igneous
Basalt Porphyry
Porphyritic Basalt
Hardness: 6 (Mohs scale); Color: Dark grey to black groundmass with light-colored (white or cream) phenocrysts; Luster: Dull or sub-metallic; Crystal structure: Aphanitic (fine-grained) matrix with visible large macro-crystals; Cleavage: Generally poor; Specific Gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Luster
- Dull or sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black groundmass with light-colored (white or cream) phenocrysts; Luster: Dull or sub-metallic; Crystal structure: Aphanitic (fine-grained) matrix with visible large macro-crystals; Cleavage: Generally poor; Specific Gravity: 2.8–3.0
Formation & geological history
Formed from volcanic activity where magma begins cooling slowly underground, allowing large crystals (phenocrysts) to form, followed by a rapid eruption and quick cooling of the remaining molten material. Found in lava flows from the Cenozoic to the Precambrian eras.
Uses & applications
Used primarily in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Distinctive specimens are prized by rockhounds and used for ornamental paving or decorative garden stones.
Geological facts
The term 'porphyry' comes from the Ancient Greek word for purple, as the first specimens described were a deep purple color with white crystals. This rock specifically demonstrates two distinct cooling rates in a single volcanic event.
Field identification & locations
Identify in the field by its 'chocolate chip cookie' appearance: a dark, dense, heavy basaltic background with light-colored, angular crystals embedded within. Commonly found in volcanic regions such as the Pacific Northwest, Hawaii, and Iceland.
