Tag Archives: pumice

Pumice: the floaty, foamy, fragile stone and its uses

Often found in cosmetics shops and beauty tip articles, this funnily-named rock has a very violent origin.

Image credits Kai Schreiber / Flickr.

Pumice is a lightly colored rock with a very foamy structure. It’s so porous, in fact, that most specimens can float on water (until they eventually become waterlogged and sink). The secret to its structure lies in the birth of pumice: violent volcanic eruptions.

Out with a bang!

Some volcanoes pop off quite violently. It happens to those whose magma is very thick, viscous, and has a high content of volatiles (mostly water and some carbon dioxide). During such an explosive eruption, highly-pressurized magma inside the volcano is ejected to the surface or underwater. Here, it rapidly cools and depressurizes. The whole process is a lot like throwing a mind in a bottle of coke, and at this stage, the lava looks a lot like the foam. The volatiles inside it bubble up as the lava cools and hardens, creating pumice.

So you can think of pumice as being frozen foam du lava. Its very name shares a root with the Latin word ‘spumam’ (‘foam’).

Pumice stone is usually created by underwater volcanoes. Particularly large eruptions can spawn whole islands or rafts of the stuff, but even humbler events can generate enough material to threaten cargo ships. Under certain conditions it can also form in subaerial (i.e. not-underwater) settings. If the source magma has a high level of volatile materials, a finer-grained variety known as pumicite can form instead. Less viscous magmas, in which gases can form bubbles more readily, create denser (and non-floaty) scoria. However, if there’s no eruption, there won’t be any pumice — it’s an extrusive ‘igneous’ mineraloid, so all deposits are centered around areas of current or past volcanic activity.

Image credits Jon Zander / Wikimedia.

However, if there’s no eruption, there won’t be any pumice — it’s an extrusive ‘igneous’ mineraloid — so all deposits are centered around areas of current or past volcanic activity.

Pumice stone isn’t exactly a rock in the strictest geological sense of the word because it has no internal crystalline structure. It cools too quickly for its atoms to arrange themselves the way they’d like to, so it has an overall amorphous (disorganized), glass-like structure. It can contain crystals, but these will be embedded in the amorphous matrix of the pumice — its exact make-up depends on the nature of its source magma. Instead, pumice is considered to be a mineraloid or a type of volcanic glass.

So let’s see what it’s good for.


Pumice soap.
Image Patrick Reijnders / Flickr.

It’s probably best-known for its cosmetic uses. Glass is quite hard, and volcanic glass isn’t any different, so it’s a very good abrasive.

The simplest way to employ a piece of raw pumice stone is to soften an area of calloused skin with warm water and then gently rub the stone on it to scrape it off. In our gentler, modern times, powdered pumice stone is often added to soaps or body gels to improve their cleaning power, or in creams and beauty products meant to exfoliate the skin. It’s a completely natural, generally chemically-inert, and has a neutral PH; it’s also more eco-friendly than synthetic alternatives such as plastic microbeads, making it quite popular in the public eye.

Cleaning, abrasives

Household cleaning products like scouring pastes and powders can also include pumice, which helps them better scrape off hardened, encrusted nastiness.

Industrial settings rely on pumice as mild abrasives in tasks where a particular surface needs scrubbing with a gentle touch. Pumice and its powders see use in glass polishing, the cleaning and texturing of electronic circuit boards, the cleaning of lithographic plates, the removal of surface oxide layers in metal surfaces meant for electroplating, the buffing of leather and fine woods, and as a tumbling agent for metal and plastic parts. The crumbly nature of pumice together with its high hardness means it can be processed without it losing effectiveness on tough surfaces.

In dentistry

Because it’s mildly abrasive and non-reactive, pumice powder is mixed into many whitening toothpastes and teeth polish products. This tradition runs back in excess of 4,000 years, with ancient Egyptians first employing the mineraloid in this role. Today, the powder sees use in dentistry as a cleaning and polishing agent, and for its antibacterial properties. The powder is also used to prepare teeth for resin fillings.

For water filtration and retention

Zoomed-in structure of pumice.
Image via Pxfuel.

Pumice can act as a pretty good filtration method due to its porous internal structure. Some advantages of pumice as a filtration medium are its effectiveness in removing particles, low filtration bed expansion, and relatively low cost of maintenance or replacement of the pumice. Being non-toxic and inert means it doesn’t dissolve or change the taste of whatever liquid it’s filtering.

The mineraloid is also quite effective at scrubbing biological material (such as hydrogen sulfide, mercaptans, and other volatile organics) from wastewater. The quality of the pumice, its production, processing, and transport has a big impact on the quality of the final filters, however. If you do plan to build a pumice filter, make sure to wash the material thoroughly beforehand.

Mixing in pumice with soils can help enhance its natural water-filtration abilities, and this approach has been used in low-impact ecological projects to prevent contamination and runoff from entering streams, lakes, or the water table.

Pumice stone can also be used as a substrate or mixed in with soils for plants as it can store moisture when the plants are overwatered and gradually release it as the soil dries up. Furthermore, its porous nature means it improves water and gas circulation through the soil, giving plants easier access to the nutrients they need — this is especially important in compact soils with lots of clay or for hydroponics. Golf courses often use pumice stone to maintain grass cover and the shape of the landscape despite the heavy traffic they see.

As a chemical and mechanical absorbent

The structure of pumice makes it very good at absorbing liquids, kind of like a mineral sponge. With adequate processing, this ability can increase quite dramatically. As such, pumice sees use in a wide range of tasks where liquids need to be contained, ranging from kitty litter to dry petroleum/chemical absorbents.

One of the more niche uses of the mineraloid is in bomb mitigation. Explosions cause damage through high-velocity shock waves that blast objects or structures with immense mechanical energy. In order to protect yourself from the explosion, you need to either stand far away (the energy dissipates with distance) or to use it up (bodies absorb the energy as they are deformed).

As we’ve seen earlier, pumice is quite hard, and it has a very complex internal structure. Breaking and squashing this structure into a compact block takes a lot of energy, as the pores inside the stone get compacted in sequence (i.e. the blast needs to deform the pumice throughout its volume). All in all, this makes pumice a very effective blast mitigation compound, and it sees use in bomb encasements to prevent damage from accidental detonations.

Paints, rubber, concrete

Pumice aggregate concrete.
Image in the public domain.

When mixed into paints and coatings as a filler, pumice helps them better retain color over time, makes them more resistant, and increases resistance to burnishing, staining, and scrubbing.

It can also be mixed into rubber. Pumice-reinforced tires have better performance of ice and snow, as the material helps increase friction; it’s useful in rubber abrasive wheels for the same reason. You’re overwhelmingly likely to have handled pumice rubber before — pencil erasers are made of this material as the mineral helps remove graphite from paper.

Finally, pumice aggregate concrete, which has been in use since the ancient Romans, offers much the same mechanical properties of regular concrete but with up to one-third reduction in weight (depending on the composition). Pumice concretes, however, have improved thermal, acoustic, and elastic properties compared to regular concrete.

I really like pumice because, on first sight, it doesn’t look like much. It’s brittle and untastefully light for a stone. But in a way, it’s a very good allegory for scientific knowledge: if you take the time to learn about it, even something that seems bland and uninteresting can whiten your teeth, clear up chemical spills, and stunt explosions all at the same time.

Volcano facts and other pieces of hot science

Volcanoes are some of the most amazing geological features but quite often, they’re misunderstood or not understood at all. Here we’ll get to know them a bit better, starting with the basic facts and the moving onto cool and surprising facts, and of course, continuing with everyone’s favorite (from a distance): eruptions.

Basic Volcano Facts

1. Volcanoes are ruptures in the Earth’s crust. Our planet’s crust is split into 17 major tectonic plates, and almost all volcanoes occur at the edges between these plates.

2. There are three types of volcanoes: stratovolcano (conical volcano consisting of layers of solid lava), cinder cone volcano (steep hill of tephra that accumulates around the vent) and shield volcano (built entirely or almost entirely from fluid lava vents).

3. Volcanoes can be active (with eruptions in the past 10,000 years), dormant (no eruptions in the past 10,000 years, but could wake up) and extinct (unlikely to ever erupt again). However, active volcanoes can become dormant and extinct, and dormant volcanoes can wake up. Before 79 AD, Vesuvius was considered dormant and its eruption was catastrophic. Knowing whether a volcano is truly extinct is hard to determine.

4. We’re still not sure how many volcanoes there are in the world, but geologists identified about 1300 active volcanoes, not counting underwater volcanoes.

5. The biggest volcano on Earth is Hawaii’s Mauna Kea. At 33,500 feet (10,210 meters) it’s even taller than the Everest, but most of it is underwater, so its height relative to sea level is lower. However…

6. The tallest volcano in the solar system is on Mars. Olympus Mons on Mars is a shield volcano with a height of nearly 22 km (16 mi), almost three times higher than Mount Everest. It was able to grow this big because Mars doesn’t have active tectonic plates.

Volcanic eruption on Io. Image credits: NASA/JPL.

7. Earth isn’t the most active place in the solar system – Jupiter’s moon Io is the most volcanic body in the solar system. Astronomers recently witnessed two huge eruptions, possibly largest than any ever recorded on our planet.

8. The two most active volcanoes in the world are Etna in Italy and Hawaii’s Kilauea, depending on how you judge. Etna has been active in the past 3,500 years, but it’s still being used for agriculture because its slopes are so fertile. Kilauea has been in a state of constant eruption since 1993, and more than 90% of its surface is made from young lava.

Image via USGS.

9. Volcanoes can be scary, but supervolcanoes can be downright terrifying. St. Helens, one of the largest eruptions in history spewed up 0.25 cubic kilometers of volcanic material while the last known eruption from the Yellowstone caldera ejected 4000 times more – 1000 cubic kilometers.

Volcano Eruption Facts

10. There are three types of volcanic eruptions: magmatic eruptions (involving gas decompressions that propel the eruption forward), phreatic eruptions (superheating of steam via contact with magma, often with no ejected material) and phreatomagmatic eruptions (compression of gas within magma, the complete opposite of magmatic eruptions).

11. How dangerous are volcano eruptions? In 1815, the volcano Tambora exploded in Indonesia. All vegetation on the island was destroyed and projected into the sea. Uprooted trees mixed with pumice ash, washed into the sea and formed rafts up to 5 km (3.1 mi) across. The eruption sent material into the stratosphere, at an altitude of more than 43 km (27 mi). Over 10,000 people were killed directly by the eruption, but that was only the beginning.

The epic explosion of Mount Tambora in 1815 left a massive crater behind, 3.7 miles wide and 3,600 feet deep. (NASA)

Over 40,000 people were killed by hunger and disease in neighboring islands, and the effects were felt globally. The following year, 1816 was called “the year without a summer”, as snow fell in the summer in Boston and New York. Crops were destroyed, widespread famine was reported in Asia, Europe and the Americas. It’s impossible to estimate the total damage, but up to 100,000 people lost their lives following this eruption. A Massachusetts historian summed up the disaster: “Severe frosts occurred every month; June 7th and 8th snow fell, and it was so cold that crops were cut down, even freezing the roots.” Which leads us to another question:

12. What if a supervolcano erupts? Geologically, it won’t mean much for the planet. At a geological scale, supervolcanoes erupt all the time… but for humans, the effects would be ghastly. The tens or hundreds of thousands of lives lost will pale in comparison to what will happen. The world will be thrown into a nuclear-type winter, where food availability could become a luxury (because volcanic eruptions can block sunlight, lowering global temperatures). Famine and widespread disease will emerge for at least a couple of years, as no country has the food reserves to last that long; it’s extremely difficult to gauge the full impact such an eruption might have. However, you shouldn’t waste much sleep on this – it’s extremely unlikely for such an eruption to take place in the next few thousand of years.

13. The last known supervolcano eruption was the Toba eruption 74,000 years ago, when more than 2,500 cubic kilometers of magma were erupted. The largest eruption in recent human history was the 1815 eruption described above.

Chichester Canal circa 1828 by J. M. W. Turner. Image via Wikipedia.

14. But it’s not all bad. Volcanic eruptions make sunsets more vibrant. The eruptions spew hundreds, thousands or even millions of tons of dust and gaseous sulfur dioxide into the stratosphere. The finer dust particles remain in the atmosphere, sometimes for years, producing vivid sunsets and twilight effects.

In fact, a team of German and Greek researchers are studying paintings of sunsets after historical eruptions to discover clues about our atmosphere, and even study global warming.

Image via Wikipedia.


15. Some volcanic eruptions can create massive thunderstorms and we still don’t know exactly why. A study published in Science found that this phenomenon, also called dirty thunderstorms, appear because electrical charges are generated when rock fragments, ash, and ice particles in a volcanic plume collide and produce static charges, just as ice particles collide in regular thunderstorms.

More Volcano Facts

16. You need at least 3.35 kg of lava to boil a liter of water. Quora user Nissim Raj Angdembay calculated that for a lava of an average temperature of 950 °C, you need to use 3.35 kg of lava to boil a liter of water. Of course, this is only a theoretical calculation, and in practice, you’d need a bit more as some of the heat will be lost to the ambient.

17. There is one unique volcano, Ol Doinyo Lengai, that produces black carbonatic lava. It also isn’t as hot as other types of lava and it’s much less viscous – comparable to water.

Black carbonatic lava. Image via SwissEduc

18. The volcanic rock pumice is the only rock that can float in water. Pumice is an extrusive volcanic rock with a very high content of water and gases extruded quickly out of a volcano. The unusual foamy configuration makes it very light.

19. Volcanic energy can be harvested to warm water and even generate electric energy. Geothermal energy generates about 3% of renewable energy-based electricity.

20. The Maleo bird in the Indonesian island of Sulawesi uses volcanic heating to incubate its eggs.

21. When Paricutin in Mexico erupted from 1943-1952 (more on that a bit later), not a single person was killed by lava, rocks or flows, but three people were killed by lightning.

Paricutin. Image via Wikipedia.


22. Lava temperature varies between 700 to 1,200 °C (1,292 to 2,192 °F). Geologists do sometimes use a thermometer called a “thermocouple” to take a volcano’s temperature.

23. Lava chemistry greatly influences both the temperature and the type of eruption. Lava with greater silica content (more basic) tends to be hotter, more fluid, and erupt more “gently” – think of the Hawaiian lava flows. Lava with less silica (acidic) tends to have more explosive eruptions. They also form different types of rocks.

24. In 1943, a Mexican farmer named Dionisio Pulido started to notice something strange in his cornfield. It started as a slight depression, and soon started to fissure, eliminating volcanic material. By 1952, the volcano was already 424 meters high and damaged a 233 square km area with the ejection of stone, ash and lava. Three people were killed by lightning as described above. Today, Paricutin the volcano is 2,800 m (9,200 ft) high and is considered dormant.

GeoPicture of the Week: Kutkhiny Baty, The Weird Valley

Deep in the Kamceatka peninsula, there lies an eerie, unique valley – Kutkhiny Baty – The Weird Valley. This is how Kutkhiny Baty looks like from a helicopter:

The valley is made from whitish pumice stone. Pumice is a very light volcanic rock. Pumice is created when super-heated, highly pressurized rock is violently ejected from a volcano. The unusual foamy configuration of pumice happens because of simultaneous rapid cooling and rapid depressurization.

Image via Wildlife Russian Photos.

However, the native tribes also have a legend explaining how the valley was formed – they believed that the valley is a storage place for a god’s canoes. Kutkhu – the Lord and the Creator of Kamchatka – used to live on the Kurile Lake for some time, and used to go fishing on the lake and to the ocean in these canoes. Before leaving Kamchatka, Kutkhu put up his boats (“Baty”), and since then this place has been considered sacred among the locals.

Image source.

New type of volcanic eruption described

The general classification splits volcanic eruptions in two: explosive or effusive. An explosive eruption is, well, explosive and violent (think Mount Helens), while an effusive eruption is associated with lava flows (think Hawaii). However, in a new study conducted by New Zealand and UK researchers described another, new type of eruption.


Inside volcanoes, magma often has dissolved gases as a function of the very high pressures and chemistry of the magma. Much in the same way you open a carbonated drink – when you take the lid off, the bubbles burst out – when magma erupts as lava, the pressure is relieved and the gases exsolve (the opposite of dissolve). In explosive eruptions, this phenomena is so strong that it fragments the lava, violently ejecting it, along with anything caught along. When this happens, the ejected lava expands so quickly the resulting rock cools and degasses to form solidified pumice that can be sufficiently light to float on water.

After studying the Macauley volcano in the Pacific Ocean however, volcanologists found an entirely different story.

“By documenting the shape and density of bubbles in pumices generated by an underwater caldera volcano in the southwest Pacific Ocean – the Macauley volcano – we found large differences in the number and shape of “bubbles” in the same pebble-sized samples, different to anything previously documented,” said co-author Ian Wright, from U.K.’s National Oceanography Centre. “This range of bubble densities distinct in these pumice samples indicates that the lava erupting from the caldera was neither vigorous enough for an explosive eruption, nor gentle enough for an effusive flow,” Wright said in a statement.


Pumice floating

Pumice floating

In oceans, when pumice is located, it generally represents the spot of a volcanic eruption – an explosive eruption. The mechanism proposed for this special type of pumice though is more complicated; it suggests that rather exploding in the neck of the volcano, the formation and expansion of bubbles in the magma created a buoyant foam, rising to the seafloor and then buoyantly detaching itself from the volcano as molten pumice, but with cooler margins. The vesicles within the molten interior would have continued to expand as the pressure – this time from the weight of the seawater – reduced.

“These processes explain the unique bubble structure seen in the samples analysed, which could have only occurred with an intermediate eruption style and in an underwater setting,” said Professor Wright. “We conclude that the presence of widespread deposits of pumice on underwater volcanoes does not necessarily indicate large-scale explosive volcanism.”

The authors proposed that this type of eruption be named Tangaroan, the Maori god of the sea, and name of the research vessel used to collect the samples.