Why pumice floats

October 2018

Floating islands are a staple of maritime fantasies, alongside mermaids, and Krakens. But unlike the mermaids and Krakens, the islands actually exist. In 2012, NASA satellites photographed one in the Pacific Ocean. And this was no tiny islet either. The New Zealand Naval vessel sent to investigate reported that it was some 300 x 30 nautical miles in size. One nautical mile is 1.852km, so the island was 555 kilometers long. That’s half the length of the North Island of New Zealand and bigger than the state of Israel.

There were two reasons why this massive lump of stone was able to float on the ocean surface. Firstly it was very thin - on average two feet. Secondly the rock was pumice created by the eruption of an underwater volcano known as the Havre Seamount. Though mega-rafts like this are very rare, pumice bobbing up from undersea eruptions is quite common. Dispersed by wind and current, this pumice can stay afloat for many months or years.

Very often floating pumice gives the location of the underwater volcano it originated from. Because underwater volcanoes are difficult to discover, floating pumice has been very helpful in finding them. But the pumice itself was something of a mystery. Why can pumice rocks float for months or years, and why do they eventually sink? The answer to this question is found in the way that pumice is formed.

Pumice is an volcanic rock produced by gas-rich magma. The process is often compared to what happens when you open a bottle of carbonated water. Like a closed bottle of water containing CO2, some magmas contain a large percentage of gas in liquid form stored under pressure. As magma breaks through the Earth’s surface and the pressure suddenly drops, it is as though the cap has come off the bottle. Gas trapped in the magma violently escapes, taking with it a molten froth of rock.This molten froth cools down rapidly in seawater, and it traps some of the gas inside. This porous material is pumice. The pore spaces (known as vesicles) are actually the bubbles of gas.

The abundance of vesicles in pumice and the thin walls between them give the rock a very low specific gravity. In fact it’s lower than the specific gravity of water, which is why very large pumice rocks can float on water. But that does not answer the question why those open pores do not immediately fill with water and cause the rock to sink. And if the rocks do not sink at once, why do they sink eventually?

Recent X-ray studies carried out at Berkeley Lab have finally produced some answers. "It was originally thought that the pumice porosity is essentially sealed, "Fauria (the lead author of the recent publication - see reference blow), said, "like a corked bottle floating in the sea. But pumice pores are actually largely open and connected - more like an uncorked bottle."

What is even more interesting is that in the laboratory the pumice stones would bob up and down, sinking during the evening and resurfacing during the day. To find out what was going on inside the pumice stones, the team used wax to coat bits of water-exposed pumice samples.

They then used an X-ray imaging technique known as microtomography to study concentrations of water and gas in the pores. X-ray microtomography uses x-rays to create a cross-sections of a physical object which can be used to make 3D models. The 'Micro' prefix is because the size of the cross-section is in the range of micrometers (1 micrometer = 1 millionth of a meter) - i.e. very thin.

Researchers found that the gas-trapping processes that are in play in the pumice stones relate to "surface tension," a chemical interaction between the water's surface and the air above it that acts like a thin skin. This is the same effect that allows insects and very small animals to walk on water. Many pores in the pumice stone are very small, about the width of a human hair. Because there are many of them the surface tension is high.

The scientists were able to predict how long a pumice stone would stay afloat by calculations based on the rock size and the diffusion of trapped gas.

Michael Manga, a staff scientist in Berkeley Lab's Energy Geosciences Division and a professor in the Department of Earth and Planetary Science at UC Berkeley who participated in the study, explained, "There are two different processes: one that lets pumice float and one that makes it sink". The X-ray studies helped to quantify these processes for the first time. The study showed that previous estimates for flotation time were in some cases off by several orders of magnitude.

"Kristen (referring to Kristen Fauria) had the idea that in hindsight is obvious," Manga said, "that water is filling up only some of the pore space." The water surrounds and traps gases in the pumice, forming bubbles that make the stones buoyant. Surface tension serves to keep these bubbles locked inside for prolonged periods. But there is also a slow but continuous diffusion of gas from the pores which allows more water to enter.

The bobbing observed in laboratory experiments of pumice floatation is explained by trapped gas expanding during the heat of day. This causes the stones to temporarily float until the temperature drops.

So in a nutshell - gas trapping explains why pumice floats on water and the outward diffusion of the trapped gas eventually causes pumice to sink. In each pumice stone there are parts which are floating and parts which are sinking. Once the stone becomes sufficiently saturated with water and its specific gravity becomes higher than water the stone will sink.

This was the fate of the massive pumice raft in the Pacific, but such rafts have had a significant effect on life on earth. Some of these rafts have been tracked as they float 5,000 km or more, and in the course of their journey they are colonized by dozens of species of marine life (and, sometimes according to legend, a few shipwrecked sailors also). While some rafts go down in bubbles in mid-ocean, others wash against reefs or distant shores, depositing their living cargo in a new habitat.

Journal Reference:

Kristen E. Fauria, Michael Manga, Zihan Wei. Trapped bubbles keep pumice afloat and gas diffusion makes pumice sink. Earth and Planetary Science Letters (2017) 460, pp50.https://www.sciencedirect.com/science/article/pii/S0012821X16306896?via%3Dihub