Editing Geophysics (section)

=== Structure of the interior ===
{{Main|Structure of Earth}}
[[Image:Earthquake wave paths.svg|thumb|upright=1.3|Seismic velocities and boundaries in the interior of the [[Earth]] sampled by seismic waves |alt=Diagram with concentric shells and curved paths.]]
Evidence from [[seismology]], heat flow at the surface, and [[mineral physics]] is combined with the Earth's mass and moment of inertia to infer models of the Earth's interior – its composition, density, temperature, pressure. For example, the Earth's mean [[specific gravity]] ({{math|5.515}}) is far higher than the typical specific gravity of rocks at the surface ({{math|2.7–3.3}}), implying that the deeper material is denser. This is also implied by its low [[moment of inertia]] ({{math| 0.33 <var>M R</var><sup>2</sup>}}, compared to {{math| 0.4 <var>M R</var><sup>2</sup>}} for a sphere of constant density). However, some of the density increase is compression under the enormous pressures inside the Earth. The effect of pressure can be calculated using the [[Adams–Williamson equation]]. The conclusion is that pressure alone cannot account for the increase in density. Instead, we know that the Earth's core is composed of an alloy of iron and other minerals.<ref name=Poirier/>

Reconstructions of seismic waves in the deep interior of the Earth show that there are no [[S-waves]] in the outer core. This indicates that the outer core is liquid, because liquids cannot support shear. The outer core is liquid, and the motion of this highly conductive fluid generates the Earth's field. [[Earth's inner core]], however, is solid because of the enormous pressure.<ref name=Lowrie>{{harvnb|Lowrie|2004}}</ref>

Reconstruction of seismic reflections in the deep interior indicates some major discontinuities in seismic velocities that demarcate the major zones of the Earth: [[Earth's inner core|inner core]], [[Earth's outer core|outer core]], mantle, [[lithosphere]] and [[crust (geology)|crust]]. The mantle itself is divided into the [[upper mantle (Earth)|upper mantle]], transition zone, lower mantle and ''D′′'' layer. Between the crust and the mantle is the [[Mohorovičić discontinuity]].<ref name=Lowrie/>

The seismic model of the Earth does not by itself determine the composition of the layers. For a complete model of the Earth, mineral physics is needed to interpret seismic velocities in terms of composition. The mineral properties are temperature-dependent, so the [[geotherm]] must also be determined. This requires physical theory for [[thermal conduction]] and [[convection]] and the heat contribution of [[radionuclides|radioactive elements]]. The main model for the radial structure of the interior of the Earth is the [[preliminary reference Earth model]] (PREM). Some parts of this model have been updated by recent findings in mineral physics (see [[post-perovskite]]) and supplemented by [[seismic tomography]]. The mantle is mainly composed of [[silicates]], and the boundaries between layers of the mantle are consistent with phase transitions.<ref name=Poirier/>

The mantle acts as a solid for seismic waves, but under high pressures and temperatures, it deforms so that over millions of years it acts like a liquid. This makes [[plate tectonics]] possible.