dimension, because it's one of the biggest lava tube on Earth, but also the presence of a particular pyroclastic layer in the middle of the tunnel that can easily be followed through the tunnel.

“We were very curious about this level because we wanted to know if it has any rule into the tube evolution and formation: so we focused our study also on this layer and we discovered that the presence of this layer was very important because. And if we talk about planetary analogs this is much more important because on Mars and the Moon instead of pyroclastic material we should have regolites that can help the injection of lava in a similar way”, Ilaria Tomasi added.

This work has also been conducted by Prof. Matteo Massironi and Prof. Christine Marie Meyzen of the Department of Geosciences of University of Padova, Dr. Francesco Sauro (BiGeA Department - University of Bologna), Dr. Riccardo Pozzobon (CISAS-Padova) and Dr. Luca Penasa (INAF Padova) in collaboration with an international team composed by Prof. Jésùs Martinez-Frìas of the Institute of Geosciences-IGEO (CSIC-UCM, Madrid, Spain), Gustavo D. Santana Gomez of the Vulcan Vertical Espeleologias y Barrancos (Arrecife, Spain), Dr. Tommaso Santagata of VIGEA (Virtual Geographic Agency - Reggio Emilia, Italy) and Dr. Matteo Tonello.