a 18 The portion of the Earth C a b Lithosphere c Asthenosphere d Both a b 19 Tectonic plates are actively separating in all of the following EXCEPT a Rift zones b Sea floor spreading areas c Subducting plate boundaries d The mid ocean ridges 20 The force that causes volcanoes to explode violently is Hot convection currents in the asthenosphere that pushes magma upward a b The large amount of iron found within mafic volcanoes c Expanding pressure from volcanic gases called volatiles d Low viscosity magmas 21 The more silica in the magma the more likely a A powerful eruption will occur b The lava erupted will flow easily causing hazards The more volatile gasses can escape readily d The lava will have low viscosity 22 A volcano can spew several kinds of gas into the air but most of the gas is Water vapor and carbon dioxide Crust and entrained mantle b Nitrogen and Sulphur dioxide c Sulphur dioxide and chlorine d Nitrogen that why it s the most common gas 71 in atmosphere 23 As seen in the video when the amount of Sulphur dioxide emissions are measured COSPEC suddenly drops at an active volcano this indicates that a The vent is blocked and pressure is building b The magma has not been contaminated c Sulphur dioxide has been replaced by water vapor d We can breathe easier the volcano is unlikely to erupt