Background: 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 (5.515) is far higher than the typical specific gravity of rocks at the surface (2.7–3.3), implying that the deeper material is denser. This is also implied by its low moment of inertia ( 0.33 M R2, compared to  0.4 M R2 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.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.Reconstruction of seismic reflections in the deep interior indicate some major discontinuities in seismic velocities that demarcate the major zones of the Earth: inner core, outer core, mantle, lithosphere and crust. The mantle itself is divided into the upper mantle, transition zone, lower mantle and D′′ layer. Between the crust and the mantle is the Mohorovičić discontinuity.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 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.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.

Situation: John is a geologist. He was trying to figure out how the Earth's inner core is made. To that end, he compared earth's physical characteristics to available items. First he took a sample of surface rock, which he labeled as item B. Then he took a sphere of constant density, which he labeled as item C. Finally, he labeled the Earth as item A.

Question: Which would have higher specific gravity, item A or item B?
Answer: item A

Situation: John is a geologist. He was trying to figure out how the Earth's inner core is made. To that end, he compared earth's physical characteristics to available items. First he took a sample of surface rock, which he labeled as item B. Then he took a sphere of constant density, which he labeled as item C. Finally, he labeled the Earth as item A.

Question: Which would have lower specific gravity, item A or item B?
Answer: item B

Situation: John is a geologist. He was trying to figure out how the Earth's inner core is made. To that end, he compared earth's physical characteristics to available items. First he took a sample of surface rock, which he labeled as item B. Then he took a sphere of constant density, which he labeled as item C. Finally, he labeled the Earth as item A.

Question: Would item A have higher or lower specific gravity than item B?
Answer: higher

Situation: John is a geologist. He was trying to figure out how the Earth's inner core is made. To that end, he compared earth's physical characteristics to available items. First he took a sample of surface rock, which he labeled as item B. Then he took a sphere of constant density, which he labeled as item C. Finally, he labeled the Earth as item A.

Question: Would item B have higher or lower specific gravity than item A?
Answer: lower

Situation: John is a geologist. He was trying to figure out how the Earth's inner core is made. To that end, he compared earth's physical characteristics to available items. First he took a sample of surface rock, which he labeled as item B. Then he took a sphere of constant density, which he labeled as item C. Finally, he labeled the Earth as item A.

Question: Which would have higher moment of intertia, item A or item C?
Answer: item C

Situation: John is a geologist. He was trying to figure out how the Earth's inner core is made. To that end, he compared earth's physical characteristics to available items. First he took a sample of surface rock, which he labeled as item B. Then he took a sphere of constant density, which he labeled as item C. Finally, he labeled the Earth as item A.

Question: Which would have lower moment of intertia, item A or item C?
Answer: item A

Situation: John is a geologist. He was trying to figure out how the Earth's inner core is made. To that end, he compared earth's physical characteristics to available items. First he took a sample of surface rock, which he labeled as item B. Then he took a sphere of constant density, which he labeled as item C. Finally, he labeled the Earth as item A.

Question: Would item A have higher or lower moment of inertia than item C?
Answer: lower

Situation: John is a geologist. He was trying to figure out how the Earth's inner core is made. To that end, he compared earth's physical characteristics to available items. First he took a sample of surface rock, which he labeled as item B. Then he took a sphere of constant density, which he labeled as item C. Finally, he labeled the Earth as item A.

Question: Would item C have higher or lower moment of inertia than item A?
Answer: higher

Situation: John is a geologist. He was trying to figure out how the Earth's inner core is made. To that end, he compared earth's physical characteristics to available items. First he took a sample of surface rock, which he labeled as item B. Then he took a sphere of constant density, which he labeled as item C. Finally, he labeled the Earth as item A.

Question: Which would be denser, item A or item B?
Answer: item A

Situation: John is a geologist. He was trying to figure out how the Earth's inner core is made. To that end, he compared earth's physical characteristics to available items. First he took a sample of surface rock, which he labeled as item B. Then he took a sphere of constant density, which he labeled as item C. Finally, he labeled the Earth as item A.

Question: Which would be lighter, item A or item B?
Answer: item B