Petrologists recreate crushing mantle pressures in the lab to study rocks nobody can reach
Some of the most important rocks on Earth never make it to the surface intact, and those deep inside other planets are completely out of reach. Petrology, the science of how rocks form, gets around this by building machines that squeeze and heat samples until they behave as they would far below the crust.
The name joins Greek words for rock and study. Petrology examines what rocks are made of, their minerals, texture and structure, and above all the conditions that produced them. It draws on mineralogy, optical mineralogy and chemical analysis, and increasingly on geochemistry, geophysics and thermodynamic data to reconstruct a rock's origins.
The field divides along the lines of the three great rock families. Igneous petrology deals with rocks that crystallised from magma, whether they cooled at the surface like basalt or deep underground like granite. Metamorphic petrology studies rocks such as slate, marble and schist, transformed by heat, pressure or both from an earlier parent rock that could have been of any type. Sedimentary petrology looks at sandstone, shale and limestone, built from fragments of older rock or from biological and chemical deposits. Because the first two rely heavily on chemistry and phase diagrams, universities usually teach them together, while sedimentary work tends to be paired with stratigraphy.
A fourth branch, experimental petrology, uses high-pressure, high-temperature equipment on natural and synthetic materials. It matters most for rocks of the lower crust and upper mantle, which rarely arrive at the surface unaltered, and it is one of the few windows onto the lower mantle and the interiors of the Moon and other rocky planets. Much of today's understanding of igneous and metamorphic processes rests on these experiments.
The vocabulary has shifted over time. Lithology once meant much the same as petrography, but now it refers to describing rocks by eye in hand samples or outcrops, while petrography handles the microscopic detail. In the oil industry lithology has a practical form: mud logging, where rock cuttings flushed from a borehole are sampled, checked under a microscope magnifying ten times and drawn onto a chart of the formations being drilled.
Source: Petrology
