Lava invasion of urban areas at monogentic systems: Examples from the Chaine des Puys

Close to becoming part of a UNESCO world heritage site, the Chaine des Puys monogenetic alignment has > 80 effusive centers. Lava flows extending east enter valleys cut into the Limagne fault scarp. Flow emplacement dynamics depend on the slope of the terrain. We focus here on two cases: (i) the Grave Noire (60 ka) lavas – emplaced on steep slopes before transiting abruptly to shallow slopes; (ii) the Tiretaine (44 ka) lava – emplaced on gentle slopes, before entering a steeply sloping valley. Both flow fields underlie the city of Clermont Ferrand, and so have been used to test a GIS to assess impact due to lava inundation of an urban area close to an active monogenetic system.

The fountain-fed flows of Grave Noire poured down the Limagne fault scarp at velocities of 8 m/s. On encountering shallower slopes at 1 km, the lava stalled to form a perched pond contained by a barrier of its own breccia and ramped-up sediment. Intrusions through the barrier fed flow that advanced a further 4 km at velocities of a few tens of m/day. Distally, Tiretaine lavas built an inflated flow field with its front at the head of the Tiretaine valley. Failure of this front fed lava that entered the valley at effusion rates of several hundred m³/s. Lava descended as far as the current town of Royat, where it encountered, and ponded behind, a barrier built by Grave Noire lavas. If occurring today, these flows would impact more than 8422 addresses, 0.065 km² industrial land, 177 km of roads and 110 ha of agricultural land.

Outcrop exposure of lavas is outstanding throughout the Chaine des Puys, allowing 3D viewing of such effusive systems. At Grave Noire and Tiretaine, we have analogs for perched ponds, channels, and satellite shields experiencing flank collapse to feed flow at high local effusion rates. The juxtaposition of central France’s capital, and the lavas themselves, vividly demonstrate effusive hazards faced by urban areas located in active monogenetic systems.