GEOS 1004 Lecture Notes - Lecture 12: Shield Volcano, Subduction, Mafic
Document Summary
Lecture 12: consequences of tectonics: the rock cycle ii - volcanoes. Volcanic eruption: extrusive magma erupts from fissures or conduits, flowing downhill or erupting into the air and settling back down into layers. The style of volcanic eruption depends on the viscosity (resistance to flow) of the magma: low viscosity- runny, easily flows (water has low viscosity, high viscosity- stiff, resists flow; explosive. Viscosity is mainly controlled by presence of silica tetrahedra. (also temperature, gas content. : high content of silica- high viscosity, viscosity controls flow style, felsic- more viscous (ex. olcanic arc above subduction zone, mafic- less viscous. Shield volcano = mafic, low viscosity lava, flows downhill; minimal explosions, gently sloping layers and sides of the volcano; form at hotspots and mid-ocean ridges (rift zones) Generally not dangerous; predictable, limited in size, not explosive. 1 october 2018 strato (composite) volcano = felsic, high viscosity, lava doesn"t flow downhill. Felsic lava (rhyolite) plugs the vent, allowing magma and gases to build.
