Trapped bubbles keep pumice afloat and gas diffusion makes pumice sink

Journal Article · Wed Dec 21 00:00:00 EST 2016 · Earth and Planetary Science Letters

DOI:https://doi.org/10.1016/j.epsl.2016.11.055· OSTI ID:1479300

Fauria, Kristen E. ^[1]; Manga, Michael ^[1]; Wei, Zihan ^[2]

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1. University of California, Berkeley, CA (United States)
2. University of California, Berkeley, CA (United States); Peking University, Beijing (China)

Pumice can float on water for months to years – long enough for pumice to travel across oceans and facilitate the spread of species. Long-lived pumice floatation is unexpected, however, because pumice pores are highly connected and water wets volcanic glass. As a result, observations of long floating times have not been reconciled with predictions of rapid sinking. We propose a mechanism to resolve this paradox – the trapping of gas bubbles by water within the pumice. Gas trapping refers to the isolation of gas by water within pore throats such that the gas becomes disconnected from the atmosphere and unable to escape. We use X-ray microtomography to image partially saturated pumice and demonstrate that non-condensable gas trapping occurs in both ambient temperature and hot (500 °C) pumice. Furthermore, we show that the size distribution of trapped gas clusters matches predictions of percolation theory. Finally, we propose that diffusion of trapped gas determines pumice floatation time. Experimental measurements of pumice floatation support a diffusion control on pumice buoyancy and we find that floatation time τ scales as τ∝L2Dθ2 where L is the characteristic length of pumice, D is the gas–water diffusion coefficient, and θ is pumice water saturation. A mechanistic understanding of pumice floatation is a step towards understanding how pumice is partitioned into floating and sinking components and provides an estimate for the lifetime of pumice rafts in the ocean.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); University of California, Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1479300
• Journal Information: Earth and Planetary Science Letters, Journal Name: Earth and Planetary Science Letters Journal Issue: C Vol. 460; ISSN 0012-821X
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
buoyancy
capillary processes
percolation theory
submarine volcanism
x-ray microtomography