Petrology is the branch of geology that studies rocks, their mineralogy, composition, texture, structure and the conditions under which they form. This scientific discipline transforms the mundane appearance of a stone into a complex narrative of Earth's history, revealing that what looks like a simple gray boulder is actually a library of chemical reactions and physical forces.
The discipline has three subdivisions: igneous, metamorphic, and sedimentary petrology, each offering a unique window into the planet's dynamic processes. Igneous and metamorphic petrology are commonly taught together because both make heavy use of chemistry, chemical methods, and phase diagrams. Sedimentary petrology is commonly taught together with stratigraphy because it deals with the processes that form sedimentary rock.
Experimental petrology employs high-pressure, high-pressure apparatus to investigate the geochemistry and phase relations of natural or synthetic materials at elevated pressures and temperatures. These experiments are particularly useful for investigating rocks of the lower crust and upper mantle that rarely survive the journey to the surface in pristine condition. They are also one of the prime sources of information about completely inaccessible rocks, such as those in the Earth's lower mantle and in the mantles of the other terrestrial planets and the Moon.
Lithology was once approximately synonymous with petrography, but in current usage, lithology focuses on macroscopic hand-sample or outcrop-scale description of rocks while petrography is the specialty that deals with microscopic details. Petrologists utilize the fields of mineralogy, petrography, optical mineralogy, and chemical analysis to describe the composition and texture of rocks. The distinction between these two fields highlights the dual nature of petrological study, which requires both a broad view of the rock's appearance and a detailed examination of its internal structure.
In the petroleum industry, lithology, or more specifically mud logging, is the graphic representation of geological formations being drilled through and drawn on a log called a mud log. As the cuttings are circulated out of the borehole, they are sampled, examined typically under a 10× microscope and tested chemically when needed. This practical application of petrology is crucial for the exploration and extraction of oil and gas, providing real-time data about the rocks being drilled.