According to the United States Geological Survey, before a volcanic eruption, magma must travel upwards through rock. The travelling magma produces these precursors to an eruption: an increase in earthquakes beneath a volcano, volcanic tremors, disturbance of ground features, increased release of steam from the volcano, increased temperatures and increase of gases. Additionally, different catalysts occur beneath the earth's surface prior to an eruption.
There are three different causes of volcanic eruptions. Different things happen before each eruption, depending on the type. The first type involves rock melting beneath the earth's surface. The melted rock, or magma, increases in volume and rises to the surface, where it erupts when the surrounding rock is more dense than the magma itself. Another type of eruption involves water, sulfur dioxide and carbon dioxide mixed with magma. These elements become gaseous as magma reaches the surface, and the gases cause eruptions. As magma with water rises, the solubility of the water decreases, causing it to form gas bubbles within the magma. The higher magma rises, the more gas bubbles appear, until the high gas quantities force an eruption. Lastly, when new magma reaches an existing magma chamber, the existing magma is forced out of the chamber in an eruption.