You might remember this one from earth science class. An igneous rock is one that forms through intense, fiery heat––usually in a volcano. It starts out molten (so hot it melts into liquid), then solidifies as it cools. So, it’s rock that has “ignited.”

Igneous comes from the Latin ignis "fire." Granite and basalt are good examples of igneous rock that started out as blazing hot lava and morphed into harder stuff as their temperature dropped. FYI, the other main types of rocks are sedimentary and metamorphic and, technically, you could use igneous to describe other things that are rare or fiery, but it would sound pretty stilted and most people wouldn’t know what you meant.

Definitions of igneous

adj produced by the action of fire or intense heat

“rocks formed by igneous agents”
aqueous, sedimentary
produced by the action of water

adj produced under conditions involving intense heat

igneous rock is rock formed by solidification from a molten state”
igneous fusion is fusion by heat alone”
pyrogenic, pyrogenous

adj like or suggestive of fire

“an igneous desert atmosphere”
used of physical heat; having a high or higher than desirable temperature or giving off heat or feeling or causing a sensation of heat or burning

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