| Summary: | Pāhoehoe and ‘a‘ā basaltic lava flows are common on volcanic islands such as Hawaii, Reunion Island and Iceland. They are present in all LIP and giant lava flows identified in planetary volcanology. In terms of both areal coverage and total volume, pāhoehoe flows dominate basaltic lavas in subaerial and submarine environments on Earth. Several processes for pāhoehoe lava flow emplacement have been identified, at different scales, from the extrusion of small lobes to the formation of giant flows. Thanks to the dynamics of geological fluids, we put forward a new paradigm to explain the emplacement of pāhoehoe lavas. The kinematics of the fluid mechanics is expressed through (immaterial) steam tubes that go on to become lava tubes and then lava tunnels (TTL). Lava tunnels, once considered a speleological curiosity, have been established to play a central role in the dynamics of geological fluids. The anastomosing network of TTL is integral to the pāhoehoe’s placement dynamics, both in open channel flows and pipe flows (inflation). The pāhoehoe fluid is considered to be a reactor in its own right. This fluid harbours numerous physicochemical reactions (including fractional crystallization, exsolution, segregation, coalescence and buoyancy), and is characterized by various interfaces and transition zones between the different constantly changing states of matter (number of phases, Newtonian fluid, Bingham and Herschel-Bulkley complex fluids). Various volcano-geomorphological indicators, at various observation scales on the surface and underground, reveal the dynamics of emplacement and make it possible to identify them in current and ancient flows. The paradigm of emplacement by TTL is crucial to both pāhoehoes and constituting LIP. Les laves basaltiques pāhoehoe et ‘a‘ā caractérisent les îles-volcans comme Hawaï, La Réunion ou l’Islande, mais plus généralement les grandes provinces magmatiques et les écoulements géants en volcanologie planétaire. Plusieurs processus de mise en place des coulées pāhoehoe ont été ...
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