These are volcanic rocks from southern Bolivia. The black background represents volcanic glass, and this rock is from a short, stubby lava flow, or dome, that flowed out of a fissure and cooled so quickly that it formed glass. The alignment of long axes of minerals like biotite and hornblende show which the lava was flowing at the instant the rock “froze” and if you map those textures in the field, they can define the shape of the lava dome.
Below is a picture of a kind of igneous rock called a gabbro, made of large crystals of intergrown plagioclase and the augite variety of pyroxene. In this case, the tunnel engineers were interested in how the rock texture would affect a tunnel advance rate. Also, the fracture cutting across the crystal boundaries could have an implication for the reported unconfined compressive strength.
