The law of included fragments is a method of
relative dating
Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age (i.e., estimated age). In geology, rock or superficial dep ...
in
geology. Essentially, this law states that
clast
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...
s in a rock are older than the rock itself. 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
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
, which is a
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
that has been eroded from an older
bed
A bed is an item of furniture that is used as a place to sleep, rest, and relax.
Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many beds ...
and redeposited into a younger one.
[D. Armstrong, F. Mugglestone, R. Richards and F. Stratton, OCR AS and A2 Geology, Pearson Education Limited, 2008, p. 276 ]
This is a restatement of
Charles Lyell's original principle of inclusions and components from his 1830 to 1833 multi-volume ''
Principles of Geology
''Principles of Geology: Being an Attempt to Explain the Former Changes of the Earth's Surface, by Reference to Causes Now in Operation'' is a book by the Scottish geologist Charles Lyell that was first published in 3 volumes from 1830–1833. Ly ...
'', which states that, with
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s, if
inclusion
Inclusion or Include may refer to:
Sociology
* Social inclusion, aims to create an environment that supports equal opportunity for individuals and groups that form a society.
** Inclusion (disability rights), promotion of people with disabiliti ...
s (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 rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...
s 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.
Notes
Petrology
Stratigraphy
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