Eskola's metamorphic facies is a classification system used to describe the conditions under which metamorphic rocks form. Developed by Finnish geologist Pentti Eskola in the early 20th century, it categorizes rocks based on their mineral composition and the temperature and pressure conditions during metamorphism. The main facies include greenschist, amphibolite, and granulite, each representing different ranges of temperature and pressure. For example, greenschist forms at lower temperatures and pressures, while granulite forms under high-temperature conditions. This system helps geologists understand the geological history and processes that shaped a region.