Thursday, 29 May 2014

Pumice rafts from the July 2012 Havre Seamount eruption.

Pumice forms when hot lava from submarine volcanic eruptions encounters seawater and cools rapidly, simultaneously crystalizing and degassing to form a lightweight volcanic rock with many gas filled vesicles (bubbles) within it, which often floats on the sea surface. Big submarine eruptions can produce large volumes of pumice, forming rafts of pumice that cover hundreds of square kilometres, and drift on the ocean surface for months before dissipating or washing ashore. On 17 July 2012 the Havre Seamount, located at a depth of more than 700 m below sea-level in the Kermadec Arc to the northeast of New Zealand, underwent a dramatic eruption, producing a pumice raft covering over 400 km² in less than 24 hours.

In a paper published in the journal Nature Communications on 22 April 2014, a team of scientists led by Martin Jutzeler of the National Oceanography Centre at the University of Southampton and the Department of Geology at the University of Otago, describe the results of attempts to track this pumice raft using the Moderate-Resolution Imaging Spectroradiometer (MODIS) on the Terra (EOS AM) and Aqua (EOS PM) satellites, as well as to predict the movements of the raft using the NEMO ocean modelling framework, with a view to predicting the movements of future pumice rafts.

Prior to the event of satellite imagery, it was not possible to detect the sources of pumice rafts if they were deeper than about 100 m. The 2012 Havre Seamount eruption was the first deep-water eruption observed by satellite and monitored by an international network of seismometers simultaneously; demonstrating unambiguously that pumice rafts could come from such a source.

An Earthquake swarm with 18 quakes in excess of Magnitude 3.5 was monitored over a 12.5 hour period on 17 July 2012, while images from MODIS showed an atmospheric plume above the seamount as well as a thermal hotspot beneath the sea at its location. The plume is thought to have comprised steam only, and not to have been directly related to the pumice raft. The pumice raft drifted to the northwest in the days following the eruption, drifting far enough from the source that it could be determined no further material was being added; no further plume, raft or discoloured water was seen around Havre for at least six months after the eruption. 

A bathymetric study of the seamount in October 2012 revealed the development of a new 250 m high cone on the southeast rim of the caldera (still more than 700 m below sea-level), as well as a distinct bulge on the crater wall around 800 m below sea-level. Dredges taken at the time found fresh pumice on the seafloor.

Using Modis imigary, Jutzeler et al. were able to follow the progress of the pumice raft from 18 July to 17 November 2012; with additional observations of smaller rafts derived from the original pumice till 22 December. 

Sequential motion of pumice rafts from MODIS images in the first 3 weeks after the eruption. Images taken from Terra and Aqua satellites at 250m resolution; raft colours refer to various dates; scale bar, 40 km. Only a few rafts were hidden by cloud cover. Insert shows regional map; islands are in grey; black lines for 2,000 meters below sea-level contour; dashed rectangle shows location of main map. Jutzeler et al. (2014).


The original raft covered about 400 km², but spread out to cover 120 000-270 000 km² after a month. Assuming that the raft had an average thickness of 50-70 cm, this translates to about 0.03-0.05 km³ of dense rock (translate to better units). Though it is likely that many pumice clasts became waterlogged and sank within 24 hours of the initial eruption, and that the MODIS system was unable to detect all of the floating material, so Jutzeler et al. estimate that Havre probably initially produced about 1 km³of pumice rock, equivalent to 0.15-0.25 km³ of dense rock.

Sequential motion of pumice rafts from MODIS images. Grey polygons show broad areas where pumice clasts are dispersed relative to the Havre volcano. Vectors refer to drift directions to match next raft positions; backward vector on 10 September is due to cloud cover on next date. Main nearby islands of Raoul, Tongatapu and Minerva reef mentioned as landmarks for reference; scale bar 200 km. Jutzeler et al. (2014).

The initial raft remained coherent for the first week then split into a number of smaller segments, primarily elongated ribbons of material, many of which exceeded 100 km in length. Observations of these ribbons by sailing crews showed them to be more extensive than detected by MODIS, for example MODIS images from 5 October 2012 show ribbons extending to within 800 km of  Tongatapu Island from the southwest, while boat crews recorded encountering the rafts 230 km southwest of the island.

The break-up of the main raft and formation of ribbons of pumice appeared to be driven by sorting of the material by wind and ocean currents. The wind seldom blows in the same direction as ocean currents, so the proportion of a pumice clast above the water, which is exposed to the wind, will be pushed in a different direction to the proportion below the water, which is exposed to ocean currents. Therefore the resultant net direction of the clast’s movement will be driven by a ratio between the surface area of the clast exposed to the wind and the surface area of the clast exposed to ocean currents. Since this ratio is determined by a clast’s size and shape, over time the action of the wind and ocean currents will separate the pumice into discreet units with similar sizes and shapes. In practice the ribbons moved many times faster than the thicker mats, suggesting that the movement of these was largely determined by the wind. Examination of material in ribbons by boat crews revealed that each was indeed made up largely of clasts of similar size.

MODIS satellite images of the pumice raft from the July 2012 Havre submarine eruption. (a) Syn-eruptive plume (white trail) and pumice raft (yellow) above the submarine vent (arrow) with initial northwest drift (19 July; 01:26 UTC). Note the discoloured water adjacent to the raft (light blue). (b) The pumice raft is very elongated and swirls (25 July; 00:50 UTC). Note the persistent discoloured water adjacent to the raft. (c) Effect of wind shear and/or oceanic surface currents on elongated bands of pumice rafts. The pumice rafts are dispersed into smaller ribbons of pumice clasts; arrows show a deduced southeast-trending wind direction (8 August; 22:08 UTC). (d) Pumice raft is widely dispersed and forms very complex dispersal patterns (19 August; 00:44 UTC). Satellite resolution is 250 m; true North is up the page. The original images from MODIS46 were filtered (vibrance and saturation) to increase contrasts; scale bars are 20 km. Jutzeler et al. (2014).

While not as obviously hazardous as pyroclastic flows or lava bombs, pumice rafts can still prove disruptive to human activity and at times even dangerous. Rafts can disrupt fisheries (and make fishing impossible) for long periods of time, as well as blocking harbours or other waterways, and physically abrading the sides of ships or other objects in the water. More seriously pumice entering the water intake system of a ship is likely to block up this system causing engines to fail. Since almost all modern ships are reliant on water intake systems for engine cooling, and since failure of engines typically results in failure of electrical systems on a ship, leaving it without navigation or communication systems, ships are forced to take lengthy detours to avoid pumice rafts, often leading to significant increases in both the duration and expense of maritime journeys.

In order to better predict the movements of future pumice rafts, Jutzeler et al. attempted to build a software model of the movements of the Havre pumice using data on past currents and winds from the NEMO ocean modelling framework. This contained data for the affected area from 1988-2010, requiring pumice movements to be predicted from composite wind and current trajectories (something that would be necessary when trying to predict the future movements of a pumice raft). The model proved to be largely accurate, although it failed to predict the extent to which the pumice drifted to the north and northeast during the first three months after the event. The model extended to 180 days after the eruptive event, further than the MODIS system was able to monitor the dissipating pumice rafts, though this appeared to be fairly accurate; it was possible to monitor the movement of the rafts from the surface to some extent after the MODIS system had lost track of them, particularly by records of strandings of pumice on beaches.

Comparison between MODIS and NEMO particle trajectory data. The comparison shows good fit between observations and simulations. The dispersal of pumice clasts using ARIANE with output from the NEMO 1/12 hindcast simulation shows the location of 232,400 particles after (a) 30, (b) 60, (c) 90, (d) 120 and (e) 180 days, respectively, in the five panels. Pink dots are past tracks, black dots are last day of simulated drifting. MODIS imagery shows past tracks (light blue polygons with dark blue contour) and last day (medium blue polygons) of data. The Havre caldera volcano (yellow star) and Tongatapu island are shown. For each panel, rose diagrams correspond to the dispersal simulated with NEMO. The radial axis indicates the number of particles (out of 23 X 2,400) per 18˚ sector. Jutzeler et al. (2014).

Pumice strandings associated with the July 2012 Havre Seamount eruption were recorded in the Bay of Plenty on North Island, New Zealand, in April 2013 (roughly 260 days after the eruption), the Great Barrier Reef in September-October 2013 (roughly 440 days after the eruption) and subsequently on the beaches of New South Wales.

See also…


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