Why do some volcanic eruptions produce lava domes instead of long lava rivers?

Lava domes form when viscous magma, often rich in silica, oozes out slowly and cannot travel far before cooling. Instead of spreading into long streams, the lava stacks up over the vent like a mound. Domes can become dangerous because pressure and hot gas may build inside them, leading to collapses or explosive activity. A well-known example is Mount St. Helens after its 1980 eruption, when a lava dome grew inside the crater.