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Clasts in a rock are older than the rock formation
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.
Notes
[
edit
]