Porphyritic Basalt

Description

Porphyritic basalt is an extrusive igneous rock characterized by a porphyritic texture, meaning it contains larger, well-formed crystals (phenocrysts) embedded within a much finer-grained or glassy groundmass. The phenocrysts typically consist of minerals like plagioclase feldspar (often labradorite), olivine, or pyroxene (augite). The groundmass is composed of microscopic crystals of these same minerals, along with volcanic glass, reflecting rapid cooling. Its overall composition is mafic, rich in iron and magnesium, and low in silica.

How to Identify

Color

Typically dark gray to black. Phenocrysts can be lighter (plagioclase) or greenish (olivine).

Luster

Dull to sub-vitreous in the groundmass; phenocrysts can be vitreous to pearly.

Texture

Porphyritic, with visible phenocrysts (typically 1 mm to several cm) set in an aphanitic (fine-grained) to glassy groundmass.

Crystal Form

Phenocrysts often exhibit euhedral to subhedral crystal forms (e.g., tabular plagioclase, prismatic pyroxene, equant olivine). Groundmass crystals are anhedral and microscopic.

Cleavage

Minerals within the rock (e.g., pyroxene, plagioclase) exhibit characteristic cleavage, but the rock as a whole does not show rock cleavage. Phenocrysts of plagioclase show two cleavages at nearly 90 degrees; pyroxene shows two cleavages at nearly 90 degrees; olivine has poor cleavage.

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 feldspar and pyroxene)
• Specific Gravity: 2.8-3.1 g/cm³
• Crystal System: Monoclinic (pyroxene, plagioclase), Orthorhombic (olivine)
• 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 uneven
• Cleavage: Poor to distinct in phenocrysts (e.g., plagioclase, pyroxene), absent in groundmass
• Composition: Predominantly plagioclase feldspar (labradorite), pyroxene (augite), olivine, and sometimes minor 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: White to light gray (due to fine powder of silicates)

Physical Characteristics

• Crystal Habit: Phenocrysts are typically euhedral to subhedral (e.g., tabular plagioclase, prismatic pyroxene, equant olivine); groundmass is anhedral and microcrystalline.
• Cleavage Type: Phenocrysts of plagioclase exhibit two cleavages at nearly 90 degrees; pyroxene exhibits two cleavages at nearly 90 degrees; olivine has poor cleavage. The rock itself does not exhibit cleavage.
• Fracture Type: Conchoidal to uneven, depending on the grain size and presence of phenocrysts.
• Tenacity: Brittle
• Luster Type: Dull to sub-vitreous for the groundmass; vitreous to pearly for phenocrysts.

Formation

Porphyritic basalt forms during volcanic eruptions when magma, containing some pre-existing, larger crystals (phenocrysts) that crystallized slowly at depth, erupts and cools rapidly at or near the Earth's surface. The rapid cooling prevents the remaining melt from forming large crystals, resulting in a fine-grained (aphanitic) groundmass.

Usage

Used as aggregate in construction (road base, concrete), railway ballast, dimension stone, and in some cases, for decorative purposes due to its texture. Historically, it has been used for tools and weapons by ancient civilizations.

Age Distribution

Ranges from Precambrian to Cenozoic, common in all geological eras with significant volcanic activity.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Igneous Rock

Group

Volcanic (Extrusive)

Crystal System

Not applicable to the rock as a whole; constituent minerals have specific crystal systems (e.g., monoclinic for pyroxene and plagioclase, orthorhombic for olivine).

Chemical Formula

No single chemical formula; composed of various silicate minerals.

Composition

Mafic, rich in iron and magnesium, low in silica. Primary minerals include calcic plagioclase (labradorite), clinopyroxene (augite), and often olivine. Minor constituents include magnetite, ilmenite, and volcanic glass.

Explore Porphyritic Basalt