Under water volcanoes, both shallow and deep, erupt more often than we think. When an exceptionally large underwater volcano erupts, the lava that is produced cools quickly, trapping gas bubbles within the rock. This forms pumice, a highly vesicular, highly porous rock that is light enough to float on water. Large enough volumes of pumice can be violently ejected during eruptions, rising to the surface of the ocean to form floating rafts of pumice. Such pumice rafts have been seen after historical eruptions such as Krakatoa erupting in 1883, spewing pumice into the ocean and clogging harbors in Indonesia.
 

Figure 1: (Top) A lithograph drawing of Krakatoa's 1883 eruption that produced pumice rafts of several squared kilometers, as seen in the lower image (Bottom), taken by NOAA, from a more recent underwater eruption.

Until recently, it was thought that only shallow water or near-shore volcanic eruptions formed these floating rafts of pumice. The 2012 eruption at the Havre Seamount, a deep submarine volcano located in the southwest Pacific (near New Zealand), changed that understanding when it erupted enough pumice to form a floating pumice raft covering over 400 square kilometers of water.

Figure 2: A NASA/Simmon image of the pumice raft from the 2012 Havre Seamount eruption.

These floating pumice rafts can cause several problems for ships at sea. Floating several hundreds of kilometers away from the location of the eruption, the pumice threaten ships that intake water as part of the ships motion, possibly causing engines to stall.
 
A recent research group at the University of Southampton in the United Kingdom has developed a computer model that can track the movement of these pumice rafts, providing alerts and warnings to ship captains in the vicinity. It is hoped that the model will be applied in the similar manner that Volcanic Ash Advisory Centers track ash plumes from volcanic eruptions for air traffic safety.

Figure 3: An example of the model used to track the movement of the pumice rafts for hazard management purposes. 

This research group is hoping that their model can be developed to trace where the pumice rafts originated from, providing a better understanding of the creation of these rafts and the materials that the pumice is composed of. This has several implications in the geologic field in understanding what eruption products float and what eventually sinks, becoming part of the rock record.