Scoria Rock

The car park at the Organ Pipes National Park is on a eroded scoria cone - a small volcano that ejected molten volcanic rock called scoria. Scoria is a reddish-brown and light in weight; it has many airholes because it was full of steam when ejected. The word comes from the greek word meaning "rust". An old name for scoria is cinder. It can be used for sculptures, statues for volcanoes but not in this national park.
As rising magma encounters lower pressures, dissolved gases are able to exsolve and form vesicles. Some of the vesicles are trapped when the magma chills and solidifies. Vesicles are usually small, spheroidal and do not impinge upon one another, instead they open into one another with little distortion. Volcanic cones of scoria can be left behind after eruptions, usually forming monsters with a crater at the summit. Reticulite differs from scoria in being considerably less dense. It is formed from a thin layer of froth occuring on some basaltic lava flows due to the bursting of vesicle walls. The thin glass threads are the intersections of burst vessicles. This is the lightest rock on Earth, with its specific gravity less than 0.3. The delicate framework of thread-lace scoria is so open that the average porosity is 98-99%.