How to identify Volcanic Rock

To correctly identify Volcanic Rock (Igneous rock (extrusive)), 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, from very dark (e.g., basalt, scoria) to light (e.g., rhyolite, pumice), and intermediate shades (e.g., andesite). Colors are often influenced by the iron and magnesium content (darker) or silica content (lighter).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Typically dull to earthy in fine-grained varieties. Glassy luster in obsidian. Vesicular rocks may appear dull due to porosity.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Predominantly aphanitic (fine-grained, crystals too small to see with the naked eye), glassy (no crystals, e.g., obsidian), or vesicular (containing gas bubbles, e.g., pumice, scoria). Porphyritic textures (larger crystals, phenocrysts, set in a fine-grained groundmass) are also common.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual crystals are generally anhedral (lacking well-formed faces) due to rapid cooling. Phenocrysts in porphyritic textures may show euhedral to subhedral forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Generally absent in the rock mass due to fine-grained or glassy nature. Individual mineral grains within the rock may exhibit cleavage, but it's not a characteristic of the bulk rock.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Associated with active and extinct volcanic regions, including convergent plate boundaries (subduction zones), divergent plate boundaries (mid-ocean ridges, continental rifts), and intraplate hotspots. Found in lava flows, volcanic cones, calderas, and pyroclastic deposits.

Key Facts

Hardness

Variable, generally 5-7 on Mohs scale for constituent minerals, but the bulk rock hardness can be lower for vesicular or highly altered types. Obsidian is ~5-5.5.

Specific Gravity

Variable, typically 2.4-3.0 g/cm³ for solid varieties. Pumice can be less than 1.0 g/cm³ (floats on water) due to high porosity.

Crystal System

Not applicable to the rock as a whole, as it's a mixture of minerals or amorphous glass. Individual mineral crystals within the rock will have their own crystal systems.

Color

Wide range, from black (basalt, obsidian) to dark gray, green, red, and light gray to white (rhyolite, pumice).

Luster

Dull, earthy, vitreous (glassy for obsidian), or resinous.

Transparency

Opaque to translucent (thin sections). Obsidian can be translucent at edges.

Fracture

Conchoidal (obsidian), irregular, splintery, or hackly.

Cleavage

Absent in the bulk rock; present in individual mineral grains if they are large enough to observe.

Composition

Silicate minerals (feldspars, pyroxenes, amphiboles, olivine, quartz, micas) and volcanic glass. Chemical composition ranges from mafic (low silica, high Fe/Mg) to felsic (high silica, low Fe/Mg).

Physical characteristics

• Crystal Habit: Aphanitic (microscopic crystals), glassy (amorphous), or porphyritic (macroscopic crystals in a fine-grained groundmass).
• Cleavage Type: Not applicable to the rock as a whole. Individual minerals may exhibit cleavage (e.g., feldspar, pyroxene).
• Fracture Type: Conchoidal (obsidian), irregular, hackly, or splintery.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, vitreous (glassy), or resinous.

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