Geologists use relative dating to determine the sequence of events in Earth’s history without needing the exact numerical age of the rocks. This process relies on logical rules that establish the chronological order in which different geological features formed. Understanding the relationship between a rock body and the smaller pieces of material it contains is fundamental to establishing this timeline. Analyzing the origin of these fragments helps determine which material existed first and which formed later to enclose it.
The Principle of Included Fragments
The fundamental rule for determining the relative age of internal rock pieces is the Principle of Included Fragments. This principle states that any fragment of rock, or inclusion, found enclosed within a larger host rock must be older than the rock that contains it. The logic is straightforward: the fragments had to be pre-existing material that was later incorporated, surrounded, and solidified by the host rock.
An inclusion is a piece of rock, mineral, or fossil physically trapped inside another rock body. The act of trapping these fragments, whether through deposition or cooling, marks the formation of the younger host rock. Therefore, the included fragment represents a piece of an older rock formation that was broken up and preserved. This principle is one of the foundational concepts used in stratigraphy to build a relative chronological history of rock layers.
Inclusions in Sedimentary Rock Layers
The Principle of Included Fragments is demonstrated in the formation of clastic sedimentary rocks, which are made up of weathered pieces of older rocks. These fragments, often called clasts, are eroded from a source rock, transported by water or wind, and eventually deposited. The clasts must be older than the sedimentary layer they become a part of, as they existed as solid pieces before being moved.
Rocks like conglomerates and breccias are classic examples, composed of large, visible inclusions set within a finer-grained matrix. In a conglomerate, rounded clasts indicate they were tumbled and smoothed during long-distance transport before being cemented. Conversely, the angular fragments found in a breccia suggest a shorter transport distance and less abrasion, but the age relationship remains the same. The process of lithification—where loose sediment is compacted and cemented into solid rock—is the final event, making the resulting sedimentary layer the younger formation enclosing the older clasts.
Inclusions in Igneous Rock Bodies
The same age relationship applies to igneous rocks, which form from the cooling and solidification of molten material. As magma rises through the Earth’s crust, it can fracture and break off pieces of the surrounding solid rock, known as the country rock. These captured fragments are called xenoliths, a term that literally means “foreign rock.”
Since xenoliths were solid parts of the crust before the magma intruded, they are older than the igneous rock body that now hosts them. The liquid magma engulfs these pieces of country rock and cools around them, preserving the older fragment within the younger igneous matrix. Xenoliths are useful because they provide samples of deeper, unexposed crustal or mantle material, offering direct evidence of the rocks that existed before the intrusion event. Studying these inclusions allows scientists to determine the minimum age of the country rock and gain insights into the source region’s composition.