Igneous
Vesicular Basalt
Amygdaloidal Basalt (Mafic extrusive volcanic rock)
Hardness: 5-6 (Mohs); Color: Dark grey to reddish-brown (weathered) with light-colored mineral infills; Luster: Dull (extrusive) to vitreous (minerals); Structure: Vesicular/Aphanitic; Cleavage: None/Irregular
- Hardness
- 5-6 (Mohs)
- Luster
- Dull (extrusive) to vitreous (minerals)
Identified More igneous →
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey to reddish-brown (weathered) with light-colored mineral infills; Luster: Dull (extrusive) to vitreous (minerals); Structure: Vesicular/Aphanitic; Cleavage: None/Irregular
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. The holes (vesicles) were formed by gas bubbles trapped in the lava during eruption. Amygdules form when secondary minerals like zeolites or calcite precipitate in these voids over millions of years.
Uses & applications
Used primarily in construction as crushed stone, road base, or concrete aggregate. Vesicular varieties are often used in landscaping (lava rock) and occasionally for geological education/collection.
Geological facts
Basalt is the most common rock on Earth's surface and makes up most of the ocean floor. It is also found on the Moon and Mars, forming the dark plains known as 'maria'.
Field identification & locations
Identify by its dark color, heavy weight, and distinct 'sponge-like' holes. Found in volcanic regions like the Pacific Northwest (USA), Iceland, and Hawaii. In the field, look for 'frothy' textures near volcanic vents.
