Which Rocks term is correctly associated with formation process: “formed by extremely rapid cooling of silica-rich lava”, and also with “glassy texture with conchoidal fracture”?

Correct Answer: Option B — Obsidian. Obsidian is correct because it is formed by extremely rapid cooling of silica-rich lava. Obsidian is also associated with glassy texture with conchoidal fracture. 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 A — Sandstone: It is formed by compaction and cementation of sand grains and is associated with often shows bedding layers; those are the identifying facts of Sandstone. Option C — Limestone: It is commonly formed from calcium carbonate remains or precipitation and is associated with reacts with dilute acid; those are the identifying facts of Limestone. Option D — Shale: It is formed from compacted mud and clay and is associated with splits into thin layers called fissility; those are the identifying facts of Shale. Exam Takeaway: Link Obsidian with “formed by extremely rapid cooling of silica-rich lava” and “glassy texture with conchoidal fracture” for quick revision.