As a result, xenoliths are older than the rock which contains them. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. A similar situation with igneous rocks occurs when xenoliths are found. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.
The principle of inclusions and components 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. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault. Faults are younger than the rocks they cut accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut.
There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.
In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. The principle of intrusive relationships concerns crosscutting intrusions. Many of these involve the ability to provide the relative ages of strata or the manner in which they were formed. There are a number of important principles in geology.
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