Stratigraphy
Stratigraphy is that subarea of geology that treats the description, correlation, and interpretation of stratified Earth materials. Typically, geologists consider stratified Earth materials as layers of sediment or sedimentary rock. This definition, however, clearly encompasses other materials such as volcanic lava, ash flows, ash-fall layers, meteoritic impact ejecta layers, and soils. In fact, using this definition, any material that obeys the law of superposition during its formation could be placed in the domain of stratigraphy. Generally, internal layers within Earth (crust, mantle, and core) are not considered the type of layers studied by stratigraphers because they formed by Earth's internal differentiation processes.
Some geologists give a broader definition to the term stratigraphy. Planetary geologists sometimes view stratigraphy as if it were the study of the sequence of events on a planet or moon's surface. In addition, stratigraphy has been broadly used by some geologists who study mountain building and plate tectonics to mean the study of order of emplacement of rock units of various types, including igneous and metamorphic rocks, to which the law of superposition does not apply. In some cases, stratigraphy is used to define the study of geologic history of an area or country, but it is more correct to say that stratigraphy is the practical foundation for historical geology.
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