Question - Why are the viscosity and gas content of a magma important in determining the type of volcanic rocks that will be formed when an eruption takes place?

Why are the viscosity and gas content of a magma important in determining the type of volcanic rocks that will be formed when an eruption takes place?

Answer:

Viscosity controls the degree to which gases can escape from magma and whether or not magma will be able to escape to surface.  Viscous magma cannot flow easily through small openings and will also retain gases. Those factors will lead to an increase in pressure. When something breaks there will be an explosive eruption and the resulting rock will have a pyroclastic texture. Non-viscous magma can escape and release gases more easily, and so the magma is more likely to flow out in an effusive eruption creating a lava texture.