Volcanic activity causes magma (molten rock) to be violently ejected at the earth's surface. If magma cools slowly, as in a lava flow, a dense rock known as basalt may be the result; if cooling is extremely rapid, a volcanic glass such as obsidian may result. Gas bubbles trapped in magma are released in the same way as the carbonation in soft drinks when a container is opened. An extreme example of this is scoria, a dark rock created by the rapid cooling of basaltic magma that is filled with expanding gas bubbles.