Dislocations
Rock creep involves creation, motion, and annihilation of dislocations, which together control the rock’s strength. Understanding dislocation dynamics lets us understand how the rocks creep on time-scales of billions of years (mantle convection), as well as a few years (post-seismic creep), or even seconds (seismic waves).
Grain Boundaries
When a rock creeps, some of its deformational energy goes towards forming dislocations, and some of it goes towards forming new grain boundaries. Changes in grain boundary area or, in other words, grain size, can have dramatic effects of rock strength and lead to shear localization. Shear localization is essential for forming tectonic plate boundaries.
Mineral Mixing
Rock samples from the field and rocks deformed in the laboratory all seem weaker when they are made up of more than one type of minerals. Earth’s weakest parts of the lithosphere – mylonitized shear zones and tectonic plate boundaries – are mostly made up of polymineralic rocks. Mixing of different minerals and its effect on rocks’ strength can provide insight into formation of plate boundaries.
Unified Theory of All Creep
Physical laws that govern solid state creep are universal. We can use advancements in understanding of creep in one type of materials (rocks, metals, or ice) to understand the others, while bearing in mind that each material is exotic in its own way. Some (metals) are easier to study at laboratory conditions, deformation of others (ice) is more readily observed on human timescale, others yet (rocks) are the most abundant for direct observations on Earth and other planetary bodies.