Flexi Says: Igneous rocks form from cooling magma. Magma that erupts onto Earth’s surface is lava which is molten rock. The chemical composition of the magma and the rate at which it cools determine what rock forms as the minerals cool and crystallizes. Lava rocks are usually fine-grained (aphanitic) to glassy in texture. For example: basalt is a lava rock with low silica content while rhyolite with high silica content. Pumice stone contains carbon dioxide gas trapped in the stone.