  We'll examine plate tectonics on Earth -- its features and forces -- and
examine some concepts that may allow astronomers to ask useful questions
regarding numeric models that putatively predict tectonic activity. But
exo-planetologists should be aware that geologists are still attempting to
understand: why does Earth operates as it does, and so much differently than
its neighbors? Has it always operated this way and have other planets of the
inner Solar System ever mimicked Earth's behavior in their past? These problems
are unsolved, though some interesting speculative notions have emerged. Studies
by Foley et al. et al. (2012) and Weller and Lenardic (2018), for example,
attempt to distill the essential planetary properties that may influence if not
dictate possible tectonic states, while Yin et al. (2016) propose a model of
planetary tectonic surface features that appears remarkably precise. These
studies yield some compelling expedients for analyses of planetary objects both
within and outside our Solar System.
