Volcanic rock is any rock formed when magma from within the Earth is expelled onto the surface through volcanic eruptions and quickly cools.
Also called extrusive igneous rock, it typically has a fine-grained or glassy texture because the molten material solidifies too quickly for large crystals to form.
Common examples include basalt, obsidian, and pumice.
How it forms
Magma rises to the surface:
Molten rock, known as magma, rises from beneath the Earth's surface through volcanic vents or fissures.
Eruption and cooling:
Once the magma reaches the surface, it is called lava.
The rapid exposure to the atmosphere and surrounding temperatures causes the lava to cool and solidify very quickly.
Solidification into rock:
This rapid cooling prevents the formation of large crystals, resulting in a fine-grained or even glassy texture.
Characteristics
Fine-grained or glassy texture:
Due to rapid cooling, the crystals are often too small to be seen with the naked eye.
Vesicles:
Gas bubbles that were trapped in the lava can remain in the solidified rock, creating small holes called vesicles.
Porphyritic structure:
Some volcanic rocks may contain a few large, well-formed crystals (phenocrysts) within a finer-grained groundmass, a texture known as porphyritic.
Examples
Basalt:
A very common volcanic rock with a dark color and low silica content.
Obsidian:
A black, volcanic glass formed by extremely rapid cooling.
Pumice:
A light-colored, frothy volcanic rock with many vesicles that can even float on water.
Andesite:
A grey volcanic rock that is intermediate in composition and silica content.
Rhyolite:
A volcanic rock with a composition similar to granite, but with a much finer texture.
Tuff:
A type of pyroclastic rock formed from volcanic ash that settles back to the surface after an eruption.
