Amygdaloidal
basalt
This is a basalt, a dark coloured volcanic rock formed from
a magma of basic composition erupted on the Earth's surface.
Magmas generally contain dissolved gas, which can form
bubbles in the magma as the pressure is released on
eruption. These bubbles can get trapped in the solidified
rock. After some time, groundwater or hot solutions
connected with the volcanic activity pass through the porous
lava and deposit crystals in the open cavities, which
gradually fill up with quartz, calcite (calcium carbonate)
or other minerals called zeolites. Filled cavities in lavas
are called amygdales, and a rock full of them can be called
amygdaloidal. The amygdales are usually white in colour.