why Earth’s internal structure varies between viscous (mantle), liquid (outer core) or solid (inner core), seemingly without relationship to depth?

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Also, what is meant by liquid, viscous? Are we talking water-like liquid, oily/gelly-like for viscous?

In: Earth Science

Humans have only drilled 7.67 miles into Earth’s crust at the deepest site known as the Kola Superdeep Borehole. The Earth’s crust ranges from an estimated 3-43 miles thick. Most of the “known” composition of Earth and other planets is an ‘educated guess’ based on studying how seismic and sound waves travel through the Earth.

The ELI5 is we don’t know why the Earth has different matter phase states in the different levels but we know that the deeper you go, the higher the temperature and we know the makeup of the top most layer and how sound waves travel through the layers so we presume based on the chemical makeup, pressure, and temperature what phase state the layers are in.

The structure is due to temperature and pressure.

Essentially, below the crust is molten rock, at varying temperatures. Closer to the crust, the rock is cooler, and therefore denser and more viscous. See lava for consistency.

Further below, the rock is hotter and therefore less dense, which reduces it’s viscosity.

At the core, there is a lot of pressure generated by the weight of rock. This pressure squeezes the material tightly, and despite the high temperatures, this force is enough to force the material into a solid.