11 January, 1771: The landslide of Alleghe and the volcano hypothesis

The lake of Alleghe in the valley of Cordévole is almost 1,5 kilometres long and exactly 240 years old. The moment of the birth of the lake is well known, at 7.02 in the morning of the 11 January 1771 the river flowing through the valley became dammed by a landslide coming from the mountain Piz, subsequently forming a 100 meter deep lake.

Fig.1. General view of the valley of Cordévole with the village and lake of Alleghe, on the right on the mountain Piz the scar of the landslide is barely visible in the forest, in the background the Civetta (3.220m).

Fig.2. Historic depictions of the lake dammed by the landslide in the "Atlas Tyrolensis" of 1774 by the Tyrolese Peter Anich and Blasius Hueber. Note the boulders on the lower shore of the lake, Anich and Hueber were one of the first cartographers to use signatures to display geomorphologic features in their maps.

The Alps-traveller and chronicler Belsazar Hacquet remembers a visit to the lake in 1780:

"The river Cordévole became my guide, by following him I would find the valley of Cadore. But only after some hundred steps the river was flowing in a vast lake, existing here only for the last nine years. I walked around in eastern direction, to the villages of Sternade and Saviner until the mountain of Piz. First the lake was narrow, only by Saviner it became more than 100 venetian fathom [an old length unit used in the mining industry of these times, one fathom ca.1,8m] broad and more than thirty deep.
The last mentioned village once was situated on a hill, before it in a broad valley there were four smaller villages...[]...which became flooded by the lake, but the fourth locality, namely Marin, was buried with the village of Riete under the collapse of the mountain of Piz, last mentioned village situated previously on the top of the mountain."

"Standing on the top of the mountain, I immediately noted that the mountain possessed a volcano on top of it, and it was possible to see how deep it went. After the mountain collapsed, it could be seen that its base was composed of limestone, build up by mighty layers, dipping from the west to the east with 45 degrees. The surface of the collapse is so plain, that a man has difficulties to climb on it to the mountain."

The area of the landslide is composed by anisian dolostone (Contrin-fm) overlying, in part overthrusted, limestone - layers and marls of the Ladinian (St. Kassian-fm).

Fig.3. Detail of the geological map (Carta Geologica delle Tre Venezie, Foglio 12 "Pieve di Cadore", 1940) showing the village and the landslide of Alleghe (with the lake in the lower part of the map) and the dammed lake, lithology: pink = Anisian dolostone (Contrin-fm), blue= Anisian dolostone, lower succession (Moena-fm), brown= marls with bedded limestone-layers (St. Kassian-Formation).

The apparently strange notion by Hacquet of a volcano is however forgivable, at these times volcanic forces were believed to cause strong and sudden movements of the terrestrial surface, also the marls in the area contain tuffaceous layers, maybe Hacquet recognized this particular lithology. Notable he describes correctly the surface where the landslide "slipped away".

The landlside of Alleghe killed 48 people, livestock and destroyed part of the village of Riete and some farms. The lake that formed after the event inundated the village of Peron, only in February 1771 a provisional channel cut into the landslide deposits stabilized the lake level.

Bibliography:

HÖFLER, H. & WITT, G. (2010): Katastrophen am Berg - Tragödien der Alpingeschichte. Bruckmann Verlag: 144