A group of scientists at the University of the Balearic Islands have identified some 5,000 blocks of boulders that were dragged by tsunamis generated by earthquakes in Algeria.
Their movement pushed them, in one instance, 136 metres inland on the Illa d'Aire in Minorca. On the same islet, up to 250 tonnes were displaced and deposited 25 metres higher than the original location. In Majorca, says professor emeritus of stratigraphy, Antonio Rodríguez Perea, the maximum displacement was 60 metres.
A tsunami generated in Algeria would arrive in the Balearics after 40 to 45 minutes. On 21 May 2003, an earthquake in Algeria registered 6.9 on the Richter Scale. A tsunami took 41 minutes. The scientists have found that were twelve incidents like this between 1660 and 2003, one roughly every thirty years.
Analysis of the rocks shows evidence, however, of tsunamis that occurred up to 1,400 years ago. Fifty of the blocks have encrustations which indicate their age. There is some documentary evidence from years back which recorded tsunami events. In 1756, Nicolau Ferrer from Santanyi described a giant wave that dragged a block of rock of some two tonnes and deposited it two kilometres inland.
There is a concentration of these rocks on the eastern and southeastern coast of Majorca between Arta and Campos. In Minorca the distribution is more widespread. There are also examples in Ibiza and Formentera.
Rodríguez Perea explains that other parts of the Mediterranean have similar rocks. "This shows us that we cannot ignore the risk that these phenomena can represent and for the tourism industry in particular."