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Vesicular basalt is a dark-colored, fine-grained extrusive igneous rock characterized by numerous small, rounded to elongated cavities called vesicles. These vesicles are former gas bubbles that were trapped in the lava as it solidified. The rock's color typically ranges from dark gray to black, reflecting its mafic composition. The texture is aphanitic (fine-grained) to porphyritic, with phenocrysts (larger crystals) of olivine, pyroxene, or plagioclase feldspar sometimes visible within the fine-grained matrix. The presence and distribution of vesicles are the defining features of vesicular basalt.
How to Identify
- Color
- Typically dark gray to black, sometimes reddish-brown due to oxidation of iron-bearing minerals.
- Luster
- Dull to earthy, sometimes vitreous on fresh surfaces of individual mineral grains.
- Texture
- Vesicular (characterized by abundant gas-bubble cavities), aphanitic (fine-grained matrix), sometimes porphyritic.
- Crystal Form
- Individual crystals (phenocrysts) are typically anhedral to subhedral, often too small to be seen without magnification in the groundmass. Vesicles are typically spherical to ovoid, sometimes elongated due to lava flow.
- Cleavage
- No distinct cleavage as a rock; constituent minerals (e.g., pyroxene, plagioclase) exhibit cleavage.
- Geological Environment
- Volcanic flows, shield volcanoes, mid-ocean ridges, oceanic islands, continental flood basalts, and any setting where mafic lava erupts subaerially or subaqueously at shallow depths.
Key Facts
- Hardness: 5-6 on Mohs scale (for constituent minerals, rock itself is friable if highly vesicular)
- Specific Gravity: 2.0-2.8 (lower than non-vesicular basalt due to porosity, typically 2.7-3.0 for solid basalt)
- Crystal System: Not applicable for a rock; constituent minerals are typically monoclinic (pyroxene, plagioclase) or orthorhombic (olivine).
- Color: Dark gray, black, sometimes reddish-brown
- Luster: Dull, earthy
- Transparency: Opaque
- Fracture: Conchoidal to irregular, often rough due to vesicles
- Cleavage: Absent as a rock; constituent minerals have cleavage.
- Composition: Primarily plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and often olivine. Minor amounts of iron-titanium oxides (magnetite, ilmenite) and sometimes volcanic glass.
Quick Check
- Color: Dark gray to black
- Luster: Dull to earthy
- Streak: Gray to brownish-gray (often difficult to obtain due to texture)
Physical Characteristics
- Crystal Habit: Aphanitic groundmass with occasional porphyritic phenocrysts; vesicles are spherical to elongated.
- Cleavage Type: Not applicable for the rock; constituent minerals exhibit cleavage (e.g., two distinct cleavages at nearly 90 degrees for pyroxene, two at nearly 90 degrees for plagioclase).
- Fracture Type: Irregular to conchoidal, often rough due to the presence of vesicles.
- Tenacity: Brittle
- Luster Type: Dull to earthy
Formation
Vesicular basalt forms during volcanic eruptions when mafic lava (basaltic composition) reaches the Earth's surface. As the lava flows, dissolved gases (primarily water vapor, carbon dioxide, and sulfur dioxide) exsolve from the magma due to decreasing pressure. These gas bubbles become trapped within the rapidly cooling lava, creating voids or vesicles. The size and abundance of these vesicles depend on the gas content of the magma, the viscosity of the lava, and the rate of cooling. Rapid cooling tends to preserve more vesicles.
Usage
Vesicular basalt is commonly used as lightweight aggregate in concrete, road construction, and landscaping. Its porous nature makes it suitable for drainage systems, as a growing medium in hydroponics, and as a filter material. In some regions, it is used as building stone or for decorative purposes. Highly vesicular basalt (scoria) is also used in gas grills and as a ground cover.
Age Distribution
Ranges from Precambrian to present day, common in all geological eras with volcanic activity.
Where to Find
Hawaii, USA
Extensive basaltic lava flows, including vesicular varieties, are found throughout the Hawaiian Islands, particularly on the Big Island (Kilauea, Mauna Loa).
Iceland
A major volcanic hotspot with abundant basaltic volcanism, including numerous vesicular basalt formations.
Columbia River Basalt Group, USA
Vast flood basalts in the Pacific Northwest, often exhibiting vesicular textures.
Deccan Traps, India
One of the largest volcanic provinces on Earth, with widespread basaltic flows, including vesicular types.
Mid-ocean ridges globally
Formed at divergent plate boundaries, where basaltic lava erupts and cools, often forming pillow basalts with vesicular rinds.
Finding Tips
Look for volcanic terrains
Vesicular basalt is exclusively found in areas of past or present volcanic activity, such as lava flows, volcanic cones, and rift zones.
Examine rock surfaces for pores
The most distinctive feature is the presence of numerous small holes (vesicles) on the rock's surface and within its interior.
Check for dark color and fine grain
Basalt is typically dark and fine-grained. If it also has vesicles, it's likely vesicular basalt.
Consider density
Due to the trapped gas bubbles, vesicular basalt is often lighter in weight than non-vesicular basalt of the same size.
Similar Rocks
Basalt
Basalt
Also known as: Trap rock
Scoria
Scoria
Also known as: Volcanic cinders
Pumice
Pumice
Also known as: Volcanic foam
Amygdaloidal Basalt
Amygdaloidal Basalt
Also known as: Amygdaloid
Scientific Classification
- Mineral Class
- Igneous Rock
- Group
- Extrusive Igneous Rock
- Crystal System
- Not applicable for a rock
- Chemical Formula
- Variable, primarily silicates of iron, magnesium, calcium, and aluminum.
- Composition
- Mafic (rich in magnesium and iron, low in silica, typically 45-52% SiO2)
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