The evolution of magmatic gases depends on the P-T-X history of the magma. These factors include the composition of assimilated materials and composition of parent rock. Gases develop in magma through two different processes: first and second boiling. First boiling is defined as a decrease in confining pressure below the vapor pressure of the melt. Second boiling is defined as an increase in vapor pressure due to crystallization of the melt. In both cases, gas bubbles exsolve in the melt and aid the ascent of the magma towards the surface. As the magma ascends towards the surface, the temperature and confining pressure decrease. A decrease in temperature and confining pressure will allow an increase in crystallization and vapor pressure of the dissolved gas. Depending on the composition of the melt, this ascent can be either slow or fast. Felsic magmas are very viscous and travel to the surface of the Earth slower than mafic melts whose silica levels are lower. The amount of gas available to be exsolved and the concentrations of gases in the melt also control ascension of the magma. If the melt contains enough dissolved gas, the rate of exsolution will determine the magmas rate of ascension. Mafic melts contain low levels of dissolved gases whereas felsic melts contain high levels of dissolved gases. The rate of eruption for volcanoes of different compositions is not the controlling factor of gas emission into the atmosphere. The amount of gas delivered by an eruption is controlled by the origin of the magma, the crustal path the magma travels through, and several factors dealing with P-T-x at the Earth's surface. When felsic melts reach the surface of the Earth, they are generally very explosive (i.e. Mount St. Helens). Mafic melts generally flow over the surface of the Earth and form layers (i.e. Columbia River Basalt). Magma development under continental crust develops a different type of volcano than magmas that are generated under oceanic crust. Subduction zones produce volcanic island arcs (such as the Aleutian Islands, Alaska) and non-arc volcanism (such as Chile and California). Typically, arc volcanism is more explosive than non-arc volcanism due to the concentrations and amounts of gasses withheld in the magma underground.