Pumice is one of the most distinct and unique rock types found anywhere in the world. Almost everyone is familiar with it for its distinct look and its unique ability to float on water. Pumice is often treated as a novelty, but it is worth learning more about due to its prevalence and geologic significance.
Despite its relatively unique properties, pumice can sometimes be confused with other rock types. It comes in more colors and varieties than you might think, and can bear a close resemblance to other igneous rocks. It can be difficult to know exactly what pumice looks like and how to identify it, but it is relatively simple if you know what to look for.
Pumice is an extrusive igneous rock composed of highly vesicular volcanic glass, formed from violent eruption, depressurization, and rapid cooling of lava. It is usually lightly colored with a felsic or intermediate composition. Pumice can be readily identified by its unique ability to float on water.
While pumice is a clearly defined rock type, it comes in many shapes and sizes and can sometimes be hard to distinguish from closely related rocks like scoria. I’ll walk you through how to identify pumice, what it looks like, and where it can be found.
What Does Pumice Look Like?
Pumice is a fairly common and widespread rock, found in deposits all over the world. Most people have seen it many times in their life – sometimes without even realizing it. Unlike most igneous rocks which are made of well-defined crystals, pumice is actually a volcanic glass with no crystal structure.
Most people know pumice for its characteristic ‘bubbly’ texture, but that often isn’t enough to reliably call a rock ‘pumice’. So what does pumice look like?
Pumice is very porous, with a highly vesicular texture that often resembles a sponge. The surface looks rough and pitted with the remnants of gas bubbles. It comes in many colors but is usually off-white or light to dark gray. As a volcanic glass, it has no visible individual crystals.
The gas bubbles (known as vesicles) in pumice can vary widely in their shape and size, even in the same rock. Most of the bubbles are too small to see, but they can sometimes be large enough to put your finger through. Sometimes the bubbles are mostly spheroidal but, in many cases, they are elongated with a ‘stretched out’ appearance.
While all pumice meets this general description, there is a pretty wide spectrum of pumices that can look significantly different from one another. The difference in their appearance is driven by the mineralogy of each type of pumice and the environment in which the rock was formed.
Color is Driven by Mineralogy
Most pumice is felsic in composition, meaning it has a chemical composition similar to rhyolite or dacite. Those rocks are primarily made from light-colored minerals like quartz and feldspar, with very few dark-colored, mafic minerals.
Pumice, however, is not made of proper mineral crystals since it is a volcanic glass. It isn’t technically made of quartz or feldspar, but it is made of the same chemical building blocks. In the end, this means that pumice takes on the same general coloration that other felsic igneous rocks have. Pumice is usually light gray, light tan, or whitish.
Silica is usually abundant in pumice. If allowed the time to crystalize it would turn into quartz. Pure silica is transparent or whitish and is usually a significant contributor to pumice’s overall color.
In general, the color of pumice aligns with the type of lava from which it forms. Rhyolitic lava, high in silica and alkali metals, tends to form whitish pumice while andesitic lava, with slightly higher mafic mineral content, forms brownish pumice. On rare occasions, pumice can even be very dark gray or even black when it forms from basaltic lava.
Texture of Pumice
One of the defining features of pumice’s appearance is its texture. All pumice is highly vesicular, meaning it is riddled with gas bubbles and has a very high porosity. This texture occurs when magma with a lot of soluble gas erupts to the surface and cools rapidly, trapping gas bubbles in hardened, amorphous volcanic glass.
While all pumices are highly vesicular, it is also possible (and even common) to describe their texture in other ways. Rocks can be described as having more than one texture as long as the texture types are not mutually exclusive.
Pumice can also be described as having a ‘glassy‘ texture when speaking about the actual hard walls of the rock. When magma cools so rapidly that there is no time for crystals to nucleate or grow, the result is volcanic glass. We are all most familiar with this texture in obsidian, but it is technically true of pumice as well.
In certain rare cases, it is also possible for pumice to be porphyritic. This texture describes large, visible crystals surrounded by a fine-grained groundmass. It occurs when minerals with a higher melt point have time to crystallize in the magma before it erupts to the surface and the entire melt rapidly crystallizes around them.
How to Identify Pumice
As unique as pumice is, you might think that identifying it is a trivial matter. While that is often the case, that assumption sometimes leads people to confuse other rocks for pumice. Luckily, there are some very quick and easy ways to identify pumice and distinguish it from closely-related rock types. As with any rock, it is important to take a systematic approach when identifying pumice.
Pumice feels noticeably light in weight when compared to other rocks of its size. It is almost always light gray, brown, or whitish, with small vesicles throughout the rock. Pumice has a specific gravity of less than 1 and floats in water, distinguishing it from all other rock types.
A rock must meet all of these requirements to be considered a pumice:
- Igneous – Formed from rapidly cooling lava
- Highly Vesicular – Many small bubbles interspersed throughout the rock, very high porosity
- Glassy – Amorphous glass with no crystal grains in the groundmass
- Specific Gravity < 1 – Floats in water for an extended period of time
If your rock meets all of those criteria, it is definitely pumice. Remember that pumice can come in many different colors depending on the makeup of the lava from which it formed.
Tip: This article is part of my igneous rock identification series. To read more about how to identify all igneous rocks, check out my article here.
It can sometimes be very difficult to distinguish pumice from closely related rock types like scoria and vesicular basalt. As a general rule, scoria and vesicular basalt are much darker than pumice since they tend to form from basaltic lava. However, pumice can also sometimes be dark-colored so it isn’t sufficient to go by color alone.
The most reliable way to identify pumice is to try to float it in water. By definition, pumice has a specific gravity of less than 1 and will float in water. It is the only rock type that does this – scoria and vesicular basalt will both sink like other rocks.
If you don’t have access to water it is usually sufficient to look at the ratio of vesicles to hard rock walls. Scoria tends to have fewer (but slightly larger) vesicles than pumice with much thicker walls between them. Vesicular basalt is even further along in this trend, with large vesicles and a lot of rock between them.
When observing and identifying a rock like pumice it can often be useful to use a hand lens like this one from Amazon. This allows you to see the small features in the rock more clearly and often helps you identify the specific species of minerals present in many rock types.
What Is Pumice Made Of?
As I mentioned above, pumice is not necessarily defined by a specific mineralogy or chemical makeup. It can form from a relatively wide spectrum of lava types.
In general, pumice is made from amorphous volcanic glass. It is usually felsic or intermediate in composition. This volcanic glass is a mineraloid made of atoms and molecules that have not organized into proper crystals. In rare cases, some larger crystals of minerals like feldspar and hornblende may be present.
This description may seem a bit vague, but that is out of necessity. Pumice can form from almost any type of magma, so the possible mixtures of elements is almost endless. However, we can make some general estimates for the most common type of pumice.
Most pumice forms from felsic lava. Felsic igneous rocks are very high in quartz and feldspar, generally making up over 75% of the total rock volume. So, for light-colored pumice especially, we can assume that the chemical makeup is the same as these other felsic rocks.
For darker-colored pumice, it is safe to assume that the chemical makeup is more intermediate or even mafic in composition.
Where Is Pumice Found?
By now you might be wondering where you can find some pumice to add to your collection. Pumice is a surprisingly common rock that can be found in locations all around the world – sometimes in large deposits that are far away from any volcanic activity.
In general, pumice is found in volcanically active regions where violent eruptions eject highly gas-saturated lava into the air or water. Very large deposits can be found near stratovolcanoes with a history of explosive eruptions. Pumice can also spread great distances via ocean currents after submarine eruptions.
Most large pumice deposits are near stratovolcanos which form at convergent plate margins. These large, impressive volcanoes are prone to explosive eruptions due to the very viscous magma from which they form. Pumice deposits can be hundreds of feet thick and cover many square miles. In the U.S., it is mined extensively in the Cascades.
Sometimes pumice can be found in surprising places. Submarine eruptions can create enormous amounts of pumice that float to the surface of the ocean. These ‘pumice rafts‘ can be several square miles in size. Ocean currents can carry the pumice great distances to beaches all around the world, so it’s possible to find pieces of pumice in locations that are nowhere near its volcanic source. Eventually, however, the pumice will become saturated with water and sink to the bottom of the ocean if it hasn’t found a resting place on a beach.
You can look for pumice formations near you using this excellent interactive map from the USGS. I have a video about how to use this tool in my Practical Rock Identification System, plus even more information on how to identify pumice and other rocks.
How Does Pumice Form?
We’ve learned all about what pumice looks like, what it is composed of, and generally where it’s found, but I have only briefly touched on how it’s actually formed. The creation of pumice is a fascinating process that always follows a few simple rules but can vary significantly in the details, which is the reason pumice can look so different from one specimen to another.
Pumice forms when highly pressurized lava is violently erupted from a volcano into cool air or water at the Earth’s surface, causing simultaneous depressurization and rapid cooling. Depressurization allows for the formation of gas bubbles (usually water and CO2) which become frozen in the rapidly cooling lava.
The creation of pumice really begins with the magma beneath the Earth’s surface. Magma is very hot and under a great deal of pressure which means that it can contain an enormous amount of soluble gas. When the magma is pressurized, the gas remains in solution with the magma.
In the case of felsic magma (from which pumice is usually formed) there is a lot of silica, which makes it more viscous. It doesn’t flow as easily as basaltic magma and it tends to build up a lot of pressure before violently erupting. This scenario is what creates large stratovolcanoes.
When the eruption finally occurs, it is very fast and explosive. The magma is thrown into the air (or water, if it’s in the ocean) where it is suddenly no longer pressurized. This process is similar to when you open a can of soda. The gases that were held in solution when under pressure are suddenly allowed to ‘exsolve’ and form gas bubbles within the lava.
If allowed enough time, these gas bubbles (which normally contain mostly water and carbon dioxide) would separate out from the lava into the surrounding air. However, the sudden exposure of the lava to the cool air causes the lava to ‘freeze’ and solidify, trapping the gas bubbles in place. This effect is compounded by the increased surface area of the lava caused by the rapidly expanding gas bubbles.
This rapid cooling of the lava does not allow the atoms and molecules enough time to nucleate and grow into proper crystals. The result is volcanic glass, which is considered a mineraloid – very similar to how obsidian forms.
Pumice tends to form in fairly thick layers around stratovolcanoes. The huge, rapid ejection of material sends lava flying high into the air where it forms pumice (and often other volcanic rocks) that eventually rains down to the surface, sometimes covering many square miles.
What Is Pumice Used For?
Chances are good that you see and use pumice fairly frequently in your day-to-day life, even if you aren’t aware of it. It is used extensively for all sorts of applications ranging from construction to medicine.
Pumice is most commonly used for its abrasive qualities, with extensive applications in personal care, polishing, and cleaning. It is also widely used for horticultural purposes as a soil additive. Pumice continues to be used in construction for its large surface area which makes it light weight and good for insulation.
Since pumice is a natural glass, it is ideal for almost any application that calls for a very fine abrasive. When crushed and ground down, pumice makes for an excellent abrasive or polish, used for things like removing unwanted callouses or scrubbing away stains.
This article is part of my rock identification series. To learn more about identifying rocks, check out my full in-depth guide here.