Scientists also use direct evidence from observations of the rock layers themselves to find the relative age of rock layers.The principle of inclusions states that inclusions found in other rocks (or formations) must be older than the rock that contain them. Geologists call it relative dating — we know which one is older but do not know how old they are.This is actually pure logic and it can be applied not only in geology, but it is especially useful for geologists. Inclusions of foreign rocks that are found in igneous rocks are named xenoliths.For liquids and solids the standard is usually water at 4°C or some other specified temperature.For gases the standard is often air or hydrogen at the same temperature and pressure as the substance.If this is the case, we can not say that the inclusion is older than the rock that surrounds it. (Units) the ratio of the density of a substance to the density of a standard substance under specified conditions.The same principle is also used in relative dating of sedimentary rocks.
Scientists have good evidence that Earth is very old, approximately four and one-half billion years old.Scientific measurements such as radiometric dating use the natural radioactivity of certain elements found in rocks to help determine their age.Note that there are one larger and several smaller pieces of granite within kersantite. So we can also say that kersantite contains xenoliths of granite. It was already solid rock when it was intruded by mafic lamprophyric magma that scraped some pieces off of granitic rock and embedded them within the solidifying magma.True xenoliths are definitely older than their host rocks but sometimes igneous rocks contain cognate inclusions or restite material.
S-type granites for example (granite with a sedimentary protolith) may contain such inclusions which are genetically related to its host rock.