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Porphyritic basalt is an extrusive igneous rock characterized by a porphyritic texture, meaning it contains large, well-formed crystals (phenocrysts) embedded within a much finer-grained or glassy matrix (groundmass). The phenocrysts are typically minerals like plagioclase feldspar, olivine, or pyroxene, which crystallized slowly at depth. The groundmass is composed of the same minerals but in much smaller, often microscopic, crystals, or sometimes volcanic glass, indicating rapid cooling at the surface. The overall composition is mafic, rich in iron and magnesium, and relatively low in silica.
How to Identify
- Color
- Typically dark gray to black, sometimes with a greenish or brownish tint due to alteration of mafic minerals. The phenocrysts can be lighter (plagioclase) or darker (olivine, pyroxene) than the groundmass.
- Luster
- Dull to sub-vitreous in the groundmass; phenocrysts can exhibit vitreous to greasy luster.
- Texture
- Porphyritic, characterized by distinct, larger crystals (phenocrysts) set in a fine-grained to aphanitic (microscopic crystals) or glassy groundmass. Phenocrysts are typically 1 mm to several centimeters in size, while the groundmass crystals are less than 1 mm.
- 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 interlocking.
- Cleavage
- Not typically observed in the rock as a whole due to fine-grained groundmass. Individual phenocrysts will exhibit their characteristic cleavage (e.g., plagioclase: two directions at ~90 degrees; pyroxene: two directions at ~90 degrees; olivine: poor cleavage or conchoidal fracture).
- Geological Environment
- Formed in various volcanic settings, including oceanic islands, mid-ocean ridges, continental rifts, and subduction zones (island arcs and continental arcs). It is a common rock type in lava flows, dikes, and sills.
Key Facts
- Hardness: 6-7 (Mohs scale, for individual mineral components; rock as a whole is tough)
- Specific Gravity: 2.8-3.1 g/cm³
- Crystal System: Monoclinic (pyroxene, plagioclase), Orthorhombic (olivine) for phenocrysts; groundmass is microcrystalline or amorphous.
- Color: Dark gray to black
- Luster: Dull to sub-vitreous (groundmass), vitreous to greasy (phenocrysts)
- Transparency: Opaque (rock), translucent to opaque (phenocrysts)
- Fracture: Conchoidal to uneven (groundmass), conchoidal (olivine), uneven (pyroxene, plagioclase)
- Cleavage: Poor to absent in groundmass; phenocrysts exhibit characteristic cleavage (e.g., plagioclase: 2 directions at ~90°; pyroxene: 2 directions at ~90°)
- Composition: Predominantly plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and often olivine. Minor amounts of iron-titanium oxides (magnetite, ilmenite) and sometimes amphibole or biotite. Silica content is typically 45-52 wt% SiO2.
Quick Check
- Color: Dark gray to black
- Luster: Dull to sub-vitreous (groundmass), vitreous to greasy (phenocrysts)
- Streak: Gray to grayish-black
Physical Characteristics
- Crystal Habit: Phenocrysts are typically euhedral to subhedral, often tabular (plagioclase) or prismatic (pyroxene). Groundmass is microcrystalline or glassy.
- Cleavage Type: Not applicable to the rock as a whole. Individual phenocrysts show good to distinct cleavage (e.g., plagioclase: perfect in two directions; pyroxene: good in two directions).
- Fracture Type: Conchoidal to uneven in the groundmass; conchoidal in olivine phenocrysts.
- Tenacity: Brittle
- Luster Type: Dull to sub-vitreous for the fine-grained groundmass; vitreous to greasy for phenocrysts.
Formation
Porphyritic basalt forms when magma begins to cool slowly beneath the Earth's surface, allowing large crystals (phenocrysts) to grow. Subsequently, the magma erupts onto the surface, where the remaining melt cools rapidly, forming a fine-grained or glassy groundmass. This two-stage cooling process is essential for the porphyritic texture.
Usage
Historically, basalts have been used for construction materials (e.g., cobblestones, building blocks), road aggregate, and as a source for basalt fibers. Porphyritic basalts, due to their often attractive phenocrysts, can also be used for decorative purposes, such as countertops or facing stones, though their primary use remains in construction.
Age Distribution
Ranges from Precambrian to Cenozoic, common in all geological eras where volcanic activity occurred.
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 with plagioclase phenocrysts.
Deccan Traps, India
One of the largest volcanic provinces globally, with numerous porphyritic basalt flows.
Iceland
A hotspot and mid-ocean ridge location, producing abundant basaltic volcanism, including porphyritic varieties.
Mid-ocean ridges worldwide
Basalt is the most common rock type formed at mid-ocean ridges, and porphyritic textures can be found in many samples.
Finding Tips
Look for volcanic terrains
Porphyritic basalts are extrusive igneous rocks, so they are found in areas of past or present volcanic activity, such as lava flows, volcanic plateaus, and shield volcanoes.
Examine outcrops for texture
Carefully observe rock outcrops for the characteristic porphyritic texture – larger, distinct crystals embedded in a finer matrix. This is the key identifying feature.
Check for dark color and density
Basalts are typically dark and relatively dense. If you find a dark, dense volcanic rock with visible crystals, it's likely a porphyritic basalt.
Consider associated features
Look for columnar jointing, vesicular textures (gas bubbles), or amygdaloidal structures (vesicles filled with secondary minerals), which are common in basaltic lava flows.
Similar Rocks
Basalt
Basalt
Also known as: Volcanic Rock
Andesite
Andesite
Also known as: Volcanic Rock
Dolerite (Diabase)
Dolerite
Also known as: Microgabbro
Gabbro
Gabbro
Also known as: Plutonic Basalt
Scientific Classification
- Mineral Class
- Igneous Rock (Extrusive)
- Group
- Volcanic Rocks (Mafic)
- Crystal System
- Not applicable to the rock as a whole; constituent minerals have their own crystal systems.
- Chemical Formula
- Complex silicate, primarily (Ca,Na)(Al,Si)2Si2O8 (plagioclase), (Ca,Mg,Fe)SiO3 (pyroxene), (Mg,Fe)2SiO4 (olivine)
- Composition
- Mafic, rich in iron and magnesium, low in silica. Dominant minerals are plagioclase feldspar, pyroxene, and often olivine.
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