Build Your Own Volcano!
Some volcanoes explode with the force of an atomic bomb. Others spill rivers of gently flowing lava. Why?
As magma rises, dissolved gases expand and bubbles form. Just like when you open a soda bottle, these bubbles escape when the volcano erupts and the pressurized magma is released. The more bubbles under pressure, the more explosive the eruption.
Magma contains the chemical compound silica. More silica makes magma more viscous (thick and gooey), so it's stiffer and flows more slowly, like molasses or tar. Lava with less silica is runnier and flows more easily.