Pahoehoe Lava

Description

Pahoehoe lava is a type of basaltic lava flow characterized by its smooth, undulating, or ropy surface. The term 'pahoehoe' is Hawaiian, meaning 'smooth, unbroken lava'. It forms when highly fluid basaltic lava cools and solidifies, creating a continuous, often shiny, skin that wrinkles and folds as the molten interior continues to move beneath it. The surface can range from glassy to finely crystalline, and its color is typically dark gray to black, often with iridescent sheens due to thin oxide films. Vesicles (gas bubbles) are common, varying in size and distribution.

How to Identify

Color

Typically dark gray to black, sometimes with reddish-brown or iridescent hues due to oxidation or thin mineral films.

Luster

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

Texture

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.

Crystal Form

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.

Cleavage

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

Geological Environment

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.

Quick Check

• Color: Dark gray to black
• Luster: Dull to vitreous
• Streak: Gray to grayish-black (difficult to obtain due to hardness)

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).

Formation

Pahoehoe lava forms from highly fluid basaltic lava flows (low viscosity, typically 100-1000 Pa·s) that cool and solidify with a smooth, billowy, or ropy surface. The flow advances by the continuous budding of new lobes from the molten interior of the flow. The outer skin cools and solidifies, but the molten interior continues to flow, dragging and folding the surface into characteristic rope-like textures. This process occurs at relatively slow flow rates (typically <10 m/h) and lower shear stresses compared to 'a'a lava.

Usage

Pahoehoe lava, as a form of basalt, is widely used as a construction aggregate, road base material, and in some cases, for dimension stone. Its unique surface texture can also be appreciated in landscaping and decorative applications. Scientifically, it provides crucial insights into volcanic processes, rheology of lava, and planetary geology.

Age Distribution

Geologically recent to ancient, wherever basaltic volcanism has occurred. Most commonly observed in active volcanic regions.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Igneous Rock (Extrusive)

Group

Basalt

Crystal System

Not applicable to the bulk rock; constituent minerals are primarily monoclinic and orthorhombic.

Chemical Formula

Complex silicate, primarily (Ca,Na)(Mg,Fe,Al)(Si,Al)₂O₆ (pyroxene), (Ca,Na)(Al,Si)₄O₈ (plagioclase), (Mg,Fe)₂SiO₄ (olivine).

Composition

Silica (SiO₂) content typically 45-52 wt%. High in Fe, Mg, Ca; relatively low in K, Na. Characterized by a high proportion of mafic minerals and calcic plagioclase.

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