Wall rock

Wall rock is the rock that constitutes the wall of an area undergoing geologic activity. Examples are the rock along the neck of a volcano, on the edge of a pluton that is being emplaced, along a fault plane, enclosing a mineral deposit, or where a vein or dike is being emplaced.

In volcanoes, wall rock can often become broken off of the wall and incoporated into the erupted volcanic rock.