In geology, rock layers contain physical evidence that helps scientists determine the sequence of past events. Fragments of older material embedded within a younger rock layer are known broadly as clasts or inclusions. These fragments are the basis for relative dating, a technique that establishes a timeline of Earth’s history without needing to know the absolute age of the rocks in years. By observing these incorporated pieces, researchers can reconstruct the order in which rock formations were deposited, eroded, and solidified.
Defining Clasts and Inclusions
The terms “clast” and “inclusion” refer to fragments of pre-existing rock or mineral incorporated into a new rock body. A clast is the general term for detrital material, a fragment broken off another rock by physical weathering and erosion. Clasts are the building blocks of sedimentary rocks, ranging in size from microscopic clay particles to large boulders.
The term “inclusion” is used specifically when a fragment is fully enclosed within a larger, surrounding rock mass. In a sedimentary context, inclusions are often recognizable pebbles, gravel, or sand grains from an older layer cemented into a younger one. For example, a conglomerate containing smooth, rounded pebbles of an older rock type indicates that the older rock must have existed before the conglomerate formed.
A different type of inclusion, called a xenolith, occurs in igneous rocks, meaning “foreign rock.” Xenoliths are fragments of the surrounding “country rock” torn off and incorporated by ascending magma or lava. The intense heat of the magma can alter these fragments, but the presence of a xenolith indicates the fragment was already solid and cool before the molten rock engulfed it.
The Principle of Inclusions
The physical relationship between an inclusion and its host rock is formalized by the Principle of Inclusions, one of the fundamental rules of relative dating in stratigraphy. This principle states that any rock fragment enclosed within another rock must be older than the rock containing it. The logic is that the fragment had to exist as a solid piece before it could be broken off and enveloped by a newer, solidifying material.
This rule is used to sequence geological events, especially those involving igneous intrusions that cut through existing rock layers. For instance, if a dike of granite magma cools and solidifies, and it contains inclusions of a nearby sandstone layer, the sandstone must be older than the granite intrusion. The magma ripped off pieces of the sandstone as it pushed through, establishing a clear time sequence. Applying this principle alongside other rules, like the Principle of Superposition, helps geologists build a reliable relative age timeline for complex rock formations.
Where These Fragments Are Found
These fragments are commonly found in geological settings involving significant erosion followed by new deposition. One common location is at the base of an unconformity, a buried surface representing a gap in the geologic record, often due to erosion. At this boundary, the younger, overlying sedimentary layer often begins with a basal conglomerate or breccia.
This basal layer is packed with clasts eroded directly from the older rock beneath the unconformity. The process involves the erosion of the source layer, the transport and deposition of those fragments in a new environment, and the eventual lithification, or hardening, of the younger rock around them. The presence of this basal conglomerate indicates that the underlying rock was exposed to the surface and weathered before the younger layer was deposited.
In igneous settings, xenoliths are found within solidified magma bodies, such as volcanic flows or deep-seated plutons. As magma rises through the crust, it mechanically breaks off pieces of the surrounding rock, incorporating them into its bulk. Xenoliths of deep mantle material, carried up by rapidly ascending magmas like kimberlite, offer rare samples of the Earth’s interior that are otherwise inaccessible for study.