The questions that this post on scoria vs. pumice will tackle are: What does scoria have in common with pumice, and how do these two rocks differ?
To ensure we are all on the same page, let us start this discussion by briefly looking at what scoria and pumice rocks are. Afterward, we will cover what they have in common before finally looking at their differences.
Note: The terms scoria and pumice don’t refer to rock type or composition but the characteristic, vesicular texture of these rocks and their other aspects. Scoria or pumice can, therefore, represent rocks of different compositions.
What is scoria
Scoria is a highly vesicular, dark-colored volcanic rock whose composition mostly ranges from intermediate to mafic. It is usually a vesicular glassy rock, but some specimens are aphanitic or fine-grained.
The name scoria is of Greek origin, describing the silicate slug, cinder, or clinker byproduct of iron smelting. Commercially, this rock is called lava rock and has uses in landscaping, gardening, or as a light-weight aggregate for making construction or insulation concrete.
What is pumice?
Pumice is an extremely low-density, highly vesicular or form-like, light-colored volcanic rock that floats in water. It is mostly a glassy rock, but a few specimens are fine-grained or have an aphanitic texture.
The name pumice comes from pūmex or spūma, meaning foam. It describes the frothy appearance of these rocks. Pumiceous, on the other hand, is the term used to describe rocks that resemble pumice.
Other terms are:
- Pumicite refers to powdered pumice with particles less than 4 mm, and pumice dust refers to pumice with particles up to 0.25 mm.
- Pumice rafts refer to pumice rocks that are floating on water, whose size can range from lapilli to over a kilometer.
The common uses of pumice rocks are in beauty, personal care, scrubbing, or cleaning products. These rocks also have applications in horticulture, landscaping, and making aggregate for light-weight or insulating concrete.
What does scoria and pumice have in common?
Scoria and pumice are light-weight, highly vesicular volcanic or extrusive igneous rocks. Their texture is mostly glassy, and they form when magmas rich in volatiles quench or cool rapidly.
Scoria and pumice are light-weight, highly vesicular volcanic or extrusive igneous rocks. Their texture is mostly glassy, and they form when magmas rich in volatiles quench or cool rapidly. The quick quenching impedes crystallization, forming a natural volcanic glass.
Their hardness is comparable with a Mohs hardness scale of 5–6 and can occur as small particles, lapilli, or larger boulders.
Scoria vs. pumice: How do they differ?
When looking at how scoria differs from pumice, we will consider colors, vesicularity, density, magma composition and other aspects.
1. Chemical composition
The first difference between these two rocks is in their chemical composition. Scoria is a basic to intermediate rock with roughly 45–63% silica, while pumice is mostly an acidic rock with 63–77% silica.
Some pumice varieties can, however, have an intermediate composition with 52–63% silica and less often a basic composition with 45–52% silica.
Additionally, scoria is higher in calcium, magnesium, and iron oxides than pumice but relatively lower in alkali oxides.
2. Mineral composition
Scoria rocks have a composition ranging from mafic to intermediate. Pumice, on the other hand, is mostly a felsic rock but can have intermediate and, in rare instances, a mafic composition.
These rocks are mostly natural volcanic glasses or amorphous solids, formed when magma quickly quenches. The rapid cooling doesn’t give time for mineral crystals to form and grow.
However, these rocks can contain minute crystallites or microlites. The specific microlites that each of these rocks possesses depend on the rock’s chemical composition.
Most contain feldspars, smaller amounts of pyroxenes, and sometimes biotite, hornblende, feldspathoids/quartz, or accessory minerals.
3. Magma types
Scoria rocks often form from basaltic or andesitic magma. In contrast, most of the pumice rocks are from rhyolitic, dacitic, pantellerite, tachylitic, or phonolitic magma.
4. Color
Scoria is a dark-colored rock, usually dark-brown, reddish, or reddish purple. Freshly erupted specimens are dark gray, but iron oxidation will turn blackish.
In contrast, pumice is whitish, cream, light or medium gray, greenish, or green-brown. However, specimens of intermediate composition are dark gray or black.
5. Vesicularity and porosity
Both of these rocks have microscopic and macroscopic cavities, but pumice has mostly microvesicles and is more porous.
Pumice has an average porosity of 90% by volume, and scoria ranges from 30 to 80%. However, some pumice varieties are as low as 64% porous, while the reticulate scoria’s porosity is 95-98%.
6. Density
Scoria is denser than pumice. The density of scoria is 1.28–1.60 g/cm3, while that of pumice is less than 1 g/cm3, water’s density. Factors that influence the density of these rocks are their chemical composition, the number of vesicles they have per unit volume, and the nature of these voids.
Pumice is less dense than scoria because it has more vesicles, which have thinner glassy walls.
7. Nature of eruptions
Pumice forms from more viscous magma, resulting in highly explosive eruptions like Pealan, Plinian, or Surtsey. On the other hand, the magmas that form scoria are less viscous, resulting in lowly or moderately explosive eruptions like Strombolian, Vulcanian, and, less often, Hawaiian.
Both of these rocks can, however, form on the upper edges of lava flows if the surface quickly cools, trapping some of the escaping gas bubbles.
8. Where do they form?
Scoria rocks form mostly at divergent boundaries and intra-ocean hotspots. Pumice, on the other hand, commonly forms at convergent plate boundaries as well as in intracontinental hotspots and rifts.
Both of these rocks can, however, form in other tectonic settings, depending on their magma composition. For instance, pumice or scoria rocks of basaltic composition can also occur in convergent boundaries or intraoceanic hotspots.
Why pumice float in water and scoria sink?
Pumice floats in water because it has numerous, isolated, thinner-walled, gas-filled vesicles. The air in these secluded vesicles and their thin walls make pumice buoyant enough to float in water.
In contrast, scoria sinks in water because it has fewer, interconnected vesicles. The vesicle interconnections allow water to seep into the cavities, making these rocks sink. Additionally, this rock has thicker, glassy walls, giving it a higher density than water.
Note: Waterlogged pumice sinks because water replaces the trapped gases, which make it less buoyant.
Bottom line
Scoria is a denser, darker-colored, slightly less vesicular rock with fewer, larger, interconnected vesicles. It sinks in water, and its composition is mafic or intermediate. In contrast, pumice is a less dense, lighter-colored, more vesicular rock with smaller, isolated vesicles. It floats in water, and its composition is mostly felsic.