Volcanic is the adjective from volcano. Its first meaning is plain: belonging to a volcano, made by a volcano, or full of volcanoes — volcanic rock, volcanic ash, volcanic activity, a volcanic island, volcanic soil (which is famously rich, so farmers like it). Geography textbooks, news reports about eruptions, and travel writing about places like Iceland, Hawaii, Japan or Indonesia use it constantly. It has a second, figurative use that students should notice: applied to people or feelings, it means sudden, huge, and hard to control — "a volcanic temper", "volcanic rage", "volcanic applause". The picture is of pressure building quietly and then bursting out all at once, so it suits anger better than gentle feelings; you would not call a soft sadness volcanic. In careful writing there is a small difference worth knowing: volcanic covers everything to do with volcanoes, while geologists use volcanic rock in a narrow way for rock formed from lava that cooled at the surface, as opposed to rock that cooled deep underground. That fine distinction only matters in science class. Note that the adjective drops the o of volcano and adds -ic, so the stress moves: VOL-ca-no but vol-CAN-ic.