How to identify Vesicular Basalt

To correctly identify Vesicular Basalt (Basalt (vesicular)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically dark gray to black, sometimes reddish-brown due to oxidation.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in the groundmass; vesicles have no inherent luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Aphanitic (fine-grained) groundmass with abundant, often interconnected, vesicles. The texture is distinctly porous or frothy.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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%.

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

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