The two basic types of volcanic eruptions are effusive and explosive eruptions. In effusive eruptions, magma flows to the surface and down the sides of the volcano as lava. In explosive eruptions, magma violently rips through the top portion of the volcano and reaches the surface in pieces.
Gases in the magma chamber determine whether an eruption is explosive or effusive. As magma rises to the surface, so do bubbles created by the gases. Sometimes these bubbles pop when magma reaches the surface. When they pop, they send fractures of magma and other materials in every direction. At other times, no bubbles are formed, and lava flows gently down the volcano's slopes.