Pumice is an extremely low density, frothy, form-like or highly vesicular volcanic rock with a glass texture. This rock is highly porous, floats in water and has a rough surface.
This microvesicular, mostly pyroclastic rock forms when explosively ejected volatile rich magma rapidly cools, trapping some of the escaping gas bubbles. The rapid cooling prevents mineral crystallization, forming a glassy texture.
Some specimen can, however, form during effusive eruption or lava flow. Such form towards the outer edges where rapid cooling traps escaping gas bubbles.
Please note that pumice is a natural volcanic glass, not a mineral. Natural volcanic glasses are amorphous or uncrystallized solids.
The name pumice comes from pūmex or spūma, which means foam-like, describing this rock’s frothy or foam-like appearance.
Rocks resembling pumice are called pumiceous. Pumicite is powdered pumice with grains, flakes, threads, or shards less than 4 mm. It forms from highly explosive eruptions.
Pumice dust, on the other hand, has particles less than 0.25 mm, remains floating in the air for a long time, and also forms from extremely explosive eruptions.
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What are pumice rafts?
These are lapilli- to kilometer-sized pumice rocks floating in water. They form during submarine eruptions or eruptions that happen close to water bodies.
The largest pumice raft was spotted in Raoul Island in 2012, measuring 300 by 30 miles (482 by 48 km). It likely originated from the Havre seamount eruption.
Pumice rafts can travel hundreds of kilometers and remain floating for several decades. For example, they traveled hundreds of kilometers during the Home Reef volcano eruptions in 1944, 1979, and 2006, reaching Fiji. Krakatau in 1880 also produced rafts that remained in water for 20 years, reaching Africa.
Description, texture, color and properties
Pumice is a highly vesicular, frothy, or form-like volcanic rock with a glassy texture. Some specimens have an aphanitic texture but are rare.
It has a rough surface, is highly porous, and has a Mohs hardness scale of 5-5.5. Its density is very low, less than 1 g/cm3, and depends on porosity, vesicle nature, and packing.
This rock’s color will vary, depending on magma type. Pumice from silica-rich or acidic magma is often whitish, light gray, cream, and other lighter shades.
Specimens with intermediate composition are darker than silicic ones. Most are medium gray, dark gray, blue, greenish, or green-brown. The darkest varieties are those low in silica, which are usually black or dark dray.
Pumice rocks exist as minute particles, lapilli, or larger blocks and boulders. They have microscopic (only visible under a microscope) to macroscopic (visible without a microscope) vesicles. Most, however, have microvesicles, which give them the silky or fibrous appearance.
The shapes of vesicles in the rocks can be round, spheroidal, flattened, subparallel, tubular, or rod-like, and have fiber-like, thread-like, or thin glass filament walls.
Vesicle shape depends on vapor pressure during formation and whether there was flow or not. For instance, elongation, flattening, and distortion are signs of shear and stretch during movement before solidification.
Pumice rocks have a large surface area of 0.5 m2 per gram and have 70–80% vesicle packing. Rod-shaped vesicles can pack up 93% of the total volume, and fibrous ones can go above 93%. If vesicles are uniform, then the maximum possible packing is 74%.
Pumice chemical composition
Pumice is mostly an acidic or intermediate rock with 55-75% silica (SiO2). Specimens with a basic composition with 45–52% silica exist but are rare.
Considering magma composition, pumice mostly forms from silica-rich to intermediate magmas like rhyolitic, dacitic, pantellerite, tachylitic, or phonolitic. Those from low silica magmas like basalt is rare.
Because this rock forms from acidic or silica-rich, intermediate, and sometimes basic rocks, its chemical composition varies greatly.
Pumice rocks formed from silicic magmas will be high in light elements like silicon, oxygen, aluminum, potassium, and sodium.
Low-silica specimens are, on the other hand, relatively higher in the darker elements like iron, magnesium, and potassium. However, these specimens are lower in the lighter elements.
For instance, Degu (2021) estimates the percentage chemical composition of pumice to be 71.80% SiO2, Al2O3 12.35%, 1.93% Fe2O3, 1.98% CaO, 0.73% MgO, 0.14% SO3, 4.30% K2O, 4.47% Na2O, and others 4.05%. This composition represents a silica-rich or acidic variety.
Mineral composition
Pumice is felsic or intermediate, rarely mafic. It is mostly volcanic glass with a microvesicular texture, but it can have small crystals or microlites of feldspar or plagioclase, hornblende, augite, orthopyroxenes, and zircon, among others.
These small to microscopic crystals form earlier as magma rises at a time when cooling rates are slower.
Finally, pumice may have secondary minerals like calcite, zeolites, prehnite, or opal. These minerals form later when mineral-rich hydrothermal fluid seeps into vesicles, forming amygdules.
How does pumice form?
Pumice forms when volatile-rich, mostly silica-rich magmas like rhyolite, dacite, or andesite erupting on or near the Earth’s surface are quenched quickly, trapping some of the escaping gas bubbles. These trapped gas bubbles and the rapid cooling of magma form a very light, highly vesicular, frothy, or form-like glassy rock.
A drop in pressure as magma ascends towards the Earth’s surface causes the dissolved volatiles to exsolve, forming gas bubbles. A good analogy is how a soda or any carbonated drink fizzes when you open it.
If you suddenly freeze the fizzling soda, it will form a foam-like solid with numerous gas bubbles.
Explosive eruptions
Pumice rocks mostly form during explosive eruptions. These eruptions eject ash, lapilli, blocks, or bombs. The ejected bubbly or frothy magma cools quickly while in flight, trapping some of the escaping gas bubbles. This forms a frothy or highly vesiculated rock whose bubbles have thin, glassy walls.
Most explosive eruptions that form pumice are Plinian, like the Mt. St. Helen eruption on May 18, 1980, in the US, or the 1991 Mt. Pinatubo eruption in the Philippines.
Plinian eruptions are hazardous and often cause fatalities. Plinian eruptions in marine or submarine environments will affect aquatic life and form floating rafts that can disrupt shipping.
Hazards caused by Plinian eruptions are falling ash, blocks, bombs, ground-hugging, fast-moving pumice or pyroclastic flows, and nuées ardentes. Pumice flows are pyroclastic flows made of over 50% pumice lapilli and blocks.
Other types of explosives that can form these rocks are Pelean and Surtsey. For instance, a Pelean eruption like the one at Mount Lamington in 2021 formed a pumice cone. Surtsey eruptions are common in Surtsey in Iceland, Taal Volcano in the Philippines, and Fukutoku-Okanoba in Japan.
2. Effusive eruption and lava flows
Pumice can also form during an effusive eruption close to the surface of flowing or oozing viscous magma. The quick cooling near the surface of the magma will trap escaping air bubbles, forming a frothy or foamy rock.
If it forms from lava flows or during effusive eruptions, the pumice will often occur with other rocks. For instance, it will occur with obsidian if it is of rhyolitic magma composition.
Where is pumice found?
Pumice forms mostly above subduction zones on convergent plate boundaries and intracontinental hotspots and rifts. It can form in continental (aerial), marine, or submarine environments.
In the US, pumice occurs in Nevada, Oregon, Arizona, Idaho, California, New Mexico, and Kansas. These states contribute most of the over 510, 000 tons of pumice produced in the US in 2019, estimates USGS.GOV. To meet its needs, the US imports about 110,000 tons of this frothy rock from Greece.
Other pumice producers are Italy, Greece, Chile, Iran, and Syria. It also occurs in France, Germany, Turkey, Russia, Hungary, Iceland, New Zealand, and Spain.
Other countries are Jordan, Indonesia, Japan, South Arabia, New Zealand, Uganda, Ethiopia, Ecuador, Cameroon, Guatemala, Afghanistan, and the Caribbean Islands.
What are the uses of pumice?
Pumice is inert, porous, abrasive, and a hard rock with a Mohs hardness scale of 5-5.5. This rock is also light in weight and retains its properties even when crushed. These properties make it an excellent choice for making low-weight aggregates. It is also ideal for use in horticulture, landscaping, or garden decoration.
Pumice also has many other uses that include personal care, beauty, and cleaning or scrubbing products.
Frequently asked questions
Since this rock occurs as loose rocks and it is quarried using the open pit mining technique. The process involves removing the overlaying dirt and using shovels to extract it. These stones then go through shakers or crushers (if necessary) before sieving, grading, and packaging.
Data from USGS.GOV shows it costs $33 per ton if locally sourced and up to $44 when imported. Pumice for horticultural use costs $1.5 per quart (about 2 pounds), while the prices of stones for cleaning and beauty use go from $2-$10 or $10-$50 if electric powered.
This is an important question that concerns scoria and pumice. Pumice floats in water because it has more vesicles, which are isolated, gas-filled, and thin-walled, making it more buoyant or lower in density than water. However, it can sink if waterlogged. Scoria, on the other hand, has fewer, thicker-walled, interconnected vesicles, making it denser. Water can seep into the interconnected vesicles, further making scoria less buoyant.