Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Vesicular basalt is a dark-colored, fine-grained extrusive igneous rock characterized by numerous spherical or elongated cavities (vesicles). These vesicles are former gas bubbles that were trapped in the lava as it solidified. The rock is typically black to dark gray, but can also be reddish-brown due to oxidation of iron-bearing minerals. The groundmass is composed predominantly of plagioclase feldspar, pyroxene, and olivine, with minor amounts of iron-titanium oxides. The presence of vesicles significantly reduces the density of the rock, making it relatively lightweight compared to non-vesicular basalt.
How to Identify
- Color
- Typically dark gray to black, sometimes reddish-brown due to oxidation.
- Luster
- Dull to earthy in the groundmass; vesicles have no inherent luster.
- Texture
- Aphanitic (fine-grained) groundmass with abundant, often interconnected, vesicles. The texture is distinctly porous or frothy.
- Crystal Form
- Individual mineral crystals within the groundmass are generally too small to be seen with the naked eye. Phenocrysts (larger crystals) of olivine or plagioclase may occasionally be present.
- Cleavage
- No macroscopic cleavage planes are observed in the rock as a whole due to its fine-grained, massive nature and vesicular texture. Individual mineral grains within the groundmass possess their characteristic cleavages.
- Geological Environment
- Forms in volcanic settings, specifically from effusive eruptions of mafic lava flows (e.g., shield volcanoes, mid-ocean ridges, continental flood basalts) and explosive eruptions that produce scoria cones.
Key Facts
- Hardness: 5-6 on Mohs scale (for constituent minerals, rock as a whole is friable if highly vesicular)
- Specific Gravity: 1.8-2.8 (significantly lower than non-vesicular basalt due to porosity; non-vesicular basalt is 2.8-3.0)
- 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 to earthy
- Transparency: Opaque
- Fracture: Conchoidal to irregular (in solid portions), but often breaks along vesicle walls.
- Cleavage: None as a rock; constituent minerals have cleavage.
- Composition: Mafic, dominated by plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and olivine, with minor iron-titanium oxides (magnetite, ilmenite). SiO2 content typically 45-52 wt%.
Quick Check
- Color: Dark gray to black
- Luster: Dull/Earthy
- Streak: Gray to brownish-gray
Physical Characteristics
- Crystal Habit: Aphanitic groundmass with trapped gas bubbles (vesicles).
- Cleavage Type: Not applicable for the rock; constituent minerals exhibit good cleavage (e.g., pyroxene: two at ~90 degrees; plagioclase: two at ~90 degrees).
- Fracture Type: Irregular to conchoidal in solid parts, often porous and friable.
- Tenacity: Brittle
- Luster Type: Dull to earthy
Formation
Vesicular basalt forms during volcanic eruptions when gas-rich magma (lava) rapidly cools and solidifies on or near the Earth's surface. As the lava decompresses during eruption, dissolved gases (primarily water vapor, carbon dioxide, and sulfur dioxide) exsolve from the melt, forming bubbles. If the lava solidifies before these gas bubbles can escape, they become trapped, creating a porous, frothy texture known as vesicles. The size and abundance of vesicles depend on the gas content of the magma, the viscosity of the lava, and the rate of cooling.
Usage
Historically used as a lightweight aggregate in construction, for landscaping, and as a filtration medium. In some cultures, it has been used for grinding tools due to its abrasive nature. Modern uses include lightweight concrete, road aggregate, and as a substrate for hydroponics.
Age Distribution
Ranges from Precambrian to Holocene, 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 (Hawaii) from active volcanoes like Kilauea and Mauna Loa.
Iceland
A hotspot over the Mid-Atlantic Ridge, Iceland is almost entirely composed of basaltic rocks, with abundant vesicular basalts from numerous volcanic eruptions.
Columbia River Basalt Group, USA
Massive flood basalt province covering parts of Washington, Oregon, and Idaho, containing vast quantities of basalt, including vesicular flows.
Deccan Traps, India
One of the largest volcanic provinces on Earth, consisting of multiple layers of basaltic lava flows, many of which are vesicular.
Mid-Ocean Ridges
Formed at divergent plate boundaries, these underwater mountain ranges are sites of continuous basaltic volcanism, producing pillow basalts, which often exhibit vesicular textures.
Finding Tips
Look in Volcanic Regions
Vesicular basalt is exclusively found in areas of past or present volcanic activity. Look for lava flows, volcanic cones, and volcanic plains.
Examine Outcrops
Inspect rock outcrops in volcanic terrains. Vesicular basalt often forms the upper, more gas-rich portions of lava flows.
Check for Porosity
The most distinctive feature is its porous, 'bubbly' texture. If the rock is dark and lightweight for its size, it's likely vesicular basalt.
Distinguish from Scoria/Pumice
While similar, vesicular basalt is generally denser than scoria and pumice, and its vesicles are typically less abundant and less interconnected than in pumice. Scoria is often more reddish and has a higher proportion of vesicles than typical vesicular basalt.
Similar Rocks
Scoria
Scoria
Also known as: Volcanic Cinder
Pumice
Pumice
Also known as: Volcanic Foam
Basalt
Basalt
Also known as: Volcanic Rock
Scientific Classification
- Mineral Class
- Igneous Rock
- Group
- Extrusive Mafic Volcanic Rock
- Crystal System
- Not applicable (rock)
- Chemical Formula
- Variable, primarily (Ca,Na)(Al,Si)2Si2O8 (plagioclase), (Ca,Mg,Fe)SiO3 (pyroxene), (Mg,Fe)2SiO4 (olivine)
- Composition
- Mafic, rich in iron and magnesium, low in silica. Predominantly plagioclase, pyroxene, and olivine.
Explore Vesicular Basalt
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.