igneous
Porphyritic basaltic-andesite cobble
Porphyritic andesite to basaltic andesite; plagioclase-rich volcanic rock
AI-generated identification · may be incorrectDark brown-gray, fine-grained groundmass with conspicuous white plagioclase phenocrysts and possible altered feldspar/vesicle fillings; typically hardness 5–6, dull to faintly vitreous luster, and no obvious cleavage in the whole rock.…
Identified More igneous →
Explore Porphyritic basaltic-andesite cobble in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark brown-gray, fine-grained groundmass with conspicuous white plagioclase phenocrysts and possible altered feldspar/vesicle fillings; typically hardness 5–6, dull to faintly vitreous luster, and no obvious cleavage in the whole rock. The rounded shape indicates water abrasion, likely with weathered iron-oxide staining.
Formation & geological history
Formed when intermediate volcanic lava cooled relatively quickly at or near Earth’s surface, preserving larger feldspar crystals in a finer matrix; commonly Cenozoic to older volcanic age depending on locality. The specimen was later transported as a stream or beach cobble.
Uses & applications
Used locally as aggregate, riprap, landscaping stone, and occasionally in polished-stone collections; generally unsuitable for jewelry because of its low uniformity and fractures.
Geological facts
The white crystals are most likely plagioclase, while the dark matrix may contain pyroxene, amphibole, and volcanic glass or microcrystalline feldspar. Precise identification requires a fresh surface, streak, magnetism, and ideally thin-section or X-ray analysis.
Field identification & locations
Look for a dark volcanic matrix containing large, blocky white feldspar crystals; distinguish it from granite by its lack of visible interlocking quartz and mica. Common in volcanic regions worldwide, including the Andes, Cascades, Japan, and island arcs.
