Its mass of 4.01 × 1024 kg is 67% the mass of the Earth.[1] It has a thickness of 2,900 kilometres (1,800 mi)[1] making up about 84% of Earth's volume. It is predominantly solid but in geological time it behaves as a viscous fluid. Partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle at subduction zones produces continental crust.[2]