Law of included fragments

Part of a series on
Geology
Science of the solid Earth
Index Outline Category Glossary History (Timeline)
Key components Minerals Rock (Igneous Sedimentary Metamorphic) Sediment Plate tectonics Strata Weathering Erosion Geologic time scale
Laws, principles, theories Stratigraphic principles Principle of original horizontality Law of superposition Principle of lateral continuity Principle of cross-cutting relationships Principle of faunal succession Principle of inclusions and components Walther's law
Topics Composition Geochemistry Mineralogy Sedimentology Petrology Structure of Earth Landform structures Geomorphology Glaciology Structural Geology Volcanology Geologic history Geological history of Earth
Research Branches of geology Geologist (List) Methods Geological survey
Applications Engineering Mining Forensics Military
Planetary geology Lists of geological features of the Solar System Geology of solar terrestrial planets By planet and body Mercury Venus Moon Mars Vesta Ceres Io Titan Triton Pluto Charon
Geology portal
v t e

The law of included fragments is a method of relative dating in geology. Essentially, this law states that clasts in a rock are older than the rock itself.^[1] One example of this is a xenolith, which is a fragment of country rock that fell into passing magma as a result of stoping. Another example is a derived fossil, which is a fossil that has been eroded from an older bed and redeposited into a younger one.^[2]

This is a restatement of Charles Lyell's original principle of inclusions and components from his 1830 to 1833 multi-volume Principles of Geology, which states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them.