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Concept
Lithology
Summary
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.
Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations, lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7.
The naming of a lithology is based on the rock type. The three major rock types are igneous, sedimentary, and metamorphic. Igneous rocks are formed directly from magma, which is a mixture of molten rock, dissolved gases, and solid crystals. Sedimentary rock is formed from mineral or organic particles that collect at the Earth's surface and become lithified. Metamorphic rock forms by recrystallization of existing solid rock under conditions of great heat or pressure.
Igneous rocks are further broken into three broad categories. Igneous rock composed of broken rock fragments created directly by volcanic processes (tephra) are classified as pyroclastic rock. Pyroclastic rocks are further classified by average fragment (clast) size and whether the fragments are mostly individual mineral crystals, particles of volcanic glass, or rock fragments. Further classifications, such as by chemical composition, may also be applied. Igneous rocks that have visible mineral grains (phaneritic rocks) are classified as intrusive, while those that are glassy or very fine-grained (aphanitic) are classified as extrusive rock. Intrusive igneous rocks are usually classified using the QAPF classification, which is based on the relative content of quartz, alkali feldspar, plagioclase, and feldspathoid.
Official source
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