Complex sentence: Magma from the planet's mantle rises through the planet's crust, if the magma from the mantle reaches the planet's surface it behaves differently depending on the viscosity of the molten constituent rock.
Split into simpler sentences: Magma from the planet's mantle rises through the planet's crust. If magma from the mantle reaches the planet's surface, it behaves differently depending on the viscosity of the molten constituent rock.