Hunga Tonga-Hunga Ha’apai 2015 eruption

Stories about volcanoes are always a crowd pleaser, no more so than the growth of a new volcanic island, something that is not witnessed every day, but is relatively common. Recently, many of the UK media outlets have been running stories about the growth of the new island volcano Hunga Tonga-Hunga Ha’apai, in the Pacific, such as The Mail and the BBC. However, the coverage is rarely based on science, and presents mainly photos, eye witness reports and information on diameters of the new edifice. This is valuable information for scientists, but currently, no in-depth scientific reports on the eruption are publically available.

The eruption has attracted a lot of media interest in the UK in early March, in BBC reports and some national newspapers. These were several weeks after the eruption ceased and the disruption of international and local air travel in the region. The story seemed to break in the UK only once sightseers started to visit the island, not at the time when the Tongan Government were actively reporting on it and issuing photos of the eruption.

The media have been reporting comments from several people who ventured (not to be recommended) onto the island. The sightseers have commented on how “hot” the surface was, which is not really surprising, as the magma (molten rock) which built this new island would most likely have been 900-1200 °C at the time of the eruption (although no scientific data is available to confirm this). It takes time for the rock to cool down to atmospheric temperature, especially when forming an island like structure, insulating the underlying material from the sea and atmosphere and trapping the heat.

During the eruption, the Natural Resources Division of the Ministry of Lands and Natural Resources of Tonga reported ash plumes up to 7-10 km height and rocks and ash being thrown up to 300 m into the air. The current eruptive phase ceased at the end of January 2015. The new island  is currently 120 metres in height, and 1.5 x 2.0 km wide. The dark colour of the erupted material suggests its composition is likely to be high in magnesium and iron.

The question the media is now asking is: how long will the island exist for and how unstable is it? Currently, there is very little scientific information available on the exact composition and nature of the erupted products. During the eruption, both steam and sulphuric gas were reported by the Natural Resources Division of the Ministry of Lands and Natural Resources of Tonga. However, as the photos of the eruption indicates it was highly explosive (due to water interaction) and fragmented in nature (ash), it is highly likely that a large proportion of the deposits are relatively fine grained and vesicular (contains bubbles). Thus, the resulting edifice is probably a relatively unstable pile of volcanic debris. Consequently, with continued interaction with the sea and weather, erosion will cause the removal of new unconsolidated material back into the sea. It is likely the new island may disappear at some point, but just how much material will disappear and how fast is harder to estimate.

The bottom line

The Earth surface is covered with tectonic plates, the hard rigid outer crust of our planet. Numerous plates float over its surface and interact, creating for example mountain chains and volcanic arcs. It is at the boundary of these plates that the majority of Earth volcanic eruptions occur. Plate boundaries are present around the Pacific margins, where one of the plates is sinking (subducting) beneath the other. This releases material from the down going plate, which triggers the mantle to melt and is ultimately the source of the melt which fuel the subsequent volcanic eruptions on the overlying plate, forming volcanic arcs. Hence, the region around the pacific is known as the ‘Ring of Fire’ and Tonga is one such place.

Since 24^th December 2014, there have been reports of volcanic eruptions at Hunga Tonga-Hunga Ha’apai (Smithsonian Volcano Database), located approximately 45 km North West of the main island of Tonga. The last reported eruption of this volcano was in 2009. Since December, continued steam plumes were reported by fisherman and pilots (Smithsonian Volcanic Database). On 6^th January, the Wellington Volcanic Ash Advisory Centre (VAAC), the organisation responsible for monitoring of volcanic ash in this region and reporting this to the aviation authorities, advised of an ash plume at an altitude of 3 km above Hunga Tonga-Hunga Ha’apai. Over the next few days, further reports of volcanic ash plumes to greater heights appeared, with local aviation routes disrupted.

Continents we live on were formed and evolved to the landmasses we know today. The material that created the crust was ultimately derived from the mantle of the Earth and brought to the surface through volcanic processes, although these may have been modified substantially since then. It is thought that nowadays new continental crust is generated in volcanic arcs, where subduction occurs. Should we thus be surprised when new volcanic islands are born? Not really, we are just witnessing the great natural cycle of the World we live.