Basalt is a fine-grained volcanic rock which forms from lava that has been extruded above or near the surface and is fluid enough to flow. When a thick lava flow cools it shrinks, sometimes causing it to fracture in a regular pattern, creating the columns seen in this specimen. The process is analogous to the shrinkage cracks, which form in mud as it dries. Basalt’s mineral composition is different from dacite (seen in the other Rock Garden columns), causing basalt to be darker in colour.
Peridotite xenoliths are fragments of rock, which solidified earlier and at a higher temperature, that were swept up and incorporated in the lava.