Plant Cuttings

Volcanoes, unexpected spreaders of plant life

Inspiration for these plant-based blog items can come from the most unlikely of sources. This one in particular had a strange (and offbeat…) beginning. Whilst researching an item about the use of drones in botany I noticed a link on that ‘page’ to another item entitled “How X-Rays Helped to Solve Mystery of Floating Rocks in the Ocean” in that news outlet’s Strange and Offbeat column. Strange? Certainly. Offbeat? Yes. Was I intrigued? Oh yes! And the seed for this blog item was truly sown. The floating rocks of interest are made of pumice, porous material that is ejected from volcanoes when they erupt. Being full of holes, many of which open at the surface, you might expect that pumice would fill with water (rather like a bath sponge), and sink. Yet, bizarrely, pumice is actually buoyant. So buoyant in fact that it can stay afloat on the surface of water bodies – such as lakes and oceans – for years, and travel great distances.

Pumice on 20 dollars
Image: Robert DuHamel/Wikimedia.

Being a simple soul, floating rocks sounds a bit like magic to me (in much the same category as metal boats that can move across the ocean without sinking, and aeroplanes made of metal that can stay in the air…). Although it’s not magic (it’s science), how this feat was possible was certainly a mystery. But, it seems to have been solved by Kristen Fauria et al.. Using sophisticated X-ray imaging techniques they show that the pores and capillary channels in the pumice are small enough that they trap gas within the body of the rock as the water infiltrates the pores. This trapped gas is sufficient to cause the observed buoyancy. In time diffusion of gas out of the pumice permits its replacement by water which eventually causes the pumice to sink.

All fascinating stuff, Mr Cuttings, but geology isn’t botany; where’s the plant biology (or any biology come to that)?? That comes from Scott Bryan et al. and their study of “Long-Distance Dispersal by Pumice Rafting”. Pumice rafting is the phenomenon whereby living organisms can be transported across the surface of oceans on or in floating pumice. In particular they studied pumice rafting of >80 spp. in a journey of > 5000 km over 7-8 months following the eruption of Tonga’s Home Reef Volcano in 2006. Many animals were recorded, but there’s also plenty of botanical interest with calcareous algae, macroalgae (seaweeds), and cyanobacteria amongst the pumice-associated ocean wanderers.

Whilst it’s probably impossible to know where on that journey the various passengers came on board, the prospect of long-distance biotic translocation is a real probability. They say that every cloud has a silver lining. That seems true for pumice. On the one hand the floating pumice that results from the erupted volcanic ‘cloud’ (and the uncharted  islands that may result from the associated volcanism!) can present a hazard to human users of the ocean. But, on the other, it also presents an opportunity for life forms to colonise new areas and extend species’ ranges – if the pumice reaches land before sinking. Whether that ‘alien invasion’ is good or bad depends upon many factors, but who can now look at pumice as just some sort of inert geological oddity? Pumice, a most unlikely oceanic life-preserver?

References

Fauria, K. E., Manga, M., & Wei, Z. (2017). Trapped bubbles keep pumice afloat and gas diffusion makes pumice sink. Earth and Planetary Science Letters, 460, 50–59. https://doi.org/10.1016/j.epsl.2016.11.055

Bryan, S. E., Cook, A. G., Evans, J. P., Hebden, K., Hurrey, L., Colls, P., … Firn, J. (2012). Rapid, Long-Distance Dispersal by Pumice Rafting. PLoS ONE, 7(7), e40583. https://doi.org/10.1371/journal.pone.0040583

About the author

Nigel Chaffey

Nigel is a botanist and full-time academic at Bath Spa University (Bath, near Bristol, UK). As News Editor for the Annals of Botany he contributes the monthly Plant Cuttings column to that august international botanical organ. His main goal is to inform (hopefully, in an educational, and entertaining way...) about plants and plant-people interactions.

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