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Porphyritic basalt is a dark-colored, fine-grained igneous rock characterized by a porphyritic texture. This texture means it contains larger, well-formed crystals (phenocrysts) embedded within a much finer-grained or glassy groundmass. The phenocrysts are typically composed of minerals such as plagioclase feldspar (often labradorite), olivine, or pyroxene (augite). The groundmass is primarily composed of microscopic crystals of plagioclase, pyroxene, and often olivine, along with iron-titanium oxides and sometimes volcanic glass. The overall color is typically black to dark gray.
How to Identify
- Color
- Dark gray to black.
- Luster
- Dull to sub-vitreous in the groundmass; phenocrysts can exhibit vitreous to pearly luster.
- Texture
- Porphyritic, with visible phenocrysts (typically 1 mm to several cm) set in an aphanitic (fine-grained) to glassy groundmass.
- Crystal Form
- Phenocrysts often show euhedral to subhedral crystal forms (e.g., tabular plagioclase, prismatic pyroxene, anhedral to subhedral olivine). Groundmass crystals are anhedral and microscopic.
- Cleavage
- Minerals within the rock (e.g., plagioclase, pyroxene) exhibit characteristic cleavage, but the rock itself does not have a rock cleavage. Fracture is typically conchoidal to irregular.
- Geological Environment
- Volcanic flows, dikes, sills, and volcanic cones in various tectonic settings including mid-ocean ridges, oceanic islands, continental rifts, and subduction zones (back-arc basins).
Key Facts
- Hardness: 6-7 (Mohs scale, for constituent minerals like plagioclase and pyroxene; bulk rock hardness can vary)
- Specific Gravity: 2.8-3.1 g/cm³
- Crystal System: Monoclinic (pyroxene), Triclinic (plagioclase), Orthorhombic (olivine) - refers to constituent minerals
- Color: Dark gray to black
- Luster: Dull to sub-vitreous (groundmass), vitreous to pearly (phenocrysts)
- Transparency: Opaque (rock), translucent to transparent (individual phenocrysts)
- Fracture: Conchoidal to irregular
- Cleavage: Good in constituent minerals (e.g., plagioclase, pyroxene), but not a rock cleavage
- Composition: Mafic; primarily plagioclase feldspar (labradorite), pyroxene (augite), olivine, and iron-titanium oxides. Silica content typically 45-52 wt% SiO₂.
Quick Check
- Color: Dark gray to black
- Luster: Dull to sub-vitreous (groundmass), vitreous to pearly (phenocrysts)
- Streak: Grayish-white to colorless (due to silicate minerals)
Physical Characteristics
- Crystal Habit: Phenocrysts are typically euhedral to subhedral (e.g., tabular plagioclase, prismatic pyroxene, anhedral olivine). Groundmass is aphanitic.
- Cleavage Type: Perfect to good in constituent minerals (e.g., plagioclase: 2 directions at ~90°; pyroxene: 2 directions at ~90°).
- Fracture Type: Conchoidal to irregular, depending on grain size and presence of phenocrysts.
- Tenacity: Brittle
- Luster Type: Dull to sub-vitreous for the fine-grained groundmass; phenocrysts can exhibit vitreous to pearly luster.
Formation
Porphyritic basalt forms when magma begins to cool slowly beneath the Earth's surface, allowing large crystals (phenocrysts) to grow. Subsequently, this magma erupts onto the surface, where the remaining melt cools rapidly, forming a fine-grained (aphanitic) or glassy groundmass around the larger crystals. This two-stage cooling process is characteristic of porphyritic textures.
Usage
Porphyritic basalt is used extensively as crushed stone for road construction, aggregate in concrete, railway ballast, and as dimension stone for paving and building facades. Its durability and resistance to weathering make it a valuable construction material. In some cultures, it has been used for tools and sculptures.
Age Distribution
Common throughout Earth's history, particularly associated with volcanic activity from the Precambrian to the present day.
Where to Find
Hawaii, USA
Extensive basaltic lava flows, many of which are porphyritic, are found on the Hawaiian Islands, particularly on the Big Island (e.g., Kilauea, Mauna Loa).
Columbia River Basalt Group, USA
Vast flood basalts covering parts of Washington, Oregon, and Idaho, often exhibiting porphyritic textures.
Deccan Traps, India
One of the largest volcanic provinces globally, with numerous porphyritic basalt flows.
Iceland
Active volcanic island with abundant basaltic volcanism, including porphyritic varieties.
Mid-ocean ridges
Commonly found in samples dredged from mid-ocean ridges, representing newly formed oceanic crust.
Finding Tips
Look for volcanic terrains
Porphyritic basalt is an extrusive igneous rock, so it is found in areas of past or present volcanic activity, such as lava flows, volcanic plateaus, and shield volcanoes.
Examine road cuts and quarries
These artificial exposures often reveal fresh rock surfaces where the porphyritic texture is clearly visible.
Check for phenocrysts
The key identifying feature is the presence of larger, distinct crystals (phenocrysts) embedded in a fine-grained matrix. Look for white (plagioclase), green (olivine), or black (pyroxene) crystals.
Consider the color and density
Basalts are typically dark and relatively dense. If the rock is light-colored or unusually light, it's likely not basalt.
Similar Rocks
Basalt
Basalt
Also known as: Aphanitic Basalt
Andesite
Andesite
Also known as: Porphyritic Andesite
Dolerite
Dolerite
Also known as: Diabase
Gabbro
Gabbro
Also known as: Coarse-grained Basalt equivalent
Scientific Classification
- Mineral Class
- Igneous Rock
- Group
- Volcanic (Extrusive)
- Crystal System
- Not applicable for the rock as a whole; constituent minerals have their own crystal systems.
- Chemical Formula
- Complex silicate mixture (no single formula)
- Composition
- Mafic, dominated by calcic plagioclase, pyroxene, and olivine, with minor iron-titanium oxides.
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