How to identify Lava Rock

To correctly identify Lava Rock (Volcanic rock (extrusive igneous rock)), 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

   Highly variable, commonly dark gray to black (basaltic), reddish-brown, or lighter shades (andesitic, rhyolitic).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in vesicular varieties; vitreous (glassy) in obsidian; sub-vitreous to dull in fine-grained types.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Aphanitic (fine-grained), porphyritic (larger crystals in fine matrix), vesicular (porous with gas bubbles), scoriaceous (highly vesicular), or glassy.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Crystals are typically microscopic or absent due to rapid cooling. Phenocrysts (larger crystals) may be present, exhibiting euhedral to subhedral forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No distinct cleavage planes in the rock mass; individual mineral grains within the rock may exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic flows, volcanic cones, shield volcanoes, stratovolcanoes, mid-ocean ridges, and subduction zones.

Key Facts

Hardness

Variable, typically 5-7 on Mohs scale for constituent minerals, but the rock mass can be brittle or friable depending on texture (e.g., pumice is very soft).

Specific Gravity

Highly variable: 0.5-1.0 for pumice (due to porosity), 1.5-2.5 for scoria, 2.7-3.3 for dense basalt.

Crystal System

Not applicable to the rock as a whole; individual mineral crystals within the rock will have their own crystal systems (e.g., plagioclase is triclinic, pyroxene is monoclinic).

Color

Dark gray, black, reddish-brown, green, or light-colored.

Luster

Dull, earthy, sub-vitreous, or vitreous.

Transparency

Opaque to translucent (thin sections).

Fracture

Conchoidal (obsidian), irregular, or hackly.

Cleavage

Absent in the rock mass; present in individual mineral grains.

Composition

Silicate minerals, primarily feldspars (plagioclase), pyroxenes, olivine, amphiboles, and quartz, with varying amounts of volcanic glass. Composition ranges from mafic (basalt) to intermediate (andesite) to felsic (rhyolite).

Physical characteristics

• Crystal Habit: Aphanitic (microscopic crystals), porphyritic (macroscopic phenocrysts in a fine-grained matrix), or glassy (no crystals).
• Cleavage Type: Not applicable to the rock; individual minerals may exhibit cleavage.
• Fracture Type: Conchoidal (obsidian), irregular, or hackly.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, sub-vitreous, or vitreous.

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