Scoria and pumice are two porous and lightweight volcanic rocks that look similar but have distinct differences. Both rocks form from rapidly cooling magma which traps gas bubbles, creating vesicles in the rock. However, scoria and pumice have varying appearances, densities, methods of formation, and uses.
One of the most noticeable differences between scoria and pumice is their appearance. Scoria is dark colored, usually black or dark red, with a coarse texture due to larger gas bubbles trapped in the rock. In contrast, pumice is light colored, often white or light gray, and has a smoother texture resulting from smaller vesicles. The vesicle size also affects the density of each rock type. Pumice has a highly porous, foamy structure with many tiny vesicles which makes it less dense than water, allowing it to float. Scoria has larger vesicles and is more dense, causing it to sink in water.
While scoria and pumice have some shared origins and traits, their distinct variations in density, color, texture, and other properties allow geologists to readily differentiate between these two volcanic rocks. Both types can be found in many volcanically active areas around the world and have various applications, from abrasives to construction to horticultural uses. In this article, we will explore the nuanced differences between scoria and pumice in more detail.
Appearance and Texture
Scoria
Scoria has a dark coloration, often black, dark brown, or purplish red. It has a coarse, cindery texture due to the presence of larger gas bubble openings and thicker vesicle walls between the bubbles. Sometimes the vesicles appear in layers or bands due to changes in eruption intensity. Composed of basalt or andesite lava, scoria will often include mineral crystal fragments interspersed in its glassy groundmass.
When examining scoria, you will easily see the classic volcanic rock texture with visible vesicles, cinder-like grains, and crystalline minerals in the fine-grained groundmass. The rough openings give scoria a rugged, sponge-like appearance.
Pumice
In contrast to scoria’s dark tones, pumice often appears light-colored, commonly light gray, white, or yellowish. Pumice owes its pale tones to its chemical composition of rhyolite or dacite lava which is higher in silica. This felsic lava creates a very fine-grained or glassy groundmass with a smooth, almost powdery texture when pumice is broken or abraded.
The defining characteristic of pumice is its extremely lightweight, porous structure riddled with holes. But unlike the large vesicles in scoria, the vesicles in pumice are much smaller and thinner-walled due to rapid volatile escape. This creates a frothed appearance almost like an aerated sponge cake. The tiny vesicles make pumice feel very light.
Comparison Table of the Appearance and Texture of Scoria vs Pumice
Feature | Scoria | Pumice |
---|---|---|
Color | Dark black, brown, purplish red | Light gray, white, yellow, pale tones |
Texture | Coarse, cindery | Smooth, fine, powdery |
Vesicles | Large gas bubbles, thick walls | Small, thin-walled bubbles |
Groundmass | Glassy with mineral crystals | Very fine-grained, glassy |
Overall Appearance | Rugged, vesicular, sponge-like | Light, foamy, aerated |
Density and Water Flotation
Differences in vesicle size and density make scoria and pumice easy to tell apart. Pumice is highly vesicular, full of small sealed pores making it extremely lightweight for a stone. It has a remarkably low specific gravity of around 0.4 – 2.0 due to the aerated structure. This makes pumice less dense than water so it will float!
Scoria has fewer vesicles than pumice so it is more dense, with a typical specific gravity of 1.5 – 3.0. This higher density causes particles of scoria to sink in water rather than float like pumice does. If you conduct a simple water flotation test, light pieces of pumice will float while scoria rocks will sink to the bottom.
Formation and Origins
Scoria and pumice ultimately originate from the same volcanic processes but form through slightly different mechanisms in the eruption sequence. Both rocks originate from rapidly cooling lava that was full of dissolved gas bubbles. As the lava cools quickly when outside the vent, the gases are unable to escape and get trapped, creating vesicles in the solidifying rock. But the specifics of the cooling and crystallization dictate the texture of scoria versus pumice.
Scoria Formation
Scoria specifically forms during Strombolian volcanic eruptions, which are relatively mild but still eject significant lava into the air. As globs of lava are thrown up out of the vent, they cool quickly in the air while falling back to the ground as pyroclasts.
The exterior of the blobs solidify, trapping gas bubbles inside. This creates larger, isolated vesicles and the hardened lava chunks have the classic scoria shape. Scoria also forms when volcanic lava flows onto land and crusts over while the still-molten interior continues flowing, creating layered bubble walls and a scoriaceous top crust layer.
Pumice Formation
Pumice arises from violent Plinian volcanic eruptions where incredibly thick, viscous, silica-rich magma (rhyolite or dacite) gets heated up and pressurized with gases. When explosively ejected, the lava becomes a foamy fluid filled with thin bubbles from rapid degassing.
The small vesicles form as the aerated lava solidifies into a light, frothy material. Pumice is ejected as pyroclasts and also forms from the solidification of pyroclastic flows – the hot ash, lava fragments and gases that race down the volcanic slopes at hundreds of miles per hour during an eruption. Long tubes of pumice known as pumiceous pipe also develop when the lava inside a volcanic conduit or vent cools into a glassy solid riddled with bubbles.
Uses
Scoria Uses
The coarse, cindery texture and durability of scoria make it desirable for many construction uses. Key uses include:
- Landscaping: Scoria’s interesting texture creates very attractive landscaping beds, walkways, and accents. The porous structure allows good drainage.
- Construction aggregate: Scoria is crushed and used extensively in road construction and concrete aggregates. It provides bulk while remaining lightweight.
- Decorative stone: Scoria’s distinctive natural appearance makes it ideal for decorative uses in gardens, vases, walkways, and interior decoration.
- Traction aid: Cracked and crushed scoria is spread on icy roads in some regions to create traction for vehicles.
Pumice Uses
Lightweight pumice finds use in:
- Abrasives: Finely ground pumice serves as an abrasive in soaps, polishes, and exfoliates. The sharp edges scrape nicely.
- Horticulture: Mixed into soil, pumice improves drainage and aeration. It’s also used as mulch.
- Construction: Pumice is mixed into cement and concrete to reduce the density and weight while increasing insulation properties.
- Decorative aggregate: Pumice pebbles are popular in landscaping due to the visually pleasing, variation in colors.
- Traction aid: Powdered pumice is spread on roads to enhance traction on ice or snow.
Locations
Scoria Locations
Some notable locations where scoria can be found include:
- Hawaii – Abundant scoria deposits on Mauna Kea and Mauna Loa volcanoes
- Italy – Scoria cones around Mount Etna and Vulcano islands
- North America – Scoria found near volcano sites in Arizona, New Mexico, Idaho, Oregon
- Iceland – Extensive scoria fields across glacial valleys
- New Zealand – Auckland Volcanic Field contains many scoria cones
Pumice Locations
Prime sources of pumice include:
- Greece – The South Aegean Volcanic Arc produced extensive pumice deposits that were mined on Milos, Kimolos, Santorini islands
- Japan – Pumice mined from Towada and Hakkoda mountains
- United States – Major pumice mines located in Nevada, Idaho, California, Oregon, New Mexico
- Chile – Large rhyolitic volcanic areas have massive pumice deposits, especially near Calbuco
- New Zealand – Taupo Volcanic Zone on North Island has vast ignimbrite sheets and pumice deposits
- Turkey – Pumice mined from fields near Mount Idi and Golcuk
- Canary Islands – Pumice found on Lanzarote and Tenerife islands
FAQs
How do the physical properties of scoria and pumice compare?
The main physical property differences are that pumice is less dense, lighter weight, and has smaller vesicles compared to scoria. Pumice will float in water while scoria sinks. Pumice also has a smoother, finer texture than the coarser, grainier scoria rock.
What causes the color differences between scoria and pumice?
Scoria forms from basaltic magma which is lower in silica, giving it a dark coloration. Pumice forms from rhyolitic magma which has higher silica content, resulting in lighter tones. The composition of the original erupting lava determines the color.
Can scoria and pumice occur together?
Yes, in certain volcanic eruptions it is possible for conditions to alternate between effusive and explosive phases, creating both scoria and pumice rocks simultaneously. The differences in gas content and cooling rate account for the varying textures.
Where are some notable places to find scoria and pumice?
Excellent places to find scoria include the volcanic islands of Hawaii, Iceland, and New Zealand due to common strombolian eruptions there. Famous pumice sources include the Aegean Islands, the Taupo Volcanic Zone in New Zealand, and the western United States.
What is the main use for scoria and pumice?
The primary uses are structural and decorative due to their vesicular volcanic nature. Scoria serves well as construction aggregate. Pumice is extensively used as an abrasive and horticultural soil amendment. Both are popular decorative rocks in landscaping.
Can I tell just by looking if a rock is scoria or pumice?
Sometimes it can be difficult to identify them visually. The best way is to combine the rock’s color, texture, crystal content and vesicle characteristics. An even better confirmation is to do a simple water float test since pumice floats and scoria sinks.
Are scoria and pumice considered dangerous volcanic hazards?
Not directly. But they form in explosive volcanic eruptions which can produce dangerous pyroclastic flows and surges along with ash fall. Indirectly their abundance indicates a region’s volcanic past.
Conclusion
Scoria and pumice may appear similar at first glance but upon closer inspection, their distinct differences in vesicle size, texture, color, density, formation style, and composition become apparent. The variations arise based on subtle differences in the types of volcanic eruptions and lava that formed each rock type. Next time you encounter a volcanic rock, use these guides to read the clues and correctly identify whether it is dark scoria or pale pumice! Both rocks have endless applications thanks to their lightweight nature and connection to volcanoes that we still depend on today.
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