Each volcanic eruption gives birth to a unique signature in the composition of the ash and in the glass shards that lodge within soil. Even eruptions from the same volcano have different chemical elements, even down to the shape of the shards. A volcano in Alaska that blew in the 9th century AD has a distinctly high chlorine content and this has helped identify its fingerprint across Europe and northern N America – see http://news.yahoo.com/alaska-volcano-blanketed-europe-ash-1-200-years-21…. The research was done at Queens University Belfash.
At www.sciencedaily.com/releases/2014/10/141006084908.htm … the European Science Foundation are looking at offshore archaeology – along former continental shelf systems and coastal waters, and even the odd open sea basin.