Adjective
1. igneous rock produced by eruption and solidified on or near the earth's surface; rhyolite or andesite or basalt;
"volcanic rock includes the volcanic glass obsidian"2. of igneous rock that has solidified beneath the earth's surface; granite or diorite or gabbro
3. erupting or tending to erupt;
"a geyser is an intermittently eruptive hot spring"