Multiple sector collapses at Stromboli volcano, Italy: how they work

This paper demonstrates that four large sector collapses have affected the NW flank of the Stromboli volcano in the past 13 ka. alternating with growt h phases. In order to contribute to the evaluation of the critical conditio ns which trigger lateral collapses, a reconstruction of the geometry of eac h collapse of the volcano edifice in the four stages that preceded the rela tive collapse events is also presented, and a computation of the landslide volume. This reconstruction is based on new field data plotted in three dim ensions. Prior to the initial 13-ka collapse, the volcano was 1125 +/- 100 in high above sea level. The collapse had a volume of 2.23 +/-0.87 km(3), w hereas the pre-collapse volcano volume was 218.8 +/-7.7 km(3). The next edi fice that failed was 900 +/- 70 in high a.s.l. The collapse volume was 1 +/ -0.54 km(3), with a pre-collapse volcano volume of 201.4 +/-5.4 km(3). The edifice then grew to 1000 +/- 60 in a.s.l. The third collapse had a volume of 1.08 +/-0.39 km(3) and occurred within a volcano with a volume of 209.1 +/-4.6 km(3). This was followed by a new growth phase followed by the last collapse with a volume of 0.73.+/-0.22 km(3). The volcano volume was about the same as the present one. The present active crater zone is at 780 in a. s.l. In the first three collapses, sliding surfaces cut the main magma cond uit. In the last collapse, the upper scarp coincided with the conduit locat ion. Dyking along a main NE-trending weakness zone across the volcano summi t exerted a lateral force for collapse inception. The decrease of the lands lide volumes with the age, and the concentric scarps of the four collapses, suggest that the younger sliding planes tended to become more superficial and to decrease the areal extent. This is interpreted as due to: (a) succes sively weaker eruptive products from dominantly lavas to dominantly pyrocla stics; and (b) the feedback effects between collapses and dykes that inject ed along the lateral segments of the first collapse slide plane.