The metamorphic rock record through Earth's history

Regionally metamorphosed rocks provide a direct record of past tectonic activity. Pressures, temperatures, and timescales of metamorphism in the rock record vary as functions of age. With the caveat of imperfection in the rock record, these differences challenge the assumption that Earth's tectonic processes have always operated as they do today. The purpose of this chapter is to summarize these changes in the metamorphic rock record and some of the secular changes in tectonics (or lack thereof) that they might record.