First, if you don’t mind, let us review what (on the molecular level) is a solid as in a solid rock, and what isn’t (say, a liquid such as water): the molecules within a liquid have enough thermal energy (that weakens intermolecular forces) so they can move about freely, but not so much that they can escape as in a gas; molecules in a solid are held strongly in place by intermolecular forces that keep them firmly joined. Thus, liquids flow and solids don’t. (Yes, there are solids like obsidian and glass that have liquid-like molecular structures, but the differences between them and real liquids, even on a Wikipedia level, are nearly incomprehensible without a good grounding in materials science, so let’s not go there.)
Turning a solid rock into magma (a solid + liquid +gas mix) will flow because the fluid phase containing the solid bits flows, carrying the solids with it; in terms of our present discussion, this is cheating – it's best to describe magmas as transporting solid rocky material or minerals, the rocky bits themselves don’t flow.
No matter how hard you or I push against two ends of a rock, we’re not going to get it to bend or flow. However, there’s a whole class of geoscientists, affectionately known as “rock squeezers” to the rest of us, who do just this: they insert rocks into cylindrical pressure vessels, apply enormous forces to each end (emulating the tectonic forces found deep in the earth), vary temperatures, sometimes even try to twist the cylinder, and then wait around until the rock breaks (or their equipment fails) and see what happened to it. Broadly, any change in shape of their sample (ie, deformation) is due to actual rock flow, and then comes the fun part of inspecting the sample to see just how it did this.
As every mineralogy student learns, without generally knowing why, the component molecules within a mineral are arranged in a framework called a crystal lattice structure: depending on which molecules and atoms are involved, these frameworks have specific geometries that allow for the best fit of their molecules in a certain space. If a molecule is too big or too small or too different in electric charge to be held in the framework, it won’t be, but will tend to go off and join others of its ilk in forming a different kind of mineral altogether. Depending on the geometry of the fit of minerals, the lattice can be very strong (like with diamond), very weak (like gypsum), or strong in some directions and weak in others (like micas that slide around on weak parallel surfaces but are very strong across these surfaces). It all depends on the intermolecular forces holding them together: intermolecular forces within the crystal lattice vary in strength as pressure-temperature conditions change.
Though they all aspire, no crystal lattice is perfect: all contain stray atoms that don’t fit well, and when pushed and shoved around by tectonic forces or the forces in a pressure vessel, the lattice itself can bend or crack – lattices don’t like this. The molecular forces that hold lattices together come unglued, bonds are broken, tangled, confused. Disorganized lattices collect bands of internal dislocations in their structure and if the pressure isn’t removed, one of three general kinds of things can happen:
--At lower temperatures and pressures, distinct mineral crystals within a rock can rub, rotate, and grind against each other, milling themselves down to smaller sizes, cracking weakened lattices when it suits them, and then flowing as a solid mix of finely milled material. This is a process called cataclasis, a kind of mechanical process for intergranular flow. Fault zones in the earth’s crust love to move via cataclasis before they, crack!, rupture apart and cause an earthquake.
--At high temperatures and pressures, the means of deforming a crystal lattice itself come into play via a number of processes we can group together as sorts of “creep” phenomena, that is, via diffusion, dislocation, and dissolution. If the pressure becomes constant for a while or temperatures are high enough, the minerals recrystallize into a more stable orientation, annealing into “new” more perfect lattices – and when they do this, the crystals shift into a more stable position, that is, they flow.
In a lot of nasty conditions, rocks flow using a combination of the two methods described above: they’ll grind themselves into a mess of very fine-grained crystals, and then with very little encouragement, the milled mess will itself start to anneal, forming altogether new crystals (called “neoblasts”). So by crushing, milling, recrystallizing and forming new crystals, rocks succeed in flowing.
--A third process of solid rock flow involves the strange behaviors of minerals at very high pressures such as those within the mantle itself. The most common mineral of the upper mantle is olivine; under sufficient pressure, olivine is capable of dissolving water (yes, water!) into its lattice structure, and by doing so, the lattice is much more easily deformed. At even higher pressures (such as at the upper-lower mantle transition and below), the common lattice of olivine succumbs, and a new, more compressed sort of lattice is stable: olivine transforms to ringwoodite and thence to perovskite at the upper/lower mantle transition. When volumes change, there’s always the option of material flow.
So – here we’ve very quickly gone through processes by which solid rocks flow on a molecular level. Within the earth’s mantle, flow occurs by the deformation and en masse movement of zillions of individual mineral crystals: no single crystal flows far and it doesn’t flow fast, but altogether this flow is THE flow that causes the tectonic plates to move. Consider the scales involved: how do immense tectonic plates move? They move via the deformation of lattice structures, at molecular rates, one crystal at a time.
Photo: Flowing limestone and schist within a ductile shear zone, Aliakmon River.
More reading for the courageous: