How to identify Vesicular Basalt

To correctly identify Vesicular Basalt (Basalt (vesicular texture)), 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 of iron-bearing minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes vitreous on fresh fracture surfaces of individual mineral grains.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Aphanitic (fine-grained) groundmass with abundant vesicles. The vesicles are typically spherical to ovoid, but can be irregular. The texture is distinctly porous.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual crystals are generally too small to be seen without magnification in the groundmass. Phenocrysts (larger crystals) of olivine or plagioclase may be present but are less common than in non-vesicular basalt.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No distinct cleavage as a rock; individual mineral components (e.g., pyroxene, plagioclase) exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic flows, lava fields, shield volcanoes, mid-ocean ridges, oceanic islands, continental flood basalts, and volcanic cones. Forms in subaerial or shallow submarine environments where gas exsolution is significant.

Key Facts

Hardness

5-6 on Mohs scale (for constituent minerals, rock itself is friable if highly vesicular)

Specific Gravity

2.5-2.9 (lower if highly vesicular, can be <1 for scoria)

Crystal System

Monoclinic (pyroxene), Triclinic (plagioclase), Orthorhombic (olivine) - refers to constituent minerals, not the rock as a whole

Color

Dark gray, black, reddish-brown

Luster

Dull, earthy

Transparency

Opaque

Fracture

Conchoidal to irregular (for solid parts), very irregular due to vesicles

Cleavage

None as a rock; constituent minerals have cleavage

Composition

Mafic igneous rock, primarily plagioclase feldspar (labradorite), pyroxene (augite), and often olivine. Minor amounts of magnetite, ilmenite, and apatite.

Physical characteristics

• Crystal Habit: Aphanitic groundmass with vesicles; phenocrysts, if present, are typically euhedral to subhedral.
• Cleavage Type: Not applicable to the rock as a whole; constituent minerals exhibit cleavage (e.g., pyroxene: two at ~90 degrees; plagioclase: two at ~90 degrees).
• Fracture Type: Irregular to conchoidal in solid portions, highly irregular due to vesicles.
• Tenacity: Brittle
• Luster Type: Dull to earthy

Explore Vesicular Basalt