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Porphyritic andesite 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 typically consist of plagioclase feldspar (often labradorite or andesine), pyroxenes (augite, hypersthene), hornblende, and sometimes biotite. The groundmass is generally aphanitic (fine-grained) to cryptocrystalline, and can range in color from light gray to dark gray, reddish-brown, or black. Its intermediate silica content places it between basalt and dacite.
How to Identify
- Color
- Typically light to dark gray, reddish-brown, or black. The color is influenced by the proportion of mafic minerals and the degree of alteration.
- Luster
- Dull to earthy in the groundmass; phenocrysts may exhibit vitreous to pearly luster.
- Texture
- Porphyritic, with visible phenocrysts (typically 1 mm to several cm) set in a fine-grained to aphanitic groundmass. The groundmass may sometimes be glassy.
- Crystal Form
- Phenocrysts are typically euhedral to subhedral, often prismatic (e.g., plagioclase, hornblende, pyroxene).
- Cleavage
- Not a characteristic of the rock as a whole, but individual phenocrysts will exhibit their characteristic cleavage (e.g., plagioclase: two directions at ~90 degrees; pyroxene: two directions at ~90 degrees; hornblende: two directions at ~56 and ~124 degrees).
- Geological Environment
- Volcanic arcs associated with subduction zones (e.g., Andes Mountains, Pacific Ring of Fire). It forms in stratovolcanoes and lava flows.
Key Facts
- Hardness: 6-7 (Mohs scale, for constituent minerals like plagioclase and pyroxene; rock as a whole is tough).
- Specific Gravity: 2.5-2.8 g/cm³.
- Crystal System: Not applicable for the rock as a whole; individual phenocrysts have their own crystal systems (e.g., plagioclase: triclinic; pyroxene: monoclinic or orthorhombic; hornblende: monoclinic).
- Color: Light to dark gray, reddish-brown, black.
- Luster: Dull to earthy (groundmass), vitreous to pearly (phenocrysts).
- Transparency: Opaque (rock), translucent to opaque (phenocrysts).
- Fracture: Conchoidal to irregular.
- Cleavage: Not a rock property; phenocrysts exhibit characteristic cleavage.
- Composition: Intermediate igneous rock, primarily composed of plagioclase feldspar (andesine to labradorite), pyroxenes (augite, hypersthene), and/or hornblende. Minor amounts of biotite, magnetite, and quartz may be present. Silica content: 52-63 wt% SiO2.
Quick Check
- Color: Gray to dark gray, reddish-brown, or black.
- Luster: Dull to earthy groundmass, vitreous to pearly phenocrysts.
- Streak: White to light gray (not typically tested for rocks).
Physical Characteristics
- Crystal Habit: Phenocrysts are typically euhedral to subhedral, often prismatic or tabular. Groundmass is aphanitic to cryptocrystalline.
- Cleavage Type: Not applicable for the rock; phenocrysts show distinct cleavage (e.g., plagioclase: perfect in two directions).
- Fracture Type: Conchoidal to irregular, depending on the grain size and mineralogy.
- Tenacity: Brittle.
- Luster Type: Dull to earthy for the groundmass; vitreous to pearly for phenocrysts.
Formation
Porphyritic andesite forms from the rapid cooling of intermediate-composition magma (52-63 wt% SiO2) that has undergone a two-stage cooling process. Initially, slow cooling at depth allows for the growth of larger, well-formed crystals (phenocrysts). Subsequent eruption and rapid cooling at or near the Earth's surface result in a fine-grained to aphanitic groundmass surrounding these phenocrysts.
Usage
Historically, andesite has been used as a building material, for road aggregate, and in some cultures, for tools and sculptures. Its hardness and durability make it suitable for construction. Modern uses are similar, primarily as crushed stone for construction, road base, and fill.
Age Distribution
Common in Cenozoic and Mesozoic volcanic arcs, but can occur in any geological period with appropriate tectonic settings.
Where to Find
Andes Mountains, South America
The type locality for andesite, found extensively along the entire length of the mountain range, associated with active and extinct volcanoes.
Pacific Ring of Fire
Numerous occurrences in volcanic arcs around the Pacific Ocean, including Japan, Indonesia, the Philippines, Alaska, and the Cascade Range (USA).
Aegean Volcanic Arc, Greece
Present in volcanic islands such as Santorini and Milos.
Caribbean Volcanic Arc
Found in islands like Montserrat, Martinique, and St. Vincent.
Finding Tips
Look for Volcanic Terrains
Porphyritic andesite is an extrusive igneous rock, so it will be found in areas of past or present volcanic activity, particularly stratovolcanoes and lava flows.
Examine Outcrops for Texture
Look for rocks with a distinct two-stage texture: larger, well-formed crystals (phenocrysts) embedded in a much finer-grained matrix. Plagioclase phenocrysts are often white or light-colored and tabular.
Check for Associated Minerals
The presence of plagioclase, pyroxene, and hornblende phenocrysts is characteristic. Quartz phenocrysts are rare or absent in true andesite, distinguishing it from dacite.
Consider Color and Density
Andesite is typically intermediate in color (mesocratic to leucocratic) and density, falling between basalt (darker, denser) and rhyolite (lighter, less dense).
Similar Rocks
Basalt
Basalt
Also known as: Volcanic rock
Dacite
Dacite
Also known as: Quartz Andesite
Rhyolite
Rhyolite
Also known as: Acidic volcanic rock
Diorite
Diorite
Also known as: Plutonic equivalent of andesite
Scientific Classification
- Mineral Class
- Igneous Rock (Extrusive)
- Group
- Volcanic Rocks
- Crystal System
- Not applicable for the rock; constituent minerals have their own systems.
- Chemical Formula
- Variable, complex silicate composition (no single formula).
- Composition
- Intermediate, calc-alkaline igneous rock. Dominant minerals are plagioclase feldspar (andesine-labradorite), pyroxenes (augite, hypersthene), and/or hornblende. Minor biotite, magnetite, and quartz may be present. Silica content ranges from 52% to 63% by weight.
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