Earth's solid mantle flows, just far more slowly than pitch
Nobody has ever drilled through Earth's crust, yet we know the planet has a liquid outer core and a solid inner one. Earthquake waves gave it away. The rock of the mantle is solid, but over vast timescales it creeps and churns, and that slow flow moves the continents.
Geologists describe the interior in two overlapping ways. By chemistry there is a silicate crust, an iron- and magnesium-rich mantle, and an iron-nickel core. By mechanical behaviour there is a rigid lithosphere of crust plus uppermost mantle, a softer asthenosphere on which tectonic plates ride, a stiffer lower mantle, the liquid outer core and the solid inner core. The crust is only 5 to 10 kilometres thick under oceans and up to 70 under continents; the mantle reaches down 2,890 kilometres, making it the thickest layer.
The mantle is solid yet not rigid. Its viscosity ranges from roughly 10 to the power 21 up to 10 to the power 24 pascal-seconds, rising with depth; pitch, famously slow, is around 10 to the power 8. Pressure at its base reaches about 140 gigapascals. Convection in this hot rock drives the motion of the plates above. Mineral grains about 4.4 billion years old show Earth has had a solid crust almost from the start.
Most of this was deduced from seismic waves. Their speeds change between layers, so they bend and bounce like light in a prism or mirror. The Mohorovičić discontinuity marks a sharp jump in wave speed at the base of the crust. Shear waves cannot cross liquid, which reveals the fluid outer core. In 1936 Inge Lehmann discovered the inner core, which transmits shear waves and so must be solid; its boundary lies about 5,150 kilometres down. Laboratory experiments squeezing iron-nickel alloy between diamond tips at around 4,000 kelvin suggest the inner core may consist of giant crystals aligned north to south.
Churning iron in the outer core, steered by Earth's rotation, generates the magnetic field. Inside the core it averages about 2.5 milliteslas, 50 times stronger than at the surface. That field shields life from radiation and keeps the solar wind from stripping away the atmosphere, and one estimate says the core will not freeze for about 91 billion years.
Source: Internal structure of Earth
