igneous
Vesicular Basalt (commonly mistaken for Slag)
Scoriaceous Basalt (composed primarily of Plagioclase Feldspar and Pyroxene)
Hardness: 5-6 (Mohs scale); Color: Dark grey, black, or reddish-brown; Luster: Dull to vitreous (glassy) in vesicles; Structure: Aphanitic (fine-grained) with large vesicles; Specific Gravity: 2.8-3.0
- Hardness
- 5-6 (Mohs scale)
- Color
- Dark grey, black, or reddish-brown
- Luster
- Dull to vitreous (glassy) in vesicles
Identified More igneous →
Explore Vesicular Basalt (commonly mistaken for Slag) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark grey, black, or reddish-brown; Luster: Dull to vitreous (glassy) in vesicles; Structure: Aphanitic (fine-grained) with large vesicles; Specific Gravity: 2.8-3.0
Formation & geological history
Formed through the rapid cooling of magnesium-rich and iron-rich lava. The holes (vesicles) are created by gas bubbles that were trapped in the magma as it solidified near the Earth's surface.
Uses & applications
Used in construction as road aggregate, railroad ballast, and ground cover in landscaping. Highly porous varieties like scoria are used in gas grills and as lightweight concrete additives.
Geological facts
Vesicular basalt is the most common rock on the surface of the Moon (Lunar Maria) and covers a significant portion of the Earth's ocean floor. If the vesicles fill with minerals like quartz or calcite later, it becomes 'amygdaloidal basalt'.
Field identification & locations
Identify in the field by its high density relative to its holey appearance and dark color. Common in volcanic regions like Iceland, Hawaii, and the Pacific Northwest. Note: If the specimen has circular gas bubbles and a glassy/metallic flow texture, it may be industrial iron-ore slag.
