Basalt: Formation & Identification

01 / Origin story

How does basalt form?

Basalt is an extrusive igneous rock, formed when lava cools quickly at the Earth's surface. It's the most common volcanic rock on the planet — most of the ocean floor is basalt, formed continuously at mid-ocean ridges where new crust is created.

02 / Read the clues

How can you identify basalt?

Use several clues together.
One photograph shows one specimen, not every variation.

Enlarged crop of the Basalt specimen — Texture study · enlarged crop of this specimen

01Fine-grained — crystals are too small to see individually, often looks almost glassy or dull
02Dark colored: black, dark gray, or dark green
03Often has small round holes (vesicles) where gas bubbles were trapped as it cooled
04Heavy for its size

03 / Commonly confused with

Similar at first glance.

Compare the materials side by side.
Photographs are not shown to the same scale.

Basalt specimen for comparison — Basalt ↗
Fine-grained — crystals are too small to see individually, often looks almost glassy or dull

Gabbro specimen for comparison — Gabbro ↗
Coarse-grained, like granite or diorite, but dark

Gabbro — chemically nearly identical to basalt, but gabbro cooled slowly underground (intrusive) so it has large visible crystals, while basalt cooled fast at the surface and stays fine-grained. Same recipe, different cooling speed, same relationship as granite/rhyolite.

04 / Out in the world