When a lava freezes extremely quickly at the Earth's surface so that no crystals are able to form, what rock will be created?

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BlackTom AI · Accepted Answer

The question asks which rock forms when lava cools extremely rapidly at the surface, so that no crystals can form.
Option 1: Granite. Granite is an intrusive igneous rock that crystallizes slowly underground, resulting in visible mineral grains. This does not match rapid surface cooling, so granite is incorrect.
Option 2: Obsidian (Glass). Obsidian is a volcanic glass produced when felsic to intermediate lava erupts or extrudes at the surface and cools so quickly that mineral crystals fail to develop. This description fits the scenario of no crystal formation due to rapid cooling, making obsidian a correct match.
Option 3: Gabbro. Gabbro is an intrusive igneous rock that forms when magma cools slowly underground, yielding a coarse-grained texture. It clearly does not form from rapid surface cooling, so it is incorrect.
Option 4: Basalt. Basalt is an extrusive igneous rock that forms from rapid cooling at the surface, but it typically contains small crystals (aphanitic texture) rather than being a completely glassy solid. While rapid cooling is involved, the complete absence of crystals described in the question points away from basalt. Therefore, basalt is not the best match for “no crystals.”

FA25: GEOL-103 Sec 02 - Earth Systems Quiz 1.2: Igneous Rocks (Due 9/23)

When a lava freezes extremely quickly at the Earth's surface so that no crystals are able to form, what rock will be created?

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