Pumice are fragments if frothy, solidified magma foam which is thrown into the air during a volcanic eruption. It may get carried very far from the source area. Pumice is so light that it floats on water but after sometime when the vesicles get saturated with water, it will sink.

Much of the pumice found in Iceland is in Hekla, the Veiðivötn volcanic area.

Information provided by Patrick M. Colgan, Northeastern University
http://www.casdn.neu.edu