The magnetic field observed at the Earth's surface is produced by internal and external sources. The internal component originates in the liquid outer core, where large convection cells in the iron-nickel alloy work much like a dynamo to produce an approximately dipolar field. The north and south magnetic poles are aligned roughly with the spin axis as a result of the mechanical interaction between the Earth's rotation and core convection. The magnetic field produced outside the core is electromagnetically filtered by the intervening mantle. While the mantle comprises mainly silicate minerals such as olivine and pyroxene, it possesses appreciable electrical conductivity.