chapter  5
42 Pages

Volcanoes and Volcanic Eruptions

Viscosity A fl uid’s resistance to fl ow is termed viscosity . Silica-rich magma, like molasses or refrigerated honey, does not fl ow easily and has a high viscosity. In contrast, magma with a relatively low silica content fl ows more easily, like warm honey, and has a low viscosity. Viscosity is affected by temperature as well as composition ( Figure 5.5 ). Basaltic lava, which has a relatively low silica content, has a low viscosity when it erupts onto the surface; however, as it fl ows away from the vent and cools, its viscosity increases, causing the fl ow to move more slowly and change in form.