II.

Types of Lava

Most lava, on cooling, forms silicate rocks—rocks that contain silicon and oxygen. Lava is classified according to which silicate rocks it forms: basalt, rhyolite, or andesite. Basaltic lava is dark in color and rich in magnesium and iron, but poor in silicon. Rhyolitic lava is light colored and poor in magnesium and iron, but rich in silicon. Andesitic lava is intermediate in composition between basaltic and rhyolitic lava. While color is often sufficient to classify lava informally, formal identification requires chemical analysis in a laboratory. If silica (silicon dioxide) makes up more than 65 percent of the weight of the lava, then the lava is rhyolitic. If the silica content is between 65 percent and 50 percent by weight, then the lava is andesitic. If the silica content is less than 50 percent by weight, then the lava is basaltic. See also Igneous Rock: Classification by Composition.

Many other physical properties, in addition to color, follow the distinctions between basaltic, andesitic, and rhyolitic lava. For example, basaltic lava has a low viscosity, meaning it is thin and runny. Basaltic lava flows easily and spreads out. Rhyolitic lava has a high viscosity and oozes slowly like toothpaste. The viscosity of andesitic lava is intermediate between basaltic and rhyolitic lava. Similarly, basaltic lava tends to erupt at higher temperatures, typically around 1000° to 1200° C (1800° to 2200° F), while rhyolitic lava tends to erupt at temperatures of 800° to 1000° C (1500° to 1800° F). Dissolved gases make up between 1 percent and 9 percent of magma. These gases come out of solution and form gas bubbles as the magma nears the surface. Rhyolitic lava tends to contain the most gas and basaltic lava tends to contain the least.