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Vesicular rhyolite is a light-coloured volcanic rock containing small holes, or vesicles, formed by gas bubbles trapped in cooling lava. It may resemble compressed pumice and can contain visible white feldspar crystals.
Volcanic igneous: Formed when silica-rich lava cooled and solidified near the Earth’s surface.
Source location: Banks Peninsula
Sizes: Select Small specimen, Hand specimen or Shelf specimen from the drop-down menu.
Note: Images are examples of specimens only. Sizes, shapes, colours and textures will vary. Some samples have an orange-brown rind on one surface.
More detail
Structure
Vesicular rhyolite has a fine-grained or partly glassy groundmass containing small gas cavities. Some specimens also contain larger white feldspar crystals within the finer rock.
Type
Volcanic (extrusive) igneous rock – a porous form of rhyolite, which is the volcanic equivalent of granite. Very porous specimens may grade into pumice.
Physical properties
Colour: Usually cream, white, pale grey, pink or light brown, sometimes with reddish or greenish tones.
Lustre: Usually dull or slightly glassy. Some pieces may develop a gentle sheen when polished.
Hardness: Generally hard, although porous, weathered or glassy surfaces may scratch more easily.
Density: Usually lighter than solid rhyolite because of its gas cavities. Weight varies with the amount of porosity.
Fracture: Uneven around the cavities. Glassy areas may break with curved or sharp edges.
General properties
Composition: Mainly silica-rich volcanic material, commonly containing quartz and feldspar, with smaller amounts of volcanic glass and other minerals.
Texture: Fine-grained, porous and sometimes porphyritic. The cavities may be rounded, stretched or slightly flattened by lava movement.
Durability: Compact pieces are reasonably durable, but porous or weathered specimens can be more fragile than dense rhyolite.
Weight: Generally lighter than solid rhyolite, although individual specimens vary considerably.
Water Absorption: Dense pieces absorb little water, while open cavities and cracks can allow water to enter.
Uses in New Zealand and worldwide
Collector’s Specimens: Valued for its light colour, gas cavities and visible crystals.
Geological Displays: Useful for demonstrating gas bubbles, crystal growth and lava movement in volcanic rocks.
Landscaping: Compact pieces can be used in rockeries, gardens and decorative displays.
Aggregate and Fill: Crushed rhyolite may be used for aggregate, road material or fill where its strength is suitable.
Lightweight Volcanic Material: Very porous rhyolitic material may be used as lightweight aggregate or an abrasive.
Geological Interest: Studied to understand viscous lava flows, pumice formation and the volcanic history of Banks Peninsula.