Basalt rock is the
most common igneous rock at the Earth's surface.
But it is not seen much because it covers ocean floors.
It is a dark rock, rich in ferromagnesian minerals such as pyroxene
While most often it is
very fine grained, it also happens that it is porphyritic, and it
can have empty cavities from gas or air bubbles, in which case its
texture is vesicular.
Basalt columns at Giant's Causeway in northern Ireland. By
Lena via Flickr.com
It is mostly
known to be extrusive,
it can also sometimes be an intrusive
By Jamie Slomski via Flickr.com
The extrusive version forms from lava flows
from shield volcanoes.
By Kris Williams via Flickr.com
The intrusive version is interesting because it forms by partial melting -
the process where only part of a rock melts and forms a magma that is
then moved upwards.
By Xevi V via Flickr.com
Fragments of unmelted rock can be carried to the
surface with magma, giving us an opportunity to observe the mantle.