Which Rocks term is correctly associated with rock classification: “a volcanic glass”, and also with “formed by extremely rapid cooling of silica-rich lava”?

Correct Answer: Option A — Obsidian. Obsidian is correct because it is a volcanic glass. Obsidian is also associated with formed by extremely rapid cooling of silica-rich lava. A further identifying fact is that Obsidian is glassy texture with conchoidal fracture. Another useful fact is that Obsidian is associated with historically used for sharp cutting tools. Why the other options are incorrect: Option B — Sandstone: It is a clastic sedimentary rock and is associated with formed by compaction and cementation of sand grains; those are the identifying facts of Sandstone. Option C — Limestone: It is a sedimentary carbonate rock and is associated with commonly formed from calcium carbonate remains or precipitation; those are the identifying facts of Limestone. Option D — Shale: It is a fine-grained sedimentary rock and is associated with formed from compacted mud and clay; those are the identifying facts of Shale. Exam Takeaway: Link Obsidian with “a volcanic glass” and “formed by extremely rapid cooling of silica-rich lava” for quick revision.