How to identify Pahoehoe Lava

To correctly identify Pahoehoe Lava (Basaltic Lava Flow), 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 with reddish-brown or iridescent hues due to oxidation or thin mineral films.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to vitreous (glassy) on fresh surfaces, especially if rapidly quenched. Often has a somewhat waxy or greasy appearance on weathered surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, billowy, undulating, or distinctly ropy surface. The ropy texture is the most diagnostic feature. Internally, it is typically aphanitic (fine-grained) to glassy, with occasional phenocrysts.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Generally massive, forming extensive flow fields. Individual crystals are typically microscopic within the groundmass. Phenocrysts (larger crystals) of olivine, pyroxene, or plagioclase may be present but are not always visible without magnification.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No macroscopic cleavage due to its fine-grained to glassy nature. Individual mineral grains within the basalt will exhibit their characteristic cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Formed in active volcanic regions where basaltic magma erupts effusively, such as shield volcanoes (e.g., Hawaii, Iceland), mid-ocean ridges, and continental flood basalt provinces. It is characteristic of low-viscosity, high-temperature lava flows.

Key Facts

Hardness

5-6 on Mohs scale (for constituent minerals like plagioclase, pyroxene, olivine; bulk rock hardness can vary)

Specific Gravity

2.7-3.1 g/cm³

Crystal System

Typically fine-grained to glassy, so no macroscopic crystal system. Constituent minerals are mostly monoclinic (pyroxene, plagioclase) or orthorhombic (olivine).

Color

Dark gray to black

Luster

Dull to vitreous

Transparency

Opaque

Fracture

Conchoidal (if glassy) to irregular/hackly

Cleavage

Absent in the bulk rock; present in individual mineral grains (e.g., plagioclase, pyroxene)

Composition

Mafic, dominated by plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and often olivine. Minor amounts of iron-titanium oxides (magnetite, ilmenite) and sometimes volcanic glass.

Physical characteristics

• Crystal Habit: Aphanitic (fine-grained) to glassy groundmass, with occasional porphyritic texture (phenocrysts).
• Cleavage Type: Not applicable to the bulk rock; individual minerals have distinct cleavage (e.g., plagioclase: 2 directions at ~90°, pyroxene: 2 directions at ~90°).
• Fracture Type: Conchoidal (especially in glassy varieties) to irregular/hackly.
• Tenacity: Brittle
• Luster Type: Dull, earthy, or vitreous (glassy).

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