igneous
Porphyritic Basalt (or Andesite) Pebble
Porphyritic Mafic Volcanic Rock
Hardness: 6-7 (Mohs scale); Color: Dark grey to greenish-grey matrix with white inclusions; Luster: Dull/Sub-vitreous; Structure: Porphyritic (two distinct grain sizes); Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark grey to greenish-grey matrix with white inclusions
- Luster
- Dull/Sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to greenish-grey matrix with white inclusions; Luster: Dull/Sub-vitreous; Structure: Porphyritic (two distinct grain sizes); Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of volcanic lava on the surface, where larger crystals (phenocrysts, likely plagioclase feldspar) formed slowly at depth before the eruption, while the fine-grained matrix cooled quickly upon eruption. Often smoothed into pebbles by river or beach erosion.
Uses & applications
Commonly used as aggregate in road construction, landscaping decor, and occasionally as a tumbling stone for hobbyists. Larger deposits are crucial for heavy-duty construction material.
Geological facts
The 'polka-dot' appearance is a classic example of a two-stage cooling process in geology. Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor.
Field identification & locations
Identify in the field by looking for a fine-grained, dark background with distinct, rectangular or lath-shaped white crystals. They are commonly found on Pacific Northwest beaches and in volcanic regions globally.
