Petrology. Igneous, sedimentary and metamorphic / Петрология. Изверженные, осадочные и метаморфические породы

Leaving aside the atmosphere and hydrosphere, rocks are the stuff of which the earth is made. Because of this, the study of rocks, petrology (from the Greek petra, “rock,” and logos, “discourse or explanation”), occupies a central position among the earth sciences. The study of rocks is the source of virtually all of our ideas about the history of Earth. Knowledge about rocks, their origin and ages, and their distribution is potentially capable of contributing to the solution of a wide variety of problems that run the gamut of geological interests.1. Most evolutionary biologists believe that living matter evolved from nonliving matter more than 3 billion years ago in a reducing atmosphere, because it is thought that primitive cells would have been defenseless against oxidation. This suggestion implies that there was little or no oxygen in the atmosphere of the early earth, a theory supported by the absence of free oxygen in the atmospheres of other planets in our solar system. However, early Precambrian (Archean) iron ores contain oxidized iron, in the form of magnetite, a fact that might be interpreted to mean that the early atmosphere contained substantial oxygen. Similarly, Precambrian weathering zones or soils have a red appearance caused by oxidized (hematitic) iron. Did these ores and soils contain magnetite or hematite from the beginning, or was more reduced iron gradually oxidized as free oxygen increased in the atmosphere in the later Precambrian and after? Can we use these rocks to infer how much oxygen was present in the early atmosphere? 2. The relative abundance of different sedimentary materials forming at present is drastically different from rock abundances found in the geologic record. <...>