Igneous
Porphyritic Andesite or Dacite (tumbled pebble)
Porphyritic Igneous Rock (likely Andesite or Dacite)
Hardness: 6-7; Color: Light gray-green groundmass with darker grayish/black and lighter phenocrysts; Luster: Dull (unless polished); Crystal structure: Porphyritic (larger crystals in a fine-grained matrix).
- Hardness
- 6-7
- Luster
- Dull (unless polished)
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Physical properties
Hardness: 6-7; Color: Light gray-green groundmass with darker grayish/black and lighter phenocrysts; Luster: Dull (unless polished); Crystal structure: Porphyritic (larger crystals in a fine-grained matrix).
Formation & geological history
Formed by extrusive volcanic activity where magma cooled in two stages: slowly underground to form the larger phenocrysts, then rapidly on the surface to form the fine-grained matrix. Often found in volcanic arcs.
Uses & applications
Commonly used as crushed stone, in landscaping, or as aggregate in construction.
Geological facts
The term 'porphyritic' refers to the distinct difference in crystal sizes, indicating a complex cooling history.
Field identification & locations
Identified by its 'chocolate chip cookie' appearance, with distinct larger crystals embedded in a fine-grained background. Found in riverbeds or beaches near volcanic terranes.
