When emerging on the surface, lava is incandescent and about 1,300 to 2,200 degrees Fahrenheit. It can be as liquid as a syrup and flow rapidly or stiff, almost not flowing. Lava's viscosity depends mainly on its silica content. Lavas with higher silica content often show greater viscosity. Lava flows advance slowly, often less than a meter per day.
Mafic or basaltic lavas, such as Hawaiian aa and pahoehoe lavas, are high in magnesium, calcium and iron, with lower silica and aluminum content, in comparison to other lava types. Andesitic or intermediate lavas, which occur in the Andes region, are rich in aluminum and iron. Dacite and rhyolite lavas are high in silica and sodium and low in iron and manganese. Mount St Helens, Washington, is an example of volcano that produces dacite lavas. Trachyte lavas are rich in aluminum, sodium and potassium. Mount Kilimanjaro, in Africa, is an example of this type of lava.
Magma, which is lighter than the solid rock above it, rises to the earth's surface due to the higher pressure and temperature in the interior of the earth. Some volcanic eruptions are more explosive than others. It depends on chemical composition and quantity of gases present in the magma. Explosions are often more violent when the magma is more viscous, because gases cannot escape, and help to build up pressure. More dangerous than the lava flows are the pyroclastic flows, which often happen during a volcanic eruption. A pyroclastic flow is a mixture of ash, gases and solid volcanic fragments that can travel at 60 miles per hour.
Repeated eruptions increase the growth of a volcano. After an eruption, the lava solidifies, adding new mineral layers to the exterior of a volcano. Volcanoes are classified into three main shapes: stratovolcanoes, which feature symmetrical cones with steep sides; cinder cones, which are a result of lava that blasts into the air and breaks into small pieces; and shield volcanoes, which have broad, sloping sides, resembling the shield of a warrior.