The compositions of magma have a significant influence on the style and explosiveness of volcanic eruptions. The most common types of magma are basaltic, andesitic, and rhyolitic. Basaltic magma accounts for about 80 percent of all magma erupted by volcanoes. Whereas rhyolitic and andesitic magma accounts for 10 percent each. The viscosity , or the ability to flow, of a magma is dependent upon its temperature and silica and dissolved gas content.
Basaltic magmas are the least viscous and rhyolitic magmas the most viscous.
A volcanic landforms' shape is determined by the composition and characteristics of the volcanic materials and the types of eruptions that have occurred. Shield volcanoes are formed from the build-up of basaltic lava, forming a broad formation that resembles a shield lying horizontally, with an average slope of only a few degrees. Stratovolcanoes , also know as composite volcanoes, are built up of interlayers of andesitic lava and pyroclastic deposits, with very steep slopes. Eruptive Styles Dissolved gases come out of solution during the upward movement of magma, forming bubbles. The melting temperature of a basaltic magma is approximately 1400 degrees C with a silica content in the 50 percent range, giving it a relatively low viscosity.