Law of included fragments
Encyclopedia
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. 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.
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.
Relative dating
Relative dating is the science determining the relative order of past events, without necessarily determining their absolute age.In geology rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another...
in geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
. Essentially, this law states that clasts in a rock are older than the rock itself. One example of this is a xenolith
Xenolith
A xenolith is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption...
, which is a fragment of country rock that fell into passing magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
as a result of stoping
Stoping
Stoping is a process accommodating the ascent of magmatic bodies from their sources in the mantle or lower crust to the surface. The theory was independently developed by Canadian geologist Reginald Aldworth Daly and American geologist Joseph Barrell....
. Another example is a derived fossil
Derived fossil
A derived fossil or reworked fossil is a fossil found in rock made later than when the fossilized animal or plant died: it happens when a hard fossil is freed from a soft rock formation by erosion and redeposited in a currently forming sedimentary deposit....
, which is a fossil that has been eroded from an older bed and redeposited into a younger one.
This is a restatement of Charles Lyell
Charles Lyell
Sir Charles Lyell, 1st Baronet, Kt FRS was a British lawyer and the foremost geologist of his day. He is best known as the author of Principles of Geology, which popularised James Hutton's concepts of uniformitarianism – the idea that the earth was shaped by slow-moving forces still in operation...
's original principle of inclusions and components from his 1830 to 1833 multi-volume Principles of Geology
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....
, 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
Matrix (geology)
The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...
. As a result, xenoliths are older than the rock which contains them.