An underwater eruption
In mid-August 2021, satellite and aircraft-based observations detected an eruption of the Fukutoku-Okanoba volcano in the Ogasawara island chain around 1,300 kilometers south of Tokyo. This underwater volcano has erupted many times since it was first observed in 1904, however, this was a particularly strong eruption resulting in an ash and steam cloud rising high into the atmosphere. Satellite imagery soon showed that many square kilometers of the waters around the volcano were covered by a mat or “raft” of floating brown pumice stones as seen in this NASA image of the ocean surface on August 17th.
This phenomenon can also be observed after other submarine eruptions and is caused by a rapid cooling of gas-saturated magma, which results in the creation of stones filled with air bubbles. The trapped air bubbles reduce the density of the rock to a value small enough to float on the ocean surface for many months before finally becoming water-logged and sinking.
Months later – a severe impact on far away coasts
By mid-October 2021 the inhabitants of several islands in Okinawa noticed that their coastline was being impacted by a sudden arrival of masses of floating rock and by November 2nd the Okinawa Governor asked the Japanese military for help clearing port facilities. The impacts in Okinawa have been extensive and prolonged. The floating rocks affected recreational use of the beaches, fish populations and port and fishing operations, as well as the harvesting of mozuku, an edible seaweed that is extensively cultivated there. Severe effects persisted through the winter and into spring. The photo below shows a scene on the coast of Okinawa in January 2022.
An Okinawan artist has documented the appearance and behaviour of the rock covering the beaches in a dramatic video. Other impressive videos taken from above and below the ocean surface appeared in early media reports. The stones were roughly the size of golf balls and in the ocean close to the beaches, they formed a layer roughly 30 cm thick.
Detective work to uncover origins of pumice rafts
Although some undersea volcanic eruptions, such as the August 2021 Fukutoku-Okanoba event, are immediately evident, submarine eruptions can occur without being noticed. In some such cases, the resulting pumice rafts may be discovered by accident much later and far from the source. Indeed the origin of eruptions can in some cases remain unknown. As recently as 2016 a pumice raft was seen in the South Pacific without a definitively known source. In August 2012 a large pumice raft found centered about 1000 km northeast of Auckland near the Kermadec Islands caught the New Zealand Navy by surprise. There was no obvious source for the pumice and scientists worked to infer the possible origin. Volcanologists examined satellite imagery to identify the origin of the pumice with a swarm of earthquakes that had occurred over a 12-hour period on July 17th at the 5 km wide undersea Havre caldera, which up to that point had not been known as an active volcano.
For the kind of detective work that can identify the source of the pumice rafts it is valuable to have data for the detailed ocean surface currents as they evolve on a daily basis. If the currents can be forecasted for some time into the future, they can also be used to make predictions of the path of the pumice stones.
Satellites enabled novel observations of the global ocean
Recent decades have seen the deployment of observing systems capable of detailed monitoring of the global ocean. There is no satellite measurement technique that can directly determine the surface currents, however beginning in 1992 the “TOPEX/Poseidon” satellite and its successor satellites have monitored the detailed topography of the ocean surface. Ocean currents tend to swirl around the highs and lows of ocean topography rather like the wind around the high and low pressure centres on a weather map.
In another key development, observational estimates of the surface winds over the ocean were first obtained from the NASA Scatterometer instrument “NSCAT”, which launched on a satellite in 1996. By 2010 daily global maps of the surface currents at fine spatial resolution were being produced based on the satellite measured ocean topography and surface wind together with observations of the motion of satellite-tracked drifting buoys.
Computer prediction of the pumice movements
A further advance can be made by using the estimates of ocean conditions based on observations as a starting point for a computer model prediction of the future in a process very analogous to the now familiar computer-modeled weather predictions. The Application Laboratory of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has in recent years produced daily maps of the ocean currents around Japan as well as forecasts of the ocean currents for several weeks.
In mid-October, when the pumice first began affecting Okinawa, JAMSTEC scientist Dr. Toru Miyama applied the ocean current data to determine how pumice stones generated at the Fukutoku-Okanoba volcano would move. To create the figure below, Miyama modeled the release of floating stones on August 14th that were initially assumed to be close to the eruption site marked by the red star. The blue areas in the figure below on the left show the predicted location of pumice stones on October 15th, while the right panel shows Miyama’s prediction for four weeks later (November 11).
The mid-October results showed that the pumice had largely moved westward toward Okinawa, thus confirming the suspected origin of the pumice as it first washed up on Okinawa’s shores. The prediction made for the subsequent four weeks showed that the much of the pumice would be caught in the Kuroshio Current (shown in the figure as the thick gray curve ending in an arrow at the northern end) and transported northward towards the east coast of the main Japanese island of Honshu, and indeed quite close to Tokyo. Strikingly the arrival of pumice was later observed near greater Tokyo starting in mid-November.
This remarkably successful forecast shows the power of the techniques developed over recent years to map and forecast the surface currents of the ocean. These scientific advances and the resulting capability to diagnose and forecast motion of floating objects has found very diverse and interesting practical applications over the last decade such as (i) projecting the fate of the debris washed into the ocean after the 2011 great East Japan tsunami, (ii) aiding the search for aircraft debris from the presumed oceanic crash of Malaysian Airlines flight MH370, which mysteriously disappeared in 2014, and even (iii) helping to substantiate the remarkable story told by a Mexican fisherman in 2012 who claimed to have survived 438 days adrift at sea.
