Porphyritic Basalt

Igneous Rock, Volcanic

Also known as: Porphyritic Lava, Basalt Porphyry

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Description

Porphyritic basalt is an extrusive igneous rock characterized by a porphyritic texture, meaning it contains larger, visible crystals (phenocrysts) embedded within a finer-grained matrix (groundmass). The phenocrysts are typically composed of minerals such as plagioclase feldspar (often labradorite), olivine, or pyroxene (augite). The groundmass is composed of microscopic crystals of these same minerals, along with volcanic glass, reflecting the rapid cooling of the lava. Chemically, it is mafic, rich in iron and magnesium, and low in silica (typically 45-52% SiO2).

How to Identify

Color: Typically dark gray to black. Phenocrysts may be lighter (plagioclase) or greenish (olivine).
Luster: Dull to earthy in the groundmass; phenocrysts can exhibit vitreous to sub-vitreous luster.
Texture: Porphyritic, characterized by large, visible crystals (phenocrysts) set in a fine-grained to aphanitic (microscopic crystals) or glassy groundmass. Phenocrysts typically range from 1 mm to several centimeters.
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: Minerals within the rock (e.g., pyroxene, plagioclase) exhibit characteristic cleavage, but the rock as a whole does not have rock cleavage. Phenocrysts of plagioclase show two cleavages at nearly 90 degrees; pyroxene shows two cleavages at nearly 90 degrees; olivine typically has no distinct cleavage.
Geological Environment: Mid-ocean ridges, oceanic islands (hotspots), continental flood basalts, and volcanic arcs. It forms from effusive volcanic eruptions or shallow intrusions.

Key Facts

Hardness: 6-7 (Mohs scale, for constituent minerals like plagioclase and pyroxene; bulk rock hardness varies)
Specific Gravity: 2.8-3.1 g/cm³
Crystal System: Monoclinic (pyroxene, plagioclase), Orthorhombic (olivine) for phenocrysts; groundmass is microcrystalline to amorphous.
Color: Dark gray to black
Luster: Dull to earthy (groundmass), vitreous to sub-vitreous (phenocrysts)
Transparency: Opaque (rock), translucent to opaque (phenocrysts)
Fracture: Conchoidal to uneven
Cleavage: Not applicable to the rock as a whole; constituent minerals exhibit cleavage (e.g., plagioclase, pyroxene)
Composition: Predominantly plagioclase feldspar (labradorite), pyroxene (augite), olivine, and sometimes minor iron-titanium oxides. Silica content (SiO2) typically 45-52%.

Quick Check

Color: Dark gray to black
Luster: Dull to earthy (groundmass), vitreous (phenocrysts)
Streak: White to grayish-white (due to fine-grained nature, not diagnostic of individual minerals)

Physical Characteristics

Crystal Habit: Phenocrysts are typically euhedral to subhedral (e.g., tabular plagioclase, prismatic pyroxene, granular olivine). Groundmass is aphanitic or glassy.
Cleavage Type: Not a rock property; constituent minerals have cleavage (e.g., plagioclase: 2 directions at ~90°; pyroxene: 2 directions at ~90°).
Fracture Type: Conchoidal to uneven, depending on the proportion of glass and crystal size.
Tenacity: Brittle
Luster Type: Dull to earthy for the groundmass; vitreous to sub-vitreous for phenocrysts.

Formation

Porphyritic basalt forms when magma begins to cool slowly at depth, allowing large, well-formed crystals (phenocrysts) to grow. Subsequently, this partially crystallized magma erupts onto the Earth's surface or intrudes shallowly, 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

Porphyritic basalt is used as an aggregate in construction, for road building, and as railway ballast. Its durability and resistance to weathering make it suitable for these applications. In some cases, attractive varieties are cut and polished for decorative purposes or as dimension stone.

Age Distribution

Found throughout Earth's history, from Precambrian to present-day volcanic activity.

Where to Find

Hawaii, USA

Active shield volcanoes (e.g., Kilauea, Mauna Loa) frequently erupt porphyritic basalts with olivine phenocrysts.

Columbia River Basalt Group, USA

Extensive flood basalts in the Pacific Northwest, often exhibiting porphyritic textures with plagioclase phenocrysts.

Deccan Traps, India

One of the largest volcanic provinces on Earth, containing vast quantities of porphyritic basalts.

Iceland

Mid-ocean ridge volcanism produces abundant porphyritic basalts, often with plagioclase or olivine phenocrysts.

Oceanic Islands and Mid-Ocean Ridges Worldwide

Commonly found in active volcanic regions associated with divergent plate boundaries and mantle plumes.

Finding Tips

Look for Volcanic Terrains

Porphyritic basalts are found in areas of past or present volcanic activity, such as lava flows, volcanic cones, and dikes.

Examine Outcrops for Texture

The key identifying feature is the porphyritic texture. Look for larger, distinct crystals embedded in a fine-grained matrix.

Check for Dark Color and Density

Basalts are typically dark and relatively dense due to their mafic mineral content.

Observe Mineral Phenocrysts

Identify the phenocrysts: white/gray tabular crystals (plagioclase), green granular crystals (olivine), or black prismatic crystals (pyroxene).

Similar Rocks

Basalt

Also known as: Fine-grained Basalt

Andesite

Also known as: Porphyritic Andesite

Dolerite (Diabase)

Dolerite

Also known as: Microgabbro

Gabbro

Also known as: Coarse-grained Basalt

Scientific Classification

Mineral Class: Igneous Rock (Volcanic)
Group: Mafic Extrusive Rock
Crystal System: Not applicable to the rock as a whole; constituent minerals have specific crystal systems.
Chemical Formula: Complex silicate mixture (no single formula)
Composition: Plagioclase feldspar (Ca-rich), pyroxene (augite), olivine, iron-titanium oxides. Minor amounts of volcanic glass may be present.

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