Hierarchy classification:

1. top of the aat hierarchies
2. Materials Facet
3. Materials (hierarchy name)
4. materials (substances)
5. [materials by composition]
6. inorganic material
7. rock (inorganic material)
8. igneous rock
9. [igneous rock by origin]
10. extrusive rock
11. pyroclastic rock
12. volcanic glass

Anotations

Scope note:

A volcanic glass that contains about 75% silica and usually has a higher water content than obsidian; found in Oregon and California. Commercially, the term refers to volcanic glass that will expand or "pop" when heated to form a lightweight aggregate.