Scoria is a term used by geologists to describe an igneous rock containing many gas bubbles, or vesicules.
Scoria forms when magma rich in dissolved gases is vented. As the magma encounters lower pressures, the gasses are able to escape and form bubbles. These bubbles are trapped when the magma cools and solidifies.
Volcanic cones of scoria can be left behind after eruptions, usually forming mountains with a crater at the summit. An example is Mount Wellington, Auckland in New Zealand.
Another material can be formed during burning eruption of a mud volcano, when heated mud forms scoria cones.