We spend our lives pressed against it, and take its stability for granted. To us, Earth is the most solid foundation of all our endeavours. It provides us with various materials to build our tools from, and anchors all our structures. To contain or separate, we use walls, made from the solid substances available to us.
This kind of solidity is useful, but it has limits. Even the most rigid materials will bend and sway like rubber in large scale constructs. To move the Earth, Archimedes would need more than a place to stand on, he would need a lever of supernatural solidity. At large scales, all known materials, even so-called solids, will start behaving like fluids.
The spheroid form of a planet is the result of the fluid-like behaviour of its substance. The surface of the Earth pushes against our feet not because it is so rigid, but because the combined pressure of the planet’s own weight cannot compress the atoms inside to any smaller volume. There is nowhere for the pressure to go except outwards, and by diffusion it all evens out into a spherical form, or actually concentric spheres of different pressures and densities. Pressure increases towards the middle of the planet, up to millions of times normal atmospheric pressure; but at the same time the relative pull of gravity diminishes to none in the middle.
At the surface, where gravity is strongest, the layering is present all around us, even where we do not see it: in the strata under our feet, or above us in layers of clouds moving in different directions. The seas have layers also, with surface currents at odds with deeper ones.
Pressure is nothing more than the combined effect of countless billions of atoms colliding with one another. In free-fall, gravity gives each particle equal acceleration regardless of mass, as famously demonstrated by Galileo. But equal speed gives the larger particle a larger momentum, which is what matters in collisions.
The separation of different materials to concentric layers is the result of the continuous struggle, the harmony of battle lines. The way “up” and “down” is one and the same, only relative to the pressure of the opposing forces. Indeed, many substances circulate endlessly between layers, like the water cycle which we depend on.
As it happens, the zero point of gravity is actually never in the exact middle of our planet. The combined center of mass of the Earth-Moon system is inside the surface of Earth, but off-center towards where the Moon happens to be at that moment. As the Earth rotates, the change in direction of the pull of the Moon causes water tides in surface waters, but also tidal movement of the earth itself.
Particles do not have to be invisibly small to behave collectively like a fluid. All that is needed is some way for the particles to flow past each other. When a container of different size objects, such as flour, nuts, or sand, is jostled or shaken, the contents get shifted so that the larger pieces are on top, and smaller ones at the bottom. Continuous vibration helps overcome static friction, but the resulting fluid is still quite viscous. It should be noted that with these kind of every-day particles, the size of the container relative to particle size would be called capillary with molecular fluids.
In our solar system, we can see an even more spectacular example of solid objects arranging themselves to concentric layers over time: the rings of Saturn. In space, particles do not need to actually touch to interact with each other, they can affect each other’s orbits just by passing closely.