Four Main Differences Between Scoria and Pumice

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There are quite a few examples of volcanic rock and ash.  Two of the more common and popular examples are scoria and pumice.  Scoria and pumice are both igneous extrusive rocks.

For a reference on igneous rocks I have written an article “How Igneous Rocks are Formed”.

Scoria and pumice will often be thought of as the same rock as they do express some similar looks.  Both are volcanic rocks that contain vesicles or small openings or pockets of air. 

These two rocks may look similar but are different and sometimes far apart in many regards.  In this article I will go over some of the very distinct differences that these two rocks have with each other.

Here are the main differences between scoria and pumice:

Scoria Displays in Dark Colors

Scoria is a coarse grained volcanic rock that looks like it has many small holes or vesicles throughout.  This rock will often display in darker colors of reds, browns, greys, and black.  This darker color in scoria is one of the biggest differences you will find between scoria and pumice.

Scoria’s darker colors come from it being composed of basalt material.  Often called vesicular basalt, scoria will be different from basalt by its course and vesicular texture.  

The color of pumice will range from white to lighter grays to even some lighter yellowish cream colors.  It is also known to have some blueish and greenish brown colors as well. 

Where it might get a bit confusing is that pumice can also be in the form of some darker grays, reds, or even black.  This is where some other identification methods might need to be considered.

Here we have some samples of Pumice. The darker sample on the left looks more like Scoria! It expresses larger vesicles and a darker color. However, when put in water, its specific gravity appears to be less than 1 as the rock actually floats like the two grey ones on the right.

Pumice Has Higher Amounts of Smaller Vesicles

Two of the most well-known rocks that have vesicles are pumice and scoria.  When pumice is formed, it happens by very violent volcanic activity.  The highly pressurized and heated rock will eject from a volcano in a sort of foamy form.  With quick cooling and the loss of pressure, smaller vesicles and thin vesicle walls will form.

There are many cases with pumice where the vesicles are very difficult to see with the eye.  The walls of those vesicles will often be translucent as well.

Typically, pumice will have more as well as smaller vesicles than scoria.  This is due to pumice forming from more rhyolitic magma containing more gases.  Soria is more known for having larger vesicles as well as thicker vesicle walls.

Don’t let the look of pumice fool you into thinking it is soft.  Pumice is a fairly hard rock despite it looking like a sponge in many cases.  If you were to put pumice on the Mohs scale of hardness, which is primarily for minerals, this rock would come in at 5 to 6.

Scoria is Often Used in Landscaping

When walking through a neighborhood you may often see those reddish or even black volcanic looking rocks in the landscaping of some yards.  This use of scoria is one of the better known and remembered uses of this particular volcanic rock.

This is not to say that pumice is not used in landscaping as well, but just not in this fashion.

Pumice is better known for its use as an abrasive tool.  Due to its texture, this tool can be used in many cleaning applications and even for removing dead skin from your feet.  On top of that, pumice will be used in building materials such as concrete, insulation, and plaster.

Scoria’s uses will be for landscaping rocks and drainage type applications.  Due to its strength and highly porous nature, things like platforms, farming applications, roads, and retaining walls are created using scoria to help in the draining of water.

Pumice is not used as landscaping rocks for a very distinct reason, and that reason has to do with its density…

This pile of volcanic looking rocks were all gathered from a location bearing scoria and other volcanic rock. Many of the rocks contain the porous vesicles and are of a darker color like scoria.

Pumice is Less Dense and Floats

When it comes to the ability to float, pumice rock does float in water and scoria rock will sink.  After some time in water, pumice can eventually fill its vesicles with water and sink.  The ability for pumice to float is another of the big and detectible differences between scoria and pumice.

Pumice floats due to the rock being highly porous with many small vesicles.  This makes pumice special as most of the volume contents in pumice will be pockets of air, giving it low density.  Scoria will have a higher density or more packed contents, making scoria sink.

The way this is measured is though what is called specific gravity.  Specific gravity is a value used to distinguish if an object, such as a rock, is denser than another reference material, such as water.   Scoria and pumice are often referenced against the substance water.

So we will take the referenced substance of water and give it the value of 1.  With scoria having a specific gravity of around 2.4, this means it will sink in water.  Pumice having a specific gravity of around .93 or less means it will float in water.

Picking up pumice can feel very deceptive as it looks like a rock but lacks the weight you would expect a rock to have. Additionally, scoria can have that same effect and also feel fairly light compared to other rocks.  However as I have mentioned, a great differentiation between the two is the floating effect of pumice compared to scoria.

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